Physics · Unit 16

Light and Optics

How light behaves

Optics is the branch of physics with the most immediately visible predictions, which is why it was worked out so early and so accurately.

The unit covers reflection and mirrors, refraction and lenses, dispersion and colour, the electromagnetic spectrum, and interference and diffraction - the evidence that light behaves as a wave.

This unit breaks down into 20 short steps and 119 questions, starting at difficulty 2 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 20
Questions: 119
Difficulty: 2-3

What this unit covers

Reflection and Mirrors
Interference and Diffraction
Electromagnetic Spectrum
Colour and Dispersion
Refraction and Lenses

Where this fits

Needs Waves and Sound. Leads into Modern Physics, where light stops being simply a wave.

Where people slip

Refraction is caused by a change of speed, not by the bending itself. Light entering glass slows, and the bend follows from that - which is why a ray hitting the surface square on changes speed and does not bend.

How the unit is structured

Light and Optics runs as 20 short steps that unlock in order. 15 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 2 and climb to 3 as you progress.

Step 1 · easierStep 20 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 119 in the unit altogether.

Colour and Dispersion

Fact or fibLevel 2
1. White light is actually a mixture of all the colours of the visible spectrum.
Answer: True
White light contains all the visible colours, which is why a prism can split it into a spectrum.
Fill the blankLevel 2
2. The splitting of white light into its separate colours as it passes through a prism is called ____.
- dispersioncorrect
- reflection
- diffraction
- absorption
Dispersion happens because each colour is refracted by a slightly different amount.
Match the pairsLevel 2
3. Match each mix of coloured light to the colour it produces.
Answer: Red + Green light = Yellow; Green + Blue light = Cyan; Red + Blue light = Magenta; Red + Green + Blue light = White
Mixing the primary colours of light (red, green, blue) in pairs makes yellow, cyan and magenta, and all three together make white.
Multiple choiceLevel 2
4. When white light passes through a prism, which colour bends (refracts) the most?
- Violetcorrect
- Red
- Yellow
- Green
Violet light has the shortest wavelength and is refracted the most, while red bends the least.
Build the sentenceLevel 3
5. Build the sentence explaining how a rainbow forms in raindrops.
Answer: Sunlight refracts reflects and disperses inside each raindrop
Sunlight refracts as it enters a raindrop, reflects off the back, and disperses into a spectrum as it leaves, forming a rainbow.
Guess the numberLevel 3
6. Light travels at 3.0 x 10^8 m/s in a vacuum. In glass with refractive index n = 1.5, its speed is v = c/n. Find v in m/s.
Answer: 200000000 m/s
v = c/n = (3.0 x 10^8)/1.5 = 2.0 x 10^8 m/s, showing light slows in denser media.

Electromagnetic Spectrum

Guess the numberLevel 2
7. In a vacuum, all electromagnetic waves travel at the speed of light. What is this speed?
Answer: 300000 km/s
Light and all electromagnetic waves travel at about 300,000 km/s in a vacuum.
Match the pairsLevel 2
8. Match each part of the electromagnetic spectrum to a common use.
Answer: Radio waves = Broadcasting TV and radio; Microwaves = Cooking food and phone signals; Infrared = Remote controls and thermal cameras; X-rays = Imaging bones in hospitals
Each part of the spectrum has typical uses, such as radio for broadcasting and X-rays for medical imaging.
Multiple choiceLevel 2
9. Which of these electromagnetic waves has the LONGEST wavelength?
- Radio wavescorrect
- X-rays
- Ultraviolet
- Visible light
Radio waves have the longest wavelengths and the lowest frequencies of all electromagnetic waves.
Odd one outLevel 2
10. Which of these is NOT part of the electromagnetic spectrum?
- Sound wavescorrect
- Microwaves
- Ultraviolet
- Gamma rays
Sound is a mechanical wave that needs a medium to travel through, so it is not part of the electromagnetic spectrum.
Choose all that applyLevel 3
11. Select all statements that are true about electromagnetic waves.
- They are transverse wavescorrect
- They travel at 3 x 10^8 m/s in a vacuumcorrect
- They consist of oscillating electric and magnetic fieldscorrect
- They need a material medium to travel
EM waves are transverse, made of oscillating electric and magnetic fields, and travel at 3 x 10^8 m/s in a vacuum without needing any medium.
Fill the blankLevel 3
12. Visible light spans wavelengths from roughly 400 nm to 700 ____.
- nmcorrect
- mm
- cm
- m
The visible spectrum runs from about 400 nm (violet) to 700 nm (red), all measured in nanometres.

