## Physics-SchoolUK.com

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#### KS3 3.4.2 Waves - Light

Although this chapter is titled "Waves", in this section we will not focus on light as a wave, but on the behaviour of light as a ray. Viewing light as a ray will make it easier for us to understand how light is reflected, refracted and dispersed.

### The ray model of light

In the ray model of light, light is considered to travel from a light source as a ray, moving in a perfectly straight line until it hits some surface at which point the ray might be reflected, refracted (more on this later) or absorbed, or maybe a little bit of all three.

What evidence exists to show that we can view light in this way?

1. The existence of sharp shadows.
If you stand with your back to a light source such as a bulb, you will see in front of you a clearly defined shadow of yourself.
You will see your shadow as a dark shape surrounded by a light area.

This occurs because your body blocks some of the rays of light, forming the dark shape, but other rays pass by your sides unhindered, forming the light area.

The following diagram shows that treating the light as "rays", where each ray travels in a straight line, allows us to predict with a diagram what we see in real life.

Notice how we draw the light rays - always a straight line with an arrow to indicate the direction of the ray.

Another good piece of evidence is the shadows that we see when there are eclipses. For example when there is a solar eclipse a shadow of the moon gradually passes across the earth's surface until, in a total eclipse, the moon blocks the sun's light completely forming a perfectly dark shadow at a point on the earth.

We can explain what we see by using the ray model of light where we draw light rays as straight lines with an arrow.

2. The evidence of light from a laser

It is difficult or impossible to look at a bulb and actually see distinct rays of light being emitted.
This is because a light source such as a bulb emitts rays of light in all directions such that we can't just see one ray at a time.

But a laser is a device which emitts light in just one direction, one ray.
I am sure we have all seen such laser rays of light whether it is from a laser pointer or from a laser light show where rays of laser light in different colours will be directed up to the sky (never pointed directly at a person!)
The light from a laser is very clear evidence that light can be viewed as a ray that travels in a perfetly straight line.

Notice in the diagram above that we represent a ray of light as a straight line with an arrow to indicate its direction. This is the way we always draw rays of light.

So in the rest of this section we will confidently use the ray model of light to explain reflection, refraction and dispersion.

### Reflection of light

Why do we see a clear reflection of ourselves when we look in a mirror?

We see a clear reflection of ourselves when we look in a mirror because
1. the mirror surface is extremely flat and smooth and
2. every ray of light that hits it gets refected such that the angle of the outgoing or "reflected" ray equals the incoming or "incident" ray.

Let's look at this with just one ray of light

You can see from the diagram that the reflected ray is reflected by the mirror such that its angle of reflection, r is the same as its angle of incidence, i.

Notice: for each ray we need to measure the two angles from the same place so we use an imaginary line which is perpendicular to the surface of the mirror. We call this line, the "normal".
Also, the statement - the angle of reflection equals the angle of incidence - is known as The Law of Reflection.

So what if we place an object in front of a perfectly smooth mirror surface?

The following diagram shows this for a simple arrow shaped object.

As you can see from the diagram, the image of the arrow shaped object is perfectly formed.
This is because due to the perfectly flat surface all of the rays have identical Normals (the diagram only shows a few of the Normals), so all of the angles of incidence and reflection are the same.

What if the surface is not extremely flat or smooth?
Well then you would get something like the following:

Now due to the uneven surface, the Normals are not all identical, they lean at a whole range of angles compared to each other. So although each ray obeys the law of reflection, they all have different angles of incidence and hence different angles of reflection.

The image is "jumbled" up and unrecognizable. Such rough surfaces do not produce perfect reflections.

Can a normally rough surface be made to produce a fairly good reflection?
Yes, sometimes.
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 in a mirror.

Most questions involving reflection are quite easy to answer, so long as you remember the Law of Reflection

### Using a ray diagram to show how a flat mirror forms an image

1. Draw a mirror as shown then draw an incident ray from an object to the mirror; draw the reflected ray (make sure to obey the law of reflection)

2. Draw another incident ray from the object and another reflected ray, again obey the law of reflection.

3. Project the two reflected rays backwards, behind the mirror until they meet. Use dashed lines since these are not real rays being behind the mirror.

The point where they meet is where the image is formed!

Notice that the image is the same distance behind the mirror as the object is in front.
But because the image is not really behind the mirror, we call it a virtual Image.

