Types of Electromagnetic Waves

In the previous section we described EM waves as "waves consisting of vibrating electric and magnetic fields", hence their name, "electro-magnetic"; also we said that they travelled as transverse waves (as oppossed to longitudinal). We also said that, like ALL waves, they transfer energy from place to place or from a source (eg the Sun) to an absorber (eg the Earth).

We listed all the many types of EM waves in the previous section, but we will do so again here, although this time we will list them in a more useful fashion; we will put them in order of their increasing wavelength (or decreasing frequency):

If you look near to the centre of the chart you will see the section "Visible Light" (the only section of this EM chart that our eyes can detect; notice how small it is) and you will see that it is shown in its characteristic 7 colours, from Red to Violet.
You will know, I hope, that we often call this section of the EM chart - the Visible Spectrum.
Although we might draw this region as consisting of 7 distinct colours, in reality the 7 colours blend "continuously" from one into the next (this is why Red blending into Yellow, passes through Orange and so on.)

Well, the same "blending" from one type of EM wave to the next occurs with the whole of the EM chart and so we call it the EM Spectrum.
So, there isn't a sudden change in type of wave, as the chart might suggest, when we go from eg Radio type to Microwave type, or from X Ray type to Gamma wave type.
A good way to illustrate the continuous nature of the EM Spectrum is to draw a wave whose wavelength changes continuously as it goes from Radio type to Gamma type.

The common property of all of the EM types that enables them to be part of a continuous spectrum is: they all travel at the same velocity, 3 x 10 ⁸ m/s in a vacuum (eg in space) or in air.

Before we leave this introductory section for the EM Spectrum we need to give a few (eg 3) examples of the transfer of energy by means of EM waves.

1. Radio waves (which include waves used for TV) are used to transfer energy from a radio/TV station to a radio/TV receiver which converts the radio wave energy into sound/light energy for us to hear and see.

2. Microwave energy is absorbed by food in a microwave oven causing the food to rise in temperature and become cooked!

3. Infra Red waves are transferred from the Sun and are absorbed by the Earth causing it to warm up.

The final fact on which to end this introduction is:
Every warm object will emit EM waves!
This might not sound believeable, but it is true; every object that has a temperature above absolute zero will emit EM waves; the hotter it is, the shorter will be the wavelength of the EM waves emitted. For example, the Sun is very hot so it emitts EM waves in the UV region and, of course, the visible region (as well as others) but a kettle containing boiling water will only emit waves in the IR region.
This fact will be important in the next section.

Different substances may absorb, transmit, refract or reflect electromagnetic waves in ways that vary with wavelength.

The above is a direct quote from the AQA Physics specification.

The quote is about how substances respond to the range of EM waves and you can see that it is saying that up to 4 things can happen when an EM wave strikes a substance; it can be absorbed, transmitted, refracted or reflected. Let's look at these in two sections:

1. absorb, transmit, reflect EM Waves

Let's consider one common substance, a piece of glass. It will:
absorb certain wavelengths of EM waves,
but it will transmit or reflect others.

Look at the following illustration:

Short wavelength UV and Visible EM waves from the Sun will pass through (or be transmitted through) our atmosphere very easily. The Visible EM waves will then be transmitted through the glass of the greenhouse and be absorbed by the plants and flowers and soil and other stuff inside the greenhouse causing them all to warm up. (The UV waves are actually absorbed by glass; evidence for this is that we don't tend to get sun tanned through closed car windows.)

Now because of this the objects in the greenhouse become warm and will naturally emit their own EM waves (see the section directly above) but because their temperature is much lower than that of the Sun, they will emit EM waves with a much longer wavelength, in the Infra Red region of the EM Spectrum.

Unlike the Visible waves, these longer wavelength Infra Red waves can NOT get through the glass, instead they get reflected by the glass causing their energy to remain inside the greenhouse making it get hotter and hotter and hotter!
This is quite literally the "Greenhouse Effect" and the same name is given to the atmospheric problem that is affecting the Earth at this time.

So, what is going on here with regard to the material Glass and how it responds to different types of EM waves?

