GCSE · Physics · AQA · Spec 8463

Visible light and colour (physics only)

Under a red party light, your friend's green T-shirt suddenly looks black. The shirt hasn't changed — the light has. Soon you'll be able to predict that, and explain it.

Physics · Light and colour

Same objects, different light

White light is a mixture of every colour. Pick an object, then the colour you think it will look. Then switch the light and sort the same five objects again.

Ordinary white light: every colour arrives at each object.

Still to sort

Looks white (0)

Reflects all the colours that arrive, equally

Looks red (0)

Only red light reaches your eye

Looks green (0)

Only green light reaches your eye

Looks blue (0)

Only blue light reaches your eye

Looks black (0)

No light at all reaches your eye from it

Where the line is: Black is not a colour of light being sent back. It's what you see when nothing is sent back to your eye.

5 of 5 still to sort.

In these examples each coloured object reflects just its own colour and absorbs the rest. That's the tidy version physics questions use; a real red object is the one that reflects red most strongly.

Exam line: Every answer has the same three moves: what light arrives, what the object reflects, what it absorbs. The light that's reflected into your eye is the colour you see.

Watch out: A white object isn't 'always white'. It reflects whatever colours reach it, so in red light it looks red.

Physics · The visible spectrum

What white light is made of

Shine white light through a triangular glass prism and it fans out into a band of colours called a spectrum. Put these colours of the spectrum in order of frequency, lowest first.

1 · Lowest frequency (longest wavelength)6 · Highest frequency (shortest wavelength)

Green
Violet
Orange
Red
Blue
Yellow

Watch out: Don't flip the two trends. Red: longest wavelength, lowest frequency. Violet: shortest wavelength, highest frequency. As one goes up, the other goes down.

Physics · What happens to light

Follow the light that hits an object

Light arrives at an object. Choose what the object does with it, and see what you end up seeing.

Does any light get through? → What happens to the colours that arrive?

4 possible appearances.

On Light arrives at an object. 2 branches to choose from.

Two ways to reflect

Specular reflectionvsDiffuse reflection

Reflection isn't only about colour — it's also about which way the light goes. Imagine a set of parallel rays hitting each surface.

Focus

What happens to parallel rays

Specular reflection

They all reflect in a single direction

Diffuse reflection

They scatter in many different directions

The insight

This is the one to remember: one direction versus scattered. Everything else follows from it.

The surface

Specular reflection

Smooth — like a mirror or still water

Diffuse reflection

Rough — like paper, even though it feels smooth

What you see

Specular reflection

A clear reflected image

Diffuse reflection

No image — just a lit-up surface

Handy way to remember the names

Specular reflection

Specular: a single direction

Diffuse reflection

Diffuse: spread out, like a scent diffusing across a room

Physics · Colour filters

What does a red filter actually do?

White light shines onto a sheet of red filter. Red light comes out of the other side.

Which is closest to what you think the filter has done?

It added red to the light, a bit like dye colouring water.It turned the other colours into red.It reflected red light towards me, like a red object does.It absorbed the other colours and transmitted the red.

How sure are you?

I'm sureI think soI'm guessing

It added red to the light, a bit like dye colouring water.

This is a common misconception

Tempting, because red appears. But a filter can't add anything to light. The red was already in the white light. The filter just let that part through and stopped the rest.

It turned the other colours into red.

This is a common misconception

The other colours aren't changed into red — they're absorbed by the filter. They go in and don't come out. Only the red that was there to begin with is transmitted.

It reflected red light towards me, like a red object does.

This is a common misconception

That's how an opaque red object works. A filter is different: you're seeing the light that passes through it. Its colour depends on the wavelengths it transmits.

It absorbed the other colours and transmitted the red.

This matches the evidence

Exactly. A colour filter absorbs some wavelengths and transmits others. A red filter transmits red, so that's the colour of the light that comes out — and the colour of the filter.

A filter's colour is the colour it transmits. Everything else is absorbed — nothing is ever added.

