Reflection and refraction of light - CCEA

• Reflection of light.

• The law of reflection.

• Properties of the image in a plane mirror.

• Refraction of light.

• Measuring angles of incidence and refraction.

• Dispersion of light in a prism.

A ray diagram shows how light travels, including what happens when it reaches a surface.

In a ray diagram, you draw each ray as:

• a straight line,

• with an arrowhead pointing in the direction that the light travels.

When measuring angles:

• it is always the angle between the ray and the normal that is measured,

• a protractor is used.

Remember to use a ruler and a sharp pencil.

The diagram below shows reflection in a plane mirror.

• On a sheet of white paper draw a pencil line – label this AB.

• Using a protractor, draw a normal at C, roughly the middle of AB.

• Draw a line at 20° to the normal.

• Position a plane mirror carefully along AB.

• Direct a ray of light from a ray box along the 20° line – this is the incident ray.

• Record the angle of incidence i in a suitable table.

• Use 2 pencil Xs to mark the position of the reflected ray.

• Take away the mirror and join these Xs back to C. This is the reflected ray. Put an arrow on it to show its direction.

• Measure the angle between the normal and the reflected ray with the protractor and record the angle of reflection r in the table.

• Repeat the experiment for a series of incident rays.

Results

Conclusion

When light is reflected by a plane mirror:

The angle of incidence = the angle of reflection.

This is known as the law of reflection.

Key fact

The law of reflection for a plane mirror: angle of incidence i = the angle of reflection r.

What is the law of reflection?

The law of reflection states that: angle of incidence i = angle of reflection r.

For example, if a light ray hits a surface with an angle of incidence of 45°, it will be reflected with an angle of reflection of 45°.

• Draw a pencil line across the top of a sheet of white paper. Mark points A and B. Select a position for the object and label that O.

• Use a pencil and ruler to draw an incident ray from O to A and from O to B. Include arrows.

• Use a ray box to direct two rays of light along the lines from object O towards points A and B.

• Mark 2 pencil Xs to mark each of the reflected rays from A and from B.

• Remove the mirror and use a pencil and ruler to join the Xs These represent the reflected rays.

• Extend the reflected rays behind the mirror until they meet at point. This is where the image was formed. Label the point I. These dotted lines are called virtual rays.

• With a 30 cm ruler, measure the perpendicular distance from O to the mirror and from I to the mirror.

Repeat the experiment for different positions of the object O.

Conclusion

For each position of the object:

• the perpendicular distance of the object O from the mirror equals the perpendicular distance of the image I from the mirror.

Another way of saying this is:

• the image is the same distance behind the mirror as the object is in front.

The image in a mirror is:

• upright – but left and right are reversed. The image is laterally inverted.

• the same height as the object.

• as far behind the mirror as the object is in front.

• virtual.

A real image is an image that can be projected onto a screen.

A virtual image is an image from which rays of light appear to diverge, and do not actually pass through.

The image appears to be behind the mirror.

A virtual image cannot be formed on a screen.

It can only be seen by looking into the mirror.

Key point

The image in a plane mirror is:

• as far behind the mirror as the object is in front.

• upright but laterally inverted.

• same size as the object.

• virtual.

When light travels from air into glass it slows down because glass is more (optically) dense than air.

This change in speed can cause the light to change direction at the boundary between the two transparent materials.

This change in direction of a beam of light as it travels from one material to another is called refraction.

The normal is a construction line drawn at right angles to the surface of the glass block.

Angle of incidence i = angle between the incident ray and the normal.

Angle of refraction r = angle between the refracted ray and the normal.

Key fact

Angles of incidence and refraction are always measured between the normal and the corresponding ray of light.

Direction of refraction

Glass is denser than air, so when light passes from air into glass it slows down.

If the ray meets the boundary at an angle to the normal, it refracts towards the normal.

Light speeds up as it passes from glass into air because air is less dense than glass.

If the ray meets the boundary at an angle to the normal, it refracts away from the normal.

The greater the change of speed of light at a boundary, the greater the amount of refraction.

Light is refracted more by glass than by water because glass is denser than water and so slows it down more.

Key facts

• When light travels from a less dense to a more dense material, it gets slower and refracts towards the normal. e.g. air to glass.

• When light travels from a more dense to a less dense material, it gets faster and refracts away from the normal. e.g. glass to air.

• The greater the change of speed, the more the light is refracted.

One way of remembering this is to use the word FAST.

If light gets Faster it refracts Away from the normal.

If light gets Slower it refracts Towards the normal.

If light is incident along the normal when it passes from air into glass it still slows down but its direction does not change – it passes straight through.

Likewise, if light is incident along the normal when it passes from glass into air it still speeds up but its direction does not change – it passes straight through.

A ray of light incident along the normal passes straight through without being refracted.

• 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 notproportional.

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.

What are the variables?

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.

What is the prediction?

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.

What apparatus is used in this practical?

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.

Possible error

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.

Results

Graph

Plot a graph of angle of incidence, i in ° on the y-axis against angle of refraction, r in ° on the x-axis and draw the line of best fit.

Conclusion

We can see from the graph that as the angle of incidence, i, increases, the angle of refraction, r, also increases.

This agrees with our prediction.

However, the angle of incidence is not directly proportional to the and angle of refraction as the line of best fit is not a straight line through the origin.

The conclusion is the angles are related but not proportional.

The incident ray is refracted towards the normal as it enters the glass prism from air.

It is then refracted away from the normal at the boundary between glass and air as it leaves the prism.

White light can be split up to form a spectrum using a prism

Light waves are refracted as they enter the glass because they are slowed down.

The spectrum is produced because different colours of light travel at different speeds in glass.

Red light is slowed down least by glass and is refracted least.

Violet light is slowed down most by glass and is refracted most.

As a result, the coloured light spreads out to form a spectrum of white light.

This is called dispersion.

Key points

• The spectrum of white light is produced because different colours of light travel at different speeds in glass:

• Red light is slowed down least by glass and is refracted least.

• Violet light is slowed down most by glass and is refracted most.

White light can be split up into the seven colours of the visible spectrum.

There is an easy way to remember the order of the colours of white light by using a sentence: Richard Of York Gave Battle In Vain.

Red, Orange, Yellow, Green, Blue, Indigo, Violet.