Firstly, the general rule is - the shorter the wavelength of the EM wave, the more penetrating it is.
Hence, the relatively long wavelength Infra Red waves emitted by the warm plants do NOT penetrate through the glass whilst the shorter wavelength Visible waves, from the Sun, do. The even shorter wavelength, X-Rays and Gamma waves, would also penetrate the glass easily.

But, secondly, substances have their own properties based on their molecular structure; glass has the peculiar (but useful) property of NOT transmitting UV waves despite their very short wavelength.

2. refract EM Waves

Still considering the glass greenhouse example - less noticeable, but happening, the visible light will get refracted by the glass when it passes into the greenhouse.

You should already know about refraction from the first page of the waves section of the syllabus.
In that section we learnt that refraction of any wave occurs if:
1. there is a change in the wave speed when it passes from one material to another, and
2. the wave strikes a boundary (that causes the change in speed) at some angle other than 90 degrees to the boundary (or Zero degrees to the Normal to the boundary).

When an EM wave such as visible light strikes a boundary such as Air into Glass, the waves are slowed down.
If the visible light strikes this boundary at exactly 90 degrees to the boundary (or Zero degrees to the Normal to the boundary) then there is NO change in direction and refraction does NOT occur.
But inside the glass the wave has still slowed down which causes the wavelength of the wave to decrease. Look at the following illustration:

As you can see, the ray diagram shows that there is no change at all to the path of the ray, but the wave diagram shows that the wavefronts are closer together inside the glass because the waves have slowed down in the denser medium, but even so, there is no change in their direction, agreeing with the ray diagram.

if, however, the ray crosses the air to glass boundary at an angle other than 90 degrees to the boundary, we see a different outcome:

We remember that the wavefronts travel slower in the glass (the denser medium).
So the part of each wavefront which enters the Glass first is slowed down whilst the other part keeps moving in the Air at the original speed. Eventually the whole wavefront is in the Glass travelling at the same reduced speed; this causes the wavefronts to bunch up together, meaning a reduction in the wavelength, and to shift direction as shown.

So the principal cause of Refraction is the change in velocity of a wave as it passes from one medium to another.

Light waves slow down when they pass from a less dense into a more dense medium, eg from Air into Glass or from Air into Water, and
light waves speed up when they pass from a more dense into a less dense medium, eg from Glass into Air or from Water into Air.

To make it simple to solve refraction problems and correctly draw refraction ray diagrams it is good to learn a few simple "Rules of Refraction". These are:

1. A ray of light passing from one medium to another along a Normal is NOT refracted.
2. A ray of light passing from a less dense medium into a more dense medium at an angle to the Normal is refracted TOWARDS its Normal.
3. A ray of light passing from a more dense medium into a less dense medium at an angle to the Normal is refracted AWAY FROM its Normal.

We can easily illustrate these 3 rules with 3 simple ray diagrams:

Before we do, a few things to clarify
- the Normal is an imaginary line at 90 degrees to the boundary; it is always drawn as a dashed line.
- the ray entering the boundary is called the Incident Ray.
- the ray on the other side of the boundary is called the Refracted Ray.
- the final ray, when two or more refractions take place, is called the Emergent Ray.

We can illustrate Rules 2 AND 3 in one diagram, as follows:

As you can see, the ray is refracted towards its Normal when it enters the glass (passing from less to more dense material, Rule 2), then it is refracted away from its Normal when it exits the glass (passing from more to less dense material, Rule 3).

According to the syllabus you need to be able to construct ray diagrams to illustrate the refraction of a wave at the boundary between two different media. To do this you need to make use of the 3 Rules of refraction. Let's look at an example:

Have a go at a few ray diagram questions yourself:

Radio Waves, UV, X and Gamma Waves

In this second section about properties of EM waves we are going to look specifically at those EM waves at the two extremes of the EM Spectrum, Radio Waves at one end, and UV, X and Gamma at the other end.

Radio Waves

We are all familiar with Radio Waves even if we haven't really thought much about them until now.
Radio waves are used to transmit TV and Radio programmes which are then received by our TV and radio receivers.
Two very special properties of EM Radio waves enables them to be produced and received.
1. How are they produced?
Back in the first few decades of the 20th century, soon after Physicists had developed a pretty good understanding of the atom, they realised that if electrons were accelerated in the form of a vibration then they emitted or produced EM radiation/waves.
Better still, they found out that the frequency of the EM wave produced exactly matched the frequency of the vibration of the electrons.
So, to produce a Radio wave of a certain frequency, all you have to do is get electrons to vibrate (or oscillate) in a piece of wire at that exact frequency.

