Reflection and refraction of light
Reflection and refraction of light
- Unit 1: Particles & Pressure
- Unit 2: Forces & Motion
- Unit 3: Energy & Heat
- Unit 4: Electricity
- Unit 5: Magnetism & Electromagnetism
- Unit 6: Waves: Sound & Light
- Unit 7: Scientific Investigations
In this topic you will:
- describe how light is reflected from a plane surface
- understand the law of reflection
- be able to draw ray diagrams to show reflection of light.
- describe how light is refracted at the boundary between air and glass or air and water
- describe how light changes speed when it passes between different substances
- draw ray diagrams to show how light is refracted.
Key words
- angle of incidence
- angle of reflection
- incident ray
- law of reflection
- normal
- perpendicular
- plane mirror
- protractor
- ray diagram
- rays
- reflection
- set square
- angle of refraction
- away from the normal
- bent
- distorted
- lenses
- medium
- refraction
- towards the normal
Reflection
When you think of reflection you probably think of using a mirror.
Most of the mirrors you use are plane mirrors. Plane means a flat surface.
To see what reflection looks like from a surface that is not plane, you can look at your own reflection from a spoon. The reflection is distorted.
Remember that light travels in straight lines called rays.
A light ray arriving at a surface, such as a mirror, is called an incident ray. An incident ray is the ray coming onto a surface.
The incident ray makes an angle with the surface of the mirror. Measure this angle from a line perpendicular to the mirror and not from the mirror itself. The line perpendicular to the mirror is called the normal.
In physics and maths, ‘normal’ means perpendicular or at right angles to something.
We can use this information to draw ray diagrams. They show what happens to the light rays during reflection.
Ray diagrams should obey the law of reflection: the angle of reflection is equal to the angle of incidence.
In science, a law is something that always applies.
We can use the law of reflection in everyday situations.
For example, mirrors can be used to see behind us.
In this picture, light from the Sun is reflected from the cyclist. This is the incident ray on the mirror. The reflected ray from the mirror goes to the driver’s eye.
Important Concept
The law of reflection applies to all types of reflected light — including from flat mirrors, water surfaces, and even curved surfaces — though the behavior differs depending on the surface shape.
Drawing ray diagrams
When drawing light rays, always use a ruler and put an arrowhead on the ray to show its direction.
1 Draw the incident ray and the mirror. Sometimes, this is done for you.
2 Use a ruler to make the incident ray meet the mirror.
3 Use a protractor or set square to draw the normal. The normal is perpendicular to the mirror where the incident ray meets the surface. The normal is usually a dashed line so it is not confused with the light ray.
4 Use a protractor to measure the angle between the incident ray and the normal. We call this angle the angle of incidence or i.
5 Measure an angle equal to the angle of incidence on the other side of the normal. This angle is called the angle of reflection or r. Draw a reflected light ray coming away from the mirror at this angle. Remember to put an arrowhead on the reflected ray.
Quick Fact
The angle of incidence is always equal to the angle of reflection when light reflects off a plane mirror. This is true regardless of the angle at which light strikes the surface.
Questions
Write the letter.
Show Answer
C
a. Extend the incident ray to meet the mirror.
b. Draw a normal where the incident ray meets the mirror.
c. Measure and write down the angle of incidence.
d. Draw a reflected ray in the correct place.
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Follow the ray construction steps: extend the ray, draw the normal, then draw a reflected ray at the same angle on the opposite side.
Copy and complete the diagram to show how light from the candle reflects from the mirror to Sofia.
You do not have to measure the angles.
Draw and label:
- the incident ray
- the normal
- the reflected ray
- the angle of incidence
- the angle of reflection
Show Answer
Draw the ray from the candle to the mirror, then reflect it to Sofia's eye. Add a dashed normal at the point of reflection and label both angles.
The diagram shows a light ray coming from the pencil. Marcus can use a mirror to see the pencil.
Copy and complete the diagram by adding a mirror and a reflected light ray to show how Marcus can see the pencil.
You do not have to measure the angles.
Show Answer
Place a mirror where the light can reflect off the floor and reach Marcus’s eye. Draw the reflected ray and label it clearly.
Think Like a Scientist
Measuring angles of incidence and reflection
In this investigation, you will explore the law of reflection. Make the room as dark as possible and work in pairs or groups of three.
Equipment you will need:
• Equipment as shown in the diagram: ray box, mirror, modelling clay, ruler, protractor, pencil, and white paper.
Step 1: Set up the equipment as shown in the diagram.
Step 2: Place the mirror vertically at the edge of the paper and mark its front edge using a pencil.
Step 3: Use a protractor to draw a normal line at the surface of the mirror.
Step 4: Shine a ray of light from the ray box onto the mirror at the normal line.
Step 5: Mark the positions of the incident and reflected rays with a pencil.
Step 6: Turn off the ray box and use the marks to draw a ray diagram.
Step 7: Measure and record the angle of incidence (i) and angle of reflection (r).
Step 8: Repeat steps 1–7 for at least four more different angles.
Show Answer
Include degree (°) units in both columns and label clearly.
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The angle of incidence.
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The angle of reflection.
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The graph should show a straight line with a slope of 1 if reflection is accurate.
