What is the relationship between the angle of incidence as the light enters the block and the angle of refraction as the light leaves the block?

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• To use ray tracing to measure the angles of incidence and refraction when light is refracted by a glass block;
• To demonstrate understanding that the angles of incidence and refraction are measured from a line at right angles to the glass surface known as the normal;
• and to use the measurements taken to plot a graph of angle of incidence against angle of refraction to show that they are related but not proportional.

There are different ways to investigate refraction in rectangular blocks.

In this required practical activity, it is important to:

• make, measure and record the angles of incidence accurately using a protractor;
• observe and use a protractor to measure angles of refraction.

In this experiment:

Independent Variable is the angle of incidence.

Dependent Variable is the angle of refraction.

Control Variables are the material of the block, the shape of the block and the colour of the light.

Remember - these variables are controlled (or kept the same) because to make it a fair test, only 1 variable can be changed, which in this case is the angle of incidence.

Light is travelling from air to glass and so is refracted towards the normal.

However, as the angle of incidence increases the refracted light will bend from a bigger initial angle, and so the angle of refraction will also be bigger.

Low voltage power pack, a 12V ray box, a single slit comb, a rectangular glass block, a sheet of white paper, a protractor, a sharp pencil.

1. Set up the ray box and slit so that a narrow, bright ray of light is produced.
2. Place the rectangular glass block on a sheet of white paper and draw around it carefully with a pencil.
3. Remove the glass block. Use the protractor to draw a normal approximately 1/3 of the way along the longest side.
4. Use a protractor to measure angles of incidence from this normal of 100, 200, 300, 400, 500, 600 and 700. Draw in the incident rays corresponding to these angles and label them A, B, C.... Record these angles of incidence in a suitable table.
5. Carefully replace the block on the outline. Direct a narrow ray of light along the line marked A. This is the incident ray for the angle of incidence, i = 100.
6. With the pencil mark two Xs to indicate the direction of the emergent ray. Mark the Xs as far apart as possible.
7. Remove the block again. Join the Xs with a pencil line, drawn using a ruler. Extend this line back to the block. This is the emergent ray: label it 'A'.
8. Use the ruler to join the incident and emergent rays together with a pencil line. This is the refracted ray. Carefully mark in the angle of refraction, r, between the refracted ray and the normal. Measure the angle of refraction with a protractor and record in the table.
9. Repeat the procedure for each of the incident rays, recording angle of incidence and corresponding angle of refraction in the table.

Hazard	Consequence	Control measures
Ray box gets hot	Minor burns	Do not touch bulb, allow time to cool
Semi-dark environment	Increased trip hazard	Ensure environment is clear of potential trip hazards before lowering lights

The main cause of error in this experiment is the measurement of the angles of incidence and refraction.

This can be kept to a minimum by:

• replacing the block carefully on its outline;
• ensuring that the power pack is set to 12 V, so that the ray box is at maximum brightness;
• doing the experiment in a dark room so that the emergent ray can be easily seen and marked.

Refraction is the bending of light (it also happens with sound, water and other waves) as it passes from one transparent substance into another.

This bending by refraction makes it possible for us to have lenses, magnifying glasses, prisms and rainbows. Even our eyes depend upon this bending of light. Without refraction, we wouldn’t be able to focus light onto our retina.

Change of speed causes change of direction

Light refracts whenever it travels at an angle into a substance with a different refractive index (optical density).

This change of direction is caused by a change in speed. For example, when light travels from air into water, it slows down, causing it to continue to travel at a different angle or direction.

How much does light bend?

The amount of bending depends on two things:

• Change in speed – if a substance causes the light to speed up or slow down more, it will refract (bend) more.
• Angle of the incident ray – if the light is entering the substance at a greater angle, the amount of refraction will also be more noticeable. On the other hand, if the light is entering the new substance from straight on (at 90° to the surface), the light will still slow down, but it won’t change direction at all.

Refractive index of some transparent substances

All angles are measured from an imaginary line drawn at 90° to the surface of the two substances This line is drawn as a dotted line and is called the normal.

If light enters any substance with a higher refractive index (such as from air into glass) it slows down. The light bends towards the normal line.

If light travels enters into a substance with a lower refractive index (such as from water into air) it speeds up. The light bends away from the normal line.

A higher refractive index shows that light will slow down and change direction more as it enters the substance.

Lenses

A lens is simply a curved block of glass or plastic. There are two kinds of lens.

A biconvex lens is thicker at the middle than it is at the edges. This is the kind of lens used for a magnifying glass. Parallel rays of light can be focused in to a focal point. A biconvex lens is called a converging lens.

A biconcave lens curves is thinner at the middle than it is at the edges. Light rays refract outwards (spread apart) as they enter the lens and again as they leave.

Refraction can create a spectrum

Isaac Newton performed a famous experiment using a triangular block of glass called a prism. He used sunlight shining in through his window to create a spectrum of colours on the opposite side of his room.

This experiment showed that white light is actually made of all the colours of the rainbow. These seven colours are remembered by the acronym ROY G BIV – red, orange, yellow, green, blue, indigo and violet.

Newton showed that each of these colours cannot be turned into other colours. He also showed that they can be recombined to make white light again.

The explanation for the colours separating out is that the light is made of waves. Red light has a longer wavelength than violet light. The refractive index for red light in glass is slightly different than for violet light. Violet light slows down even more than red light, so it is refracted at a slightly greater angle.

The refractive index of red light in glass is 1.513. The refractive index of violet light is 1.532. This slight difference is enough for the shorter wavelengths of light to be refracted more.

Rainbows

A rainbow is caused because each colour refracts at slightly different angles as it enters, reflects off the inside and then leaves each tiny drop of rain.

A rainbow is easy to create using a spray bottle and the sunshine. The centre of the circle of the rainbow will always be the shadow of your head on the ground.

The secondary rainbow that can sometimes be seen is caused by each ray of light reflecting twice on the inside of each droplet before it leaves. This second reflection causes the colours on the secondary rainbow to be reversed. Red is at the top for the primary rainbow, but in the secondary rainbow, red is at the bottom.

Use these activities with your students to explore refration further:

• Investigating refraction and spearfishing – students aim spears at a model of a fish in a container of water. When they move their spears towards the fish, they miss!
• Angle of refraction calculator challenge – students choose two types of transparent substance. They then enter the angle of the incident ray in the spreadsheet calculator, and the angle of the refracted ray is calculated for them.
• Light and sight: true or false? – students participate in an interactive ‘true or false’ activity that highlights common alternative conceptions about light and sight. This activity can be done individually, in pairs or as a whole class.

Learn more about different types of rainbows, how they are made and other atmospheric optical phenomena with this MetService blog and Science Kids post.

Learn more about human lenses, optics, photoreceptors and neural pathways that enable vision through this tutorial from Biology Online.