The following diagram illustrates the process:

In a real life situation, the "piece of wire" is a metal aerial, often mounted on top of a very high mast which is then placed on a high hill; you will see these around the countryside, used to produce/transmit EM Radio Waves for radio and TV programmes.

2. How are they received?
Receiving the Radio wave is even easier than producing it!
If a Radio wave passes close to a piece of wire, it will induce into the wire an alternating current at the exact same frequency as the Radio wave; in other words - receiving a radio wave is just the reverse of producing one.

In the diagram above, the Ammeter (the circle with the letter "A") is simply to detect the presence of the induced alternating current in the wire.
As with the transmitter diagram, the "piece of wire" is a metal aerial in a real world receiving system; you will probably have such an aerial on your roof top to receive "TV" Radio waves; you might have a portable "radio" with an extending metal rod aerial or simply a piece of wire, to receive "radio" Radio waves (radio programmes).

Now you know how EM Radio waves are produced and received.
It is all due to 2 properties of EM Radio Waves:
1. Radio waves are produced by vibrations/oscillations of electrons (an alternating current) within a piece of wire in an electrical circuit; the frequency of the Radio wave will be the same as that of the vibration of the electrons (the alternating current).
2. Radio waves will induce an alternating electrical current into a piece of wire or an electrical circuit; the frequency of the induced current will be the same as that of the incoming Radio wave. Another way of saying this is - when a Radio wave is absorbed it may create an alternating current of the same frequency in an electrical circuit.

UV, X and Gamma Waves

Another interesting property of EM waves also concerns atoms and electrons.
Electrons are arranged in locations around an atom's nucleus in what we call "energy levels".
Sometimes an electron will fall from a high energy level to a lower energy level and in so doing, it releases the "lost" amount of energy as an EM Wave. This is how Gamma waves are produced.

As we found with Radio waves, the reverse of producing the Gamma wave can also happen, ie some nuclei will absorb a Gamma wave causing an electron to rise from a low to a higher energy level, effectively storing an amount of energy within the atom.

Dangers of UV, X and Gamma Waves

An undesirable property of some types of EM wave is their ability to ionise atoms that happen to absorb them.
This is undesirable because "ionising" an atom changes its structure and if that atom is part of a living cell such as within living creatures, then that "changed" cell can become cancerous!
(It is thought that the "change" within the cell that leads to cancer is mainly due to a change (or a mutation) in the DNA of the cell.)

That is why we have to be careful when it comes to exposing ourselves to UV, X or Gamma waves.

To help people to determine their exposure risk Physicists have developed a radiation dose unit called a sievert (Sv) and one can look up the dose expected from certain procedures such as a hospital X-ray, then compare the dose to the recomended maximum annual dose.
For example a typical chest X ray involves exposure to about 0.02 milli Sv (mSv). The recomended max for the general public is 1 mSv per year. So one chest X ray is well within the recomended max.

NB. 1 Sv = 1000 mSv (the lower case "m", you should know, means "thousandth", so 1 mSv is one thousandth of a Sv.)

It should be obvious that the higher dose a person experiences of any type of radiation, the more damaging it will be for the body's tissues, potentially causing cancer, as mentioned above.
But also, the type of EM radiation will have an effect on the the exposure risk.

Generally, the shorter the wavelength of the EM radiation, the more dangerous it can be; this is because shorter wavelengths will penetrate materials more easily. So, UV waves can potentially cause skin cancer since they will only penetrate just through skin, but X radiation and especially Gamma radiation can penetrate deeper to our internal organs, making them potentially much more dangerous.

Uses and Applications of EM Waves

Electromagnetic waves have many practical applications. For example:
• radio waves – television and radio
• microwaves – satellite communications, cooking food
• infrared – electrical heaters, cooking food, infrared cameras
• visible light – fibre optic communications
• ultraviolet – energy efficient lamps, sun tanning
• X-rays and gamma rays – medical imaging and treatments.

Apart from knowing the above list, you need to be able to give a brief explanation of why each particular type of EM Wave is suitable for the application.