Physics · Put it in writing

Explain it like a physicist

A car is painted green. Explain why the car looks green in white light, and what colour it looks when it is lit only by blue light. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

White light is a mixture of all the colours (all visible wavelengths). (1 mark)
The paint reflects green light and absorbs the other colours. (1 mark)
The reflected green light reaches the eye, so the car looks green. (1 mark)
In blue light the car looks black: the blue light is absorbed and there is no green light to reflect. (1 mark)

Model answer

White light is a mixture of all the colours. The green paint reflects the green light and absorbs all the other colours, so only green light is reflected into your eye and the car looks green. In blue light there is no green light arriving to reflect. The paint absorbs the blue light, so no light is reflected from the car and it looks black.

Colour explanations always have the same bones: what arrives, what's reflected, what's absorbed, what reaches your eye.

Exam line: Say what arrives, what's reflected, what's absorbed and what reaches the eye. Leave one out and the explanation has a gap.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why a red jumper can look black — and what colour really is.

What you need to know

White light is a mixture of all the colours of the visible spectrum.
An object's colour depends on the light that arrives and on which colours it reflects or transmits.
Anything not reflected or transmitted is absorbed — objects and filters never add colour.

The big picture

White light is a mixture of all the visible colours, and each colour is its own narrow band of wavelength and frequency. From red to violet, frequency increases and wavelength decreases. Objects and filters never add colour: an opaque object reflects some wavelengths and absorbs the rest, and a filter transmits some and absorbs the rest. The light that reaches your eye is the colour you see. Smooth surfaces reflect light in a single direction (specular reflection); rough surfaces scatter it (diffuse reflection).

Key points

1Each colour of visible light has its own narrow band of wavelength and frequency; from red to violet, frequency increases and wavelength decreases.

2A triangular prism spreads white light into a spectrum, showing white light is a mixture of all visible wavelengths.

3Specular reflection: a smooth surface reflects light in a single direction. Diffuse reflection: a rough surface scatters it.

4Opaque objects transmit no light. Their colour is decided by the wavelengths they reflect most strongly; the rest are absorbed.

5Reflects all wavelengths equally → looks white. Absorbs all wavelengths → looks black.

6Transparent and translucent objects transmit light. A colour filter absorbs some wavelengths and transmits others, and its colour is the colour it transmits.

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Worked example

Problem

A stage is lit by a spotlight with a blue filter in front of it. A dancer wears a white top, a blue scarf and green trousers. What colour does each item look?

⚠ Watch out

Saying a white object always looks white. It reflects whatever colours reach it — in red light it reflects red and looks red, and through a green filter it looks green.

🧠

Memory hook

Colour is what's left over. Objects and filters are light thieves, never painters: they keep (absorb) some colours and pass on (reflect or transmit) the rest.

✓

Check yourself

White light passes through a red filter, then straight through a green one. What comes out? (Answer: nothing. The red filter transmits only red, and the green filter absorbs that red.)

Flashcards

(13)

What does each colour of visible light have its own narrow band of?

Wavelength and frequency.

Going from red to violet, how do frequency and wavelength change?

Frequency increases; wavelength decreases.

What does a triangular prism show about white light?

It spreads white light into a spectrum, showing it is a mixture of all visible wavelengths.

Specular reflection

Reflection from a smooth surface in a single direction.

Diffuse reflection

Reflection from a rough surface, where the light is scattered.

How does a colour filter work?

It absorbs certain wavelengths (colours) and transmits others.

What decides the colour of a filter?

The wavelengths it transmits.

Opaque object

An object that transmits none of the light that hits it.

What are objects that transmit light called?

Transparent or translucent.

What decides the colour of an opaque object?

Which wavelengths of light it reflects most strongly.

What happens to the wavelengths an opaque object doesn't reflect?

They are absorbed.

When does an object look white?

When it reflects all wavelengths equally.

When does an object look black?

When it absorbs all wavelengths, so none reach your eye.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Visible light and colour (physics only)

Same objects, different light

What white light is made of

Follow the light that hits an object

What does a red filter actually do?

Explain it like a physicist

More AQA GCSE Physics topics

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