Light and Sound
Section: Physics | Syllabus: Cambridge Primary Science (0846)
How We See and Light Travel
- Light travels in straight lines - it does not bend or curve, unless it reflects or refracts
- We see objects because light from a source (like the Sun or a lamp) reflects off the object and travels in a straight line into our eyes
- e.g. you can see your face in a mirror because light from the room reflects off your face, then reflects off the mirror, and travels to your eyes
- you can see a lamp indirectly on a screen because light from the lamp reflects off the screen and into your eyes
Equipment for Investigating Light
- Ray box: produces a single, narrow ray of light so you can clearly see how it reflects or refracts
- Protractor: used to measure the angle of incidence and angle of reflection/refraction
Reflection of Light
When light hits a shiny surface like a plane mirror (flat mirror), it bounces off. This is called reflection.
Key Terms
- Incident ray: the ray of light travelling TOWARDS the mirror
- Reflected ray: the ray of light bouncing AWAY from the mirror
- Normal: an imaginary line at 90° (right angle) to the mirror surface
- The Law of Reflection: the angle of incidence (between incident ray and normal) equals the angle of reflection (between reflected ray and normal)
- In a plane mirror:
- the image appears the same size as the object
- the image appears the same distance behind the mirror as the object is in front
- the image is laterally inverted (left and right are swapped)
- how much of yourself you can see depends on the mirror's position, not its size - moving closer to a small mirror lets you see more of yourself
- a rough or dirty mirror scatters light in different directions, giving a blurry, unclear image - a smooth, clean mirror reflects light in an organised way, giving a clear image
Worked Example: 2023 Oct/Nov Paper 1 Q3
Question: Safia investigates how light is reflected from two mirrors.
She draws a diagram to show the result of her investigation.
Look at the diagram showing a ray of light and two mirrors. [1 mark]
(a) The ray of light is reflected when it touches the bottom mirror.
Describe what happens when a ray of light is reflected. [1]
(b) Complete the sentences to show the number of reflections from each mirror.
The top mirror has .............. reflections.
The bottom mirror has .............. reflections. [1]
(c) Safia wants to increase the total number of reflections.
Describe what Safia does to increase the total number of reflections. [1]
Answer:
(a) the ray of light changes direction
(b) TWO (2)
(c) THREE (3)
Worked Example: June 2024 Paper 2
Question: Lily investigates what happens to a ray of light when it touches a mirror.
(a) Write down the name of the process Lily investigates.
........................................................ [1]
(b) Lily changes the size of angle A and then measures angles A, B and C. She writes her results in a table. [1]
| Angle A in degrees |
Angle B in degrees |
Angle C in degrees |
|---|---|---|
| 10 | 160 | 10 |
| 20 | 140 | 20 |
| 30 | 120 | 30 |
| 40 | 100 | 40 |
| 50 | 80 | 50 |
Describe two things that happen as angle A increases in size.
1: ..............................................................
2: ..............................................................
[2 marks]
Answer:
(a) reflection
(b) 1: angle B decreases in size
2: angle C increases in size, or angle C is the same size as angle A
Refraction of Light
Refraction is the bending of light as it passes from one transparent material into another. This happens because light travels at different speeds in different materials.
Key Terms
- Incident ray: the ray of light travelling TOWARD the boundary between two materials
- Refracted ray: the ray that has crossed the boundary and changed direction
- Normal: an imaginary line at 90° to the boundary at the point where light enters
- Angle of incidence (θi): the angle between the incident ray and the normal
- Angle of refraction (θr): the angle between the refracted ray and the normal (inside the new material)
The Rule of Refraction
- Air → water or glass: light slows down and bends toward the normal (θr < θi)
- Water or glass → air: light speeds up and bends away from the normal (θr > θi)
Real-World Examples of Refraction
| Example | What You See | Why |
|---|---|---|
| Straw in a glass of water | The straw looks bent or broken at the water surface | Light bends as it leaves the water, so our eye traces it back along a straight line to the wrong position |
| Swimming pool floor | The pool looks shallower than it really is | Light from the bottom bends away from normal entering air, making the floor appear closer |
| Glass lenses (spectacles) | Objects look clearer or magnified | The curved glass refracts light to focus it correctly onto the eye |
Dispersion: Splitting White Light
- White light is actually a mixture of all the colours of light
- When white light passes through a prism, each colour refracts (bends) by a different amount, splitting the light into a band of colours - this is called dispersion
- violet light bends the most
- red light bends the least
A prism splits white light into a band of colours, with violet bending the most and red the least
Worked Example: October 2024 Paper 1 Question 3
Question:
Ahmed uses a semi-circular glass block to investigate refraction.
Look at the diagram.
Ahmed changes the size of angle A by moving the flashlight.
He writes down the size of angle B.
Look at his results.
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