Radio waves - TV and radio
Why? Because Radio waves will travel over large distances and are little affected by buildings etc, so they can be used to communicate with people over a very wide area. Longer wavelength radio waves will reflect off the upper layers of the atmosphere (the ionoshpere) making them reach around the world.

Microwaves - satellite communications, cooking food
Why? Just as longer wavelength radio waves will reflect off the upper atmosphere, microwaves (which are just very, very short wavelength radio waves) pass straight through the atmosphere, reaching into space; this property of microwaves makes them suitable for communicating with orbiting satellites.

Microwaves (of a particular frequency) have the property of being easily absorbed by water and objects containing water; when this happens, the object heats up; this makes them ideal for use in cooking food.
Notice how this also explains why a microwave oven (always use its full name) will not "heat up" eg a woolen glove! And, why you have to make sure there is a water containing object in the oven before you turn it on, to absorb the microwaves.

Infrared – electrical heaters, cooking food, infrared cameras
Why? If we heat up, eg a bar of metal, till it is NOT quite "red hot", we will feel the heat emanating or "radiating" from it; what we feel is the heating effect of invisible InfraRed EM Waves. So, an electrical "heater" might use a coil of high resistance wire through which travels the electrical current; it does NOT have to glow red before it begins to give off heat. People, however, expect to see them glowing red hot, so inevitably that is what the designers allow to happen, but the heat felt by the user is due to invisible InfraRed Waves, not the visible Red light waves.
Similarly an electric hob which has coiled metal elements (ie not the more modern halogen style hobs) doesn't have to glow red before it produces heat.
So InfraRed is ideal for cooking applications.

Remote controls use an InfraRed LED to send out pulses of infrared "light" to control a TV, but if you look at the LED you will not see it lighting up, because the InfraRed is not visible. This is preferable to having a visible light LED which would flash visible light.

Now we know that hot objects emit invisible InfraRed waves it should be obvious how InfraRed cameras function. They use very sensitive InfraRed sensors instead of visible light sensors (used in "normal" cameras). The InfraRed camera will "see" any heat source including people or even the parts of people that are emitting the most InfraRed heat. This makes such cameras ideal for looking for people at night time when normal cameras would be of no use without visible light.

Visible light – fibre optic communications
Why? An optical fibre is a thin glass fibre which will allow light to enter at an end but it doesn't let it escape through its sides even though the glass is transparent! This might seem strange, but it is making use of a property called Total Internal Reflection, which as the name suggest, means that light always gets reflected back inside the fibre until it reaches the other end. So optical fibres can be used to carry pulses of light which contain information; they can carry the light pulses over short or very long distances and since the light never escapes through its side, bundles of fibres can be placed together allowing lots of information to be transferred along a fibre optic bundle. Visible light and fibre optics are ideal for modern digital communication whether that is for voice calls or internet data.

UV – energy efficient lamps, sun tanning
Why? With regard to the energy efficient lamps, these are those curvy tube lamps that are also known as Compact Fluorescent Lamps or CFL. They work by a process of fluorescence; UV waves are produced inside the tube which then hits a "fluorescent coating" which aborbs the invible UV and emits visible light. (Remember the above explanation of electrons being raised to higher energy levels when absorbing an EM wave, then falling to a lower level whilst emitting an EM wave.) Old style filament bulbs which worked by passing a current through a filament of wire, thus making it heat up till it glowed "white hot", consumed a much greater amount of energy compared to the modern CFL.

UV from the Sun causes our skins to tan, which is quite nice, but it can be harmful as mentioned above and can contribute to skin cancer. UV tanning lamps can be used to mimic the effect of the sun. These lamps are similar to the CFL in that they produce UV but do not have the fluorescent coating to turn the UV into visible light. People using such lamps have to consider the risk of doing so!

X-rays and gamma rays – medical imaging and treatments
Why?As already mentioned, the very short wavelength EM waves are the most penetrating which can be both a good and a bad thing. If used carefully they can be put to good use eg X rays will penetrate the soft tissues of our body but are stopped by denser bones, making them ideal for producing images to show the outlines of bones plus any fractures. Gamma rays are potentially the most dangerous due to their great penetrating ability plus their ability to ionise atoms and even kill them; but such a property can be put to good use if used VERY carefully to aim the gamma rays at cancer cells, sometimes deep within the body, to kill them; this is know as Radiotherapy and is standard practice for the treatment of cancer.

