Example 1: angle change
A ray enters glass at 40° to the normal and refracts to 25°. By how much has it changed direction?
- Difference = 40° − 25°.
- = 15°, bending towards the normal.
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IB DemystifiedMYP Sciences
Light: refraction and lenses
A swimming pool that looks shallower than it is, a pencil that seems bent in water, glasses that make the board sharp again, and a rainbow after the monsoon rain: all of these happen because light changes speed and direction as it passes into a new material.
Recommended for MYP 2 · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Refraction rules
- Refraction in the real world
- Worked examples
- Assessment tips
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- describe and explain refraction at a boundary
- use ray diagrams with the normal and angles
- describe how converging and diverging lenses affect light
- explain how the eye focuses light and how glasses correct sight
- explain dispersion and the colours of objects
- analyse refraction and lens data and plan fair tests
Before you start
You will use these skills. If any feel shaky, review them first.
- light travels in straight lines and reflection (see Light: reflection and shadows)
- measuring angles with a protractor
- reading tables and graphs
Key vocabulary
- Refraction
- The change in direction of light as it passes from one material into another.
- Normal
- A line drawn at 90° to a surface, from which angles are measured.
- Converging (convex) lens
- A lens that brings parallel rays together at a focal point.
- Diverging (concave) lens
- A lens that spreads parallel rays apart.
- Focal length
- The distance from a lens to its focal point.
- Dispersion
- The splitting of white light into a spectrum of colours.
Understanding the ideas
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Refraction
Light travels at different speeds in different materials: slower in glass and water than in air. When light crosses a boundary at an angle, this change of speed makes it change direction. Entering a denser material, it bends towards the normal; leaving it, it bends away.
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Lenses and sight
Lenses use refraction to focus light. Converging lenses bring rays together and are used in magnifying glasses, cameras and the eye; diverging lenses spread rays apart. Glasses correct sight: diverging lenses for short sight and converging lenses for long sight.
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Colour
Different colours refract by slightly different amounts, so a prism or a raindrop splits white light into a spectrum. Objects look coloured because they reflect some colours and absorb others.
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Why does it matter?
Refraction explains everyday effects such as shallow-looking water and rainbows, and it underpins technology from spectacles to microscopes, telescopes and phone cameras.
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What does it connect to?
This topic builds on reflection and links to the eye in biology, to waves and the electromagnetic spectrum, and later to optical fibres and communication.
Refraction rules
- Into a denser material (air → glass or water): light slows down and bends towards the normal.
- Into a less dense material (glass → air): light speeds up and bends away from the normal.
- Along the normal (0°): no change of direction.
- Convex lens: converging; fat in the middle; corrects long sight.
- Concave lens: diverging; thin in the middle; corrects short sight.
Refraction in the real world
Opticians test children's eyes and prescribe lenses to correct short and long sight. Cameras in phones use several tiny lenses. Rainbows are common in South Asia when the Sun comes out after monsoon showers.
Worked examples
Example 2: colour
What colour does a blue book look under red light?
- A blue book reflects only blue light.
- There is no blue light to reflect, and red is absorbed.
- It looks black.
Assessment tips
Questions on refraction often use ray diagrams, angle data and everyday examples. Expect to:
- Describe which way light bends at a boundary, using the normal.
- Explain refraction using the change of speed.
- Use lens data and explain how glasses correct sight.
- Analyse angle data and plan accurate experiments.
Common mistakes: measuring angles from the surface instead of the normal; saying light bends away from the normal when entering glass; mixing up convex and concave lenses; and saying coloured objects "make" coloured light.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: how deep is the coin?
Partially guided investigation · about 40 minutes · pairs
- Research question
- How does the depth of water in a beaker affect how much a coin at the bottom appears to rise?
- Scientific background
- Light from the coin refracts as it leaves the water, so the coin appears higher than it really is. The deeper the water, the bigger the effect.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Tall clear beaker or jar, coin, water, ruler, a small movable marker (for example a paper clip on a straw) outside the beaker, measuring cylinder.
- Method
- Put the coin at the bottom of the beaker and add water to a depth of 4 cm.
- Looking straight down, move the marker outside the beaker until it seems level with the image of the coin.
- Measure the apparent depth of the coin and compare it with the real depth.
- Repeat for water depths of 6, 8, 10 and 12 cm, three times each.
Safety. Wipe up spills straight away and keep water away from electrical equipment.
Then evaluate: why is it important to look straight down each time?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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