Other things to know about an image seen in a flat mirror:

1. The image is upright, meaning the same way up as the object.
2. The image is the same size as the object.
3. The image is laterally inverted compared to the object (eg if you stood in front of a mirror and held up your left hand, your image would hold up its right hand).

### Refraction of Light

All waves such as light can be refracted.

What do we mean by "refracted" or refraction?

Look at the following diagram - when a light ray is directed towards a rectangular glass block such that it strikes the block at an angle of 90° to the block, as shown, the ray will simply cross the boundary into the block with no change of direction; similarly if it meets the other side of the block at 90° then it will pass back into the air with no change of direction.

So far, not very exciting.

But now look at what happens if the incident light ray crosses the boundary into the block at an angle other than 90°:

When the ray of light meets the boundary at an angle of incidence other than 90° it crosses the boundary into the glass block but its direction is changed.
We call this change of direction of a light ray, refraction.

You might ask, what happens when the ray of light meets the other side of the glass block?

The following diagram shows the whole passage of the light ray into and out of the block.

As you can see, because the ray once again meets the boundary at an angle to its normal, it is refracted again.

Notice - how the final ray (the emergent ray) emerges parallel to the original incident ray. The following diagram makes this clear by "dashing" the emergent ray back so it is alongside the incident ray.

By looking at the above few diagrams we can make some conclusions which we call Rules of Refraction and they can be applied to any relevant example allowing you to work out what will happen to a light ray.

Rules of Refraction

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 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.

According to the syllabus you need to be able to construct ray diagrams to illustrate the refraction of a ray 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:

#### Refraction and Prisms

A prism is a triangular piece of transparent material, often glass.
The most common shape is the equilateral triangle prism.

Let's start by showing a ray of light directed towards such a prism:

The prism "works" or does its thing simply because of the Rules of Refraction and its shape.

If we draw a normal at the point where the ray meets the prism, we can see that the incident ray is at an angle to the normal so it will be refracted when it crosses the boundary.
But which way will it be refracted? Towards or away from the normal?

Answer - towards, because the light is travelling from a less dense medium (air) into a more dense medium (glass).

It will look like this:

At the next boundary the light is travelling from a more dense medium (glass) back into a less dense medium (air).
Which way will it be refracted?

Answer - away from the normal, as shown in the final diagram below.

As you can see, prisms can be used to control the path of rays of light, especially by altering the angles of the prism.
So prisms are used in a lot of optical instruments eg binoculars. One very famous use of a prism was when Isaac Newton used one to show that "white" light is actually made up of all the colours of the rainbow/spectrum.

#### 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."

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."

How do lenses work?

If you consider the shape of the convex lens you can see that it can be considered to be made up from a few prisms, as shown below:

If you then apply your knowledge of how light passes through prisms you can see that the rays are refracted in the way shown in the diagram above. This is how lenses work!

If you want a challenge - draw a concave lens and then draw appropriate prisms over it to confirm that this lens does what we drew earlier.

#### White light and Colour

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.

This tells us is that White Light is made up of the colours of the spectrum, so White Light is really a mix of the wavelengths (or frequencies) of the colours Red, Orange, Yellow, Green, Blue, Indigo and Violet.

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

What makes an Opaque object appear a particular colour?

First of all - what is an Opaque object?
Answer - an opaque object is one through which light does not pass.

What makes an opaque object eg a post box, appear to be red?
Or, what makes grass appear to be green?

It is very simple!
When White Light shines onto an opaque 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.

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

#### Keywords

Now that we have reached the end of this section we can focus on the keywords highlighted in the KS3 specification. You have already met each one, but it is important to learn them.

• Incident ray: The incoming ray.
• Reﬂected ray: The outgoing ray.
• Normal line: From which angles are measured, at right angles to the surface.
• Angle of reﬂection: Between the normal and reﬂected ray.
• Angle of incidence: Between the normal and incident ray.
• Refraction: Change in the direction of light going from one material into another.
• Absorption: When energy is transferred from light to a material.
• Scattering: When light bounces off an object in all directions.
• Transparent: A material that allows all light to pass through it.
• Translucent: A material that allows some light to pass through it.
• Opaqu: A material that allows no light to pass through it.
• Convex lens: A lens that is thicker in the middle which bends light rays towards each other.
• Concave lens: A lens that is thinner in the middle which spreads out light rays.
• Retina: Layer at the back of the eye with light detecting cells and where an image is formed.