Refraction and Lenses

Lenses are optical devices, made of a transparent material such as glass, that make use of the refraction properties of the material and the particular SHAPE of the lens itself to produce an image.

There are two main shapes of lens:
1. Convex shaped Lens, and
2. Concave shaped Lens.

We make use of these two types or shapes of lens because they refract light quite differently to each other and can therefore be used in various instruments such as telescopes, microscopes or spectacles ("glasses") to control the path of light.
Notice the lens symbols; these make drawing the lenses much easier, so they are what we will use from now on.
Let's now look at what these two basic lens shapes do to a simple beam of parallel rays of light.

Convex lens

Notice how the Convex lens causes rays of light that are parallel to the Principal Axis to converge at a precise point which we call the Principal Focus. This is why Convex lenses are often described as Converging Lenses.
From this finding we can write a simple definition of a Convex lens:
"A convex lens is a lens that causes parallel rays of light to converge at the principal focus."

The distance from the lens to the Principal Focus is called the Focal Length.

Concave lens

Notice how the Concave lens causes rays of light that are parallel to the Principal Axis to diverge as though they came from the Principal Focus. This is why Concave lenses are often described as Diverging Lenses.
From this finding we can write a simple definition of a Concave lens:
"A concave lens is a lens that causes parallel rays of light to diverge from the principal focus."

As for the convex lens, the distance from the lens to the Principal Focus is called the Focal Length.

The really important thing to note about the Concave lens is that the light passing through it always "spreads out" or diverges, so it never forms a REAL image.
The light "appears" to come from a point, the Principal Focus, (notice the use of dashed lines) on the same side of the lens as the light source, so Concave lenses only ever produce what we call Virtual Images. These are images that appear to be at a point but they are not REALLY there!

As you can see in the first lens diagram, Convex lenses, cause light to converge on the opposite side of the lens compared to the light source, at an actual real place, the Principal focus. So, convex lenses, unlike concave, can produce Real Images. These are images that really are at a precise point.

Now that we know the basic behaviour of the two main lens shapes we will make use of more interesting ray diagrams to figure out exactly what these lenses will do when presented with objects at various positions.

Ray Diagrams - Convex Lens

In order to successfully construct lens ray diagrams, we need to make use of 2 Construction Rays. These are simply rays that we know have they will behave when they meet a convex lens.

Convex Lens - Construction Ray 1
This is a ray that enters the lens travelling parallel to the Principal Axis

What this tells us is that any ray of light that enters a Convex lens having travelled parallel to its Principal Axis, wil always converge at the Principal focus.

Convex Lens - Construction Ray 2
This is a ray that enters the lens travelling towards its exact centre.

What this tells us is that any ray of light that enters a Convex lens travelling directly to the centre, will always pass straight through unaffected.

We will do a similar pair of diagrams for the Concave lens, but first, lets make use of these two Construction Rays to show how we can predict the location and nature and size of an image formed for an object placed at various positions from the Convex lens.

We start by drawing our convex lens and we mark the positions of the principal focii (there are actually 2 since the lens is symmetrical and light could enter from either side, couldn't it?). We also mark on the chosen position for our object.

Convex Ray Diagram 1 - Object far from F

Then we draw construction ray 1 from the top of the Object.

Then we add construction ray 2, also from the top of the Object.

Finally we can draw in the Image, the head of which is formed at the point where the two construction rays cross; the base of which is on the principal axis.

Now we can make 3 judgements about the Image.
The Image is:
Inverted, Smaller than the object and Real.

NB 1. We decalre that the Image is Real because we can see that it is formed at the point where "real" rays of light meet; if you set this up in a lab and put a piece of paper at that point, you would see an inverted image on the paper - a Real image.
NB 2. The principal focus in front of the lens gives us a good way of describing where the object is placed; so in the example above we would say - the object is far from the principal focus. Not surprisingly, when the object is moved even further from F, the image gets even smaller.
But what happens if we move the object closer to F?

Convex Ray Diagram 2 - Object closer to F
First we set up our axis with our lens; we mark on the 2 principal focii, then add the Object which we now position quite a lot closer to the left side focus.

