Colour and filters: how the colour of an opaque object depends on the wavelengths it reflects and absorbs, and how a colour filter transmits some colours and absorbs others.

What this dot point is asking

Edexcel statement 5.3 (separate physics) wants you to explain how the colour of light is related to the differential absorption, transmission and reflection of light at surfaces and through filters, including why objects appear black or white.

Why opaque objects have colour

White light is a mixture of all the colours of the visible spectrum. When it falls on a coloured object, the surface reflects certain wavelengths to your eyes and absorbs the others. The reflected wavelengths are what you see, so the object appears that colour. This differential reflection and absorption is the whole explanation of everyday colour.

Black and white objects

White and black are the two extremes of reflection and absorption. Because a black surface absorbs nearly all the light energy falling on it, it heats up more than a white surface, linking this dot point to infrared radiation and surfaces. A white surface, reflecting most light, stays cooler.

Colour under different light

This is a favourite exam idea: an object cannot reflect a colour that is not shining on it. So a red jumper looks black under pure blue light because there is no red light for it to reflect, and it absorbs the blue. Always check which colours are present in the illuminating light before deciding how the object looks.

Colour filters

A filter does not add colour; it removes the colours it absorbs and lets the rest through. So white light passing through a green filter emerges green, because only the green wavelengths are transmitted. Putting two different filters together (for example red then green) can block almost all the light, because each absorbs what the other transmits.

How Edexcel examines this

This statement is separate-physics only and is examined on both tiers within that route, usually as a two or three mark explanation question. The two reliable styles are an object viewed under coloured light (predict the colour and explain it) and a colour filter (state what it transmits and absorbs and the colour seen). Mark schemes reward the reflect-and-absorb language: an object reflects its own colour and absorbs the rest, and a filter transmits its own colour and absorbs the rest. The classic full-mark response for "a red object in blue light" states that there is no red light to reflect, so the object absorbs the blue and appears black. Examiners penalise the misconception that a filter or object "changes" light into a new colour or "adds" colour; light is only ever reflected, transmitted or absorbed. Linking black surfaces to absorbing more energy connects this dot point to the infrared radiation core practical, which examiners sometimes test together.

Try this

Q1. Explain why a white object appears white in white light. [1 mark]

• Cue. It reflects all the wavelengths of visible light.

Q2. State the colour of light transmitted by a blue filter. [1 mark]

• Cue. Blue (it absorbs the other colours).