Interference and Diffraction

Build the sentenceLevel 2
13. Build the sentence describing when diffraction is strongest.
Answer: diffraction is greatest when the gap equals the wavelength
The spreading effect of diffraction is most noticeable when the gap size is about the same as the wavelength.
Choose all that applyLevel 2
14. Which of the following are evidence that light behaves as a wave? Select all that apply.
- Interference fringes in the double-slit experimentcorrect
- Diffraction of light spreading through a narrow gapcorrect
- Light travelling in straight lines to cast a sharp shadow
- A ball bouncing off a solid wall
Interference and diffraction can only be explained by a wave model; casting a sharp shadow does not require waves.
Fill the blankLevel 2
15. When two light waves meet crest-to-crest and reinforce each other to give a brighter result, this is called ____ interference.
- constructivecorrect
- destructive
- random
- parallel
Constructive interference occurs when waves arrive in phase and their amplitudes add together.
Guess the numberLevel 2
16. Visible light has very short wavelengths. Roughly what is the wavelength of green light?
Answer: 550 nm
Visible light ranges from about 400 nm (violet) to 700 nm (red); green light is around 550 nm.
Match the pairsLevel 2
17. Match each wave term to its meaning.
Answer: Constructive interference = Waves add to make a bright band; Destructive interference = Waves cancel to make a dark band; Diffraction = Waves spread through a narrow gap; Wavelength = Distance between two wave crests
These are key wave ideas used to explain interference and diffraction patterns.
Fact or fibLevel 3
18. Longitudinal waves such as sound can be polarised.
Answer: False
Only transverse waves can be polarised because polarisation restricts oscillations to one plane; longitudinal waves vibrate along their direction of travel.

Reflection and Mirrors

Fill the blankLevel 2
19. Angles of incidence and reflection are always measured from an imaginary line called the ____, which is perpendicular to the mirror surface.
- normalcorrect
- tangent
- axis
- horizon
The normal is the line drawn perpendicular to the surface at the point where the ray hits.
Guess the numberLevel 2
20. A ray of light strikes a plane mirror with an angle of incidence of 35 degrees, measured from the normal. What is the angle of reflection?
Answer: 35 degrees
By the law of reflection, the angle of reflection equals the angle of incidence, so it is 35 degrees.
Match the pairsLevel 2
21. Match each mirror to how it affects light or its image.
Answer: Plane mirror = Same-size upright image; Concave mirror = Converges light to a focus; Convex mirror = Diverges light outward; Rear-view car mirror = Gives a wide view behind
Plane mirrors keep the image size, concave mirrors converge light, and convex mirrors diverge light to give a wide view.
Multiple choiceLevel 2
22. What does the law of reflection state about a ray of light striking a mirror?
- The angle of incidence equals the angle of reflectioncorrect
- The angle of incidence is twice the angle of reflection
- The reflected ray always bends toward the normal
- The angle of reflection is always 90 degrees
The law of reflection states the angle of incidence equals the angle of reflection, both measured from the normal.
Odd one outLevel 2
23. Three of these describe the image in a plane mirror. Which one does NOT?
- It appears larger than the objectcorrect
- It is the same size as the object
- It is upright
- It is left-right reversed
A plane mirror image is the same size, upright and laterally inverted, but it is never magnified.
Sort into groupsLevel 2
24. Sort each everyday mirror into the type of curved mirror it uses.
Answer: Make-up mirror = Concave; Car passenger-side mirror = Convex; Reflecting telescope mirror = Concave; Shop security mirror = Convex
Concave mirrors magnify (make-up mirrors, telescopes) while convex mirrors give a wide view (car side mirrors, shop security mirrors).

Refraction and Lenses

Build the sentenceLevel 2
25. Build the sentence describing what a diverging lens does to light.
Answer: a diverging lens spreads light rays apart
A concave, diverging lens spreads parallel light rays apart so they appear to come from a focal point.
Fill the blankLevel 2
26. The bending of light as it passes from one transparent material into another is called ____.
- refractioncorrect
- reflection
- diffraction
- dispersion
Refraction is the change in direction of light when it changes speed moving between media.
Guess the numberLevel 2
27. The refractive index of water tells how much it slows and bends light. What is its approximate value?
Answer: 1.33
Water has a refractive index of about 1.33, meaning light travels around 1.33 times slower in water than in a vacuum.
Match the pairsLevel 2
28. Match each optical device to how it uses a lens.
Answer: Magnifying glass = Single converging lens; Human eye = Focuses light onto the retina; Camera = Forms an image on a sensor; Short-sight glasses = Use a diverging lens
Different instruments use lenses; the eye focuses light on the retina, magnifiers use a converging lens, and short-sight glasses use a diverging lens.
Multiple choiceLevel 2
29. What happens to a ray of light as it passes from air into a denser medium such as glass?
- It bends toward the normalcorrect
- It bends away from the normal
- It always continues in a perfectly straight line
- It reflects straight back the way it came
Light slows down entering a denser medium, so it bends toward the normal.
Fact or fibLevel 3
30. Total internal reflection can only occur when light travels from a denser medium into a less dense medium and exceeds the critical angle.
Answer: True
TIR needs light going from high to low refractive index at an angle greater than the critical angle; the whole beam then reflects back inside the denser medium.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sequence recall
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise Light and Optics

119 questions across 20 steps. Start with step one and crawl at your own pace.

Play this unit

Read about Light and Optics

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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