This time we will go straight to the final diagram, adding both construction rays at the same time and inserting the Image.

Like before, we can make 3 judgements about the Image.
The Image is:
Inverted, Larger than the object (Magnified) and Real.

Notice, if you draw these diagrams yourself, you have to take great care to make sure that the two F's are positioned at exactly the same focal length from the lens; if you get one even slightly wrong, the diagram may not yield a correct result.

We will do one more Convex lens diagram before moving to the Concave lens, because there is one obvious Object position we must explore!

Convex Ray Diagram 3 - Object between the lens and F
As before we set up our axis with our lens; we mark on the 2 principal focii, then add the Object which we now position between F and the lens.

Now we add our 2 construction rays.

But we notice a strange thing! The rays do NOT cross on the other side of the lens as they had done in previous diagrams.
What do we do? Where can the Image be placed?
The answer must be - we extend the 2 refracted rays backwards until they cross!

Finally, we draw the Image from the point where the rays appear to cross.

The Image is:
Upright, Larger than the object (Magnified) and Virtual.

This object position is the only one which causes a convex lens to produce a Virtual, Upright image. Its only when the object is really close to the lens. Can you think of a lens instrument that makes use of this position? (Sherlock Holmes is often pictured using one!)

Ray Diagrams - Concave Lens

Concave Lens - Construction Ray 1

What this tells us is that any ray of light that enters a Concave lens having travelled parallel to its Principal Axis, wil always diverge as though coming from the Principal focus.

Concave Lens - Construction Ray 2
This is the same as for the Convex lens, construction ray 2.

So, using these 2 construction rays we can figure out the locations and natures of images for a variety of object positions.

Concave Ray Diagram 1 - Object far from F

Lets go straight to the diagram with the 2 construction rays, since you should by now be able to understand the use of these rays:

But where do we place the Image?
We place it, as always, at the point where the 2 rays cross, which can only be as shown below:

Therefore the Image is:
Upright, Smaller than the object and Virtual.

Concave Ray Diagram 2 - Object closer to F

As above lets go straight to the diagram with the 2 construction rays:

And you should know where we place the Image:
We place it, as before, at the point where the 2 rays cross:

And we find, once again, that the Image is:
Upright, Smaller than the object and Virtual.

There is one more Object position to try:

Concave Ray Diagram 3 - Object between F and the lens

As above lets go straight to the diagram with the 2 construction rays:

And you should know where we place the Image:
We place it, as before, at the point where the 2 rays cross:

And we find that the Image is still:
Upright, Smaller than the object and Virtual.

So unlike the Image formed by a Convex lens which is sometimes Real, sometimes Virtual, sometimes Smaller, sometimes Magnified, sometimes Inverted, sometimes Upright, the Image formed by a Concave lens is ALWAYS Upright, Smaller and VIrtual, no matter the position of the Object.

Magnification

We have seen how the size of the Image formed by a lens can vary depending on the lens type/shape and on the position of the Object.
We can calculate the exact Magnification of the Image compared to the Object using a very simple formula:

Visible Light - what we see.

At the very start of this section we said that the EM spectrum is hugely wide and that we only "see" a tiny part of it, Visible Light or the Visible Spectrum. Here is the picture that we showed at the start:

If we expand out this Visible Spectrum we see a familiar range or spectrum of colours - Red, Orange, Yellow, Green, Blue, Indigo and Violet (to remember this pattern you could use "Richard Of York Gave Battle In Vain").
But what are these "colours" really?
They are just EM waves; each colour occupies a small spread of Wavelengths.
Essentially they are no different from the other EM waves that are on either side of them in the EM Spectrum, but these are the only ones that a human eye will detect and a normal human eye interprets each wavelength as a different colour.
Notice the word "normal"; people with colour blindness have eyes that do not detect the difference between the wavelengths quite so well; they don't see the difference between a red colour and an orange colour, for example.

What is White Light?
Isaac Newton showed a long time ago that if you passed the light from the Sun (essentially "white light") through a triangular prism, the prism split the white light into the familiar colours of the spectrum, Red, Orange, etc. We call this process Dispersion of White Light.
What it tells us is that White Light is made up of the colours of the spectrum, so White Light is really an equal mix of the Wavelengths of (or the colours of) Red, Orange, Yellow...etc.