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The angle of incidence equals the angle of reflection.
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Positioning the ray box precisely and marking the rays accurately can be difficult.
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Use a fine pencil and a sharper ray box. Make sure the mirror and protractor are aligned properly.
Self-assessment
- I can recall the law of reflection.
- I can draw ray diagrams to show reflection.
- The experiment on reflection helped my understanding.
Refraction
When you look through a glass of water or through a wet window, you cannot see clearly.
Look carefully at the picture, which shows a glass of water on a table. The background, through the glass of water, appears distorted. Distorted means changed in some way from the original.
The background appears distorted because of refraction.
The material that light passes through is called a medium. Air, glass and water are each examples of a medium for light to pass through.
Light travels very fast. The speed of light in air is 300 000 kilometres per second (km/s). When light passes from air into water or glass, the light travels more slowly. The table shows how the speed of light changes according to the medium.
| Medium | Speed of light in km/s |
|---|---|
| air | 300 000 |
| water | 225 000 |
| glass | 200 000 |
The change of speed can cause the light to change direction.
Imagine you are on roller skates. You are moving at a constant speed on a hard surface. The roller skate on one foot goes onto grass. What happens? You will change direction because one roller skate is moving slower than the other.
This is what happens when a ray of light passes from air into glass or water. One side of the ray of light slows down first, causing it to change direction.
Refraction of light is defined as the change in direction of light on passing from one medium to another because of change in speed.
Important Concept
Refraction occurs when light changes speed as it moves between materials with different densities. This causes the light to bend, which is why objects appear distorted through water or glass.
Light passing from air into water or glass
This ray diagram shows what happens when light passes from air into glass or water.
The light slows down when it passes from air into glass or water. This causes it to change direction.
The light passing from air into glass or water is bent towards the normal. That means the refracted ray is closer to the normal than it would be if the incident ray just carried on in a straight line.
Notice also from the ray diagram that, when light passes from air into glass or water, the angle of incidence is greater than the angle of refraction. Both of these angles are measured from the normal.
Light passing from water or glass into air
This ray diagram shows what happens when a ray of light passes from water or glass into air.
The light speeds up when it passes from glass or water into air. This causes it to change direction.
The light passing from glass or water into air is bent away from the normal. That means the refracted ray is further away from the normal than it would be if the incident ray just carried on in a straight line.
Notice also from the ray diagram that, when light passes from glass or water into air, the angle of refraction is greater than the angle of incidence. Both of these angles are measured from the normal.
Refraction in everyday life
Refraction can be a nuisance. It can stop you from seeing clearly through wet windows.
Each individual drop of water on the window refracts light in a different direction, making it very difficult to see. This is why many vehicles have windscreen wipers. The windscreen wipers remove the water drops. It is then easier to see clearly as all the refraction from the glass is in the same direction.
Refraction can also be useful. Lenses are used in our eyes, in cameras and in glasses that people wear, to cause refraction of light.
A lens is a curved piece of glass that is designed to refract light in a known way.
Questions
a. Light travels … in water than it does in air.
b. Light travels … in air than it does in glass.
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a faster
b faster
Refraction of light happens when light … direction because of a change in … .
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Refraction of light happens when light changes direction because of a change in speed.
Copy and complete the ray diagram to show what happens when the ray of light enters the glass.
Include on your diagram:
- the normal
- the angle of incidence
- the refracted ray
- the angle of refraction
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The ray bends towards the normal when entering glass because it slows down. Draw the normal at the boundary, the angle of incidence in air, and the refracted ray inside the glass bending towards the normal.
Copy and complete the ray diagram to show what happens when the ray of light enters the air.
Include on your diagram:
- the normal
- the angle of incidence
- the refracted ray
- the angle of refraction
Show Answer
The ray bends away from the normal when it enters air because it speeds up. Draw the normal at the boundary, the angle of incidence in water, and the refracted ray in air bending away from the normal.
Think Like a Scientist
Drawing accurate ray diagrams
In this investigation, you will make accurate ray diagrams using rays of light passing through a rectangular glass block.
Equipment you will need:
• Ray box
• Power supply
• Rectangular glass block
• Large white paper
• Pencil
• Ruler
• Protractor
Step 1: Set up the equipment as shown in the top-view diagram.
Step 2: Use a pencil to draw around the glass block.
Step 3: Turn on the ray box and aim a ray of light at an angle at one surface of the glass.
Step 4: Mark the incident ray in two places — 5 cm before the block and at entry point.
Step 5: Do the same for the emerging ray on the opposite side.
Step 6: Turn off the ray box and remove the glass block.
Step 7: Use a ruler to draw both rays accurately.
Step 8: Use a protractor to draw normal lines at both surfaces.
Step 9: Measure the angles of incidence and refraction.
Step 10: Repeat for different angles. Be sure to include an incidence angle of zero.
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As the angle of incidence increases, the angle of refraction also increases, but not equally.
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The curve starts with a shallow slope and increases — not a straight line.
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The angle of refraction increases with the angle of incidence, but less sharply.
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The light passes straight through with no change in direction.
Self-assessment
- What did you do in your group?
- Did you make sure everyone in your group had a role?
- Would you feel confident to lead a group activity next time?