OK, now that we know this important fact, can we answer the next question.

What makes an object appear a particular colour?

For example what makes a post box appear to be red?
Or, what makes grass appear to be green?

It is very simple!
When White Light shines onto a surface, the surface will reflect some of the colours within the white light and it will absorb the others.
A surface will appear to be whatever colour it reflects into your eyes.
So, grass will appear to be green because it reflects Green light (and absorbs the other colours);
a post box will appear to be red because it reflects Red light (and absorbs the other colours).

NB. Not too improtant, but in case you wonder - What makes the actual grass reflect the green light or the postbox reflect the red light? This is down to the "pigment" of the surface; so, the surface of grass consists of a pigment (chlorophyl) which has the property of absorbing all wavelengths except green which it reflects; the paint on the postbox has a pigment within it which has the property of absorbing all wavelengths except red which it reflects.

What makes an object appear White or Black?

The answer to this should be pretty obvious now:
An object/surface will appear to be white if it reflects all of the colours or wavelengths within the incident White Light.
An object/surface will appear to be black if it reflects none of the colours or wavelengths within the incident White Light.

All of the discussion so far refers to colour and Opaque surfaces/objects. But what if we have a tansparent or translucent material?

Colour and transparent/translucent materials

First of all, what do we mean by these 2 words?
A transparent material is one through which objects can be seen quite clearly; normal window glass is a good example.
A translucent material is one through which objects can be seen but not clearly; frosted bathroom window glass might be a good example.

However, for our purposes we will consider them to be the same with regard to the way they affect light and its wavelengths/colours.
We are going to refer to objects such as coloured plastic or sweep wrapping sellophane. We are going to call these objects - Colour Filters.

Unlike a Colour Surface which, we have seen, will either Reflect or Absorb light colours, a Colour Filter such as a transparent/translucent plastic ruler will either Transmit or Absorb colours (or wavelengths) of white light.
Note- the word "Transmit" simply means- to pass through or to allow through.

For example, a Blue Filter (such as a blue see-through ruler) will only Transmit blue light; it will Absorb all the other colours.
A piece of "red" sellophane used to wrap a chocolate is a Red Filter which will only Transmit red light; it will Absorb all the others.

Now its time for you to have a go at a few questions.

Finally, before we end this section:

Why don't we see a clear reflection in a paving stone?

We could equally ask, why don't we see a clear reflection in, for example, a piece of wood?

When we look in a mirror we expect (and do) see a clear reflection of ourselves and of things around us, but we don't have the same expectation when we look at a concrete paving stone or a piece of wood.
So, what is the difference between the mirror surface and the concrete or wood surfaces?

The answer is very simple - it is all to do with the flatness of the surfaces.
A mirror has an unbelievably flat surface made by coating the back of a piece of already flat glass with a thin layer of silver or aluminium which makes the overall surface even flatter.
The concrete or wood, on the other hand is much rougher.

OK, so how does flatness affect reflectivity?

When light rays from an object reflect from a smooth flat surface, as shown below, they reflect off the surface "perfectly", ie they all have the same angle of incidence and thus they have the same angle of reflection. The resulting image would be coherent with (meaning - the same as) the object. This is what we expect when we use a typical mirror to produce a reflection.

Reflection from a smooth surface in a single direction is called specular reflection

But when light rays from an object reflect from a rough surface, as shown below, they do not have the same angles of incidence or reflection. The resulting image would not be coherent and would be difficult or impossible to distinguish. The image is scattered.

Reflection from a rough surface, causing scattering, is called diffuse reflection.

Can a normally rough surface be made to produce a fairly good reflection?
Yes. For example - wooden furniture can be polished (and polished, repeatedly) until it is quite reflective. This is a result of the wax in the polish filling all the dips and crevices in the wood, flattening it, making it smoother and smoother. Obviously it also helps if the wood is smoothed down as much as possible before polishing takes place.
Another simple example is water! If we look at the surface of a pond on a windy day, we tend not to see a good reflection of ourselves or our surroundings, but if we wait for a wind free day, the surface of the pond becomes perfectly flat and we see an image as good as that we see in a mirror.