14.3 Light and Colour

A free GCSE Physics lesson for AQA (8463) from OBA School: colour and wavelength, light sources, filters, the colour of opaque surfaces, transparent, translucent and opaque objects. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe how wavelength and frequency change across the visible spectrum
• Explain how colour filters work by absorbing some wavelengths and transmitting others
• Explain the colour of an opaque object in terms of the wavelengths it reflects and absorbs, in white and in coloured light
• Describe the difference between transparent, translucent and opaque objects

Key words

• Visible spectrum: The range of light the eye can detect: red, orange, yellow, green, blue, violet (about 400–700 nm).
• Wavelength: The distance from one point on a wave to the same point on the next wave.
• Colour filter: A transparent material that transmits one colour (range of wavelengths) and absorbs the others.
• Pigment: A chemical in a surface that absorbs certain wavelengths of light and reflects others.
• Transparent: Transmits light without scattering it, so you can see clear images through it.
• Translucent: Lets light through but scatters it inside, so you cannot see clear images through it.
• Opaque: Lets no light through: the light is reflected or absorbed at the surface.

Colour and wavelength

Visible light is the part of the electromagnetic spectrum the eye can detect. Each colour is a narrow band of wavelengths (and frequencies), and each band merges smoothly into the next. Going from violet to red, the wavelength increases and the frequency decreases.

• White light sources, such as the Sun and filament lamps, give out a continuous range of wavelengths across the whole visible spectrum.
• Some sources give out only a narrow range of wavelengths, so they have a definite colour, e.g. a laser pointer or a sodium street lamp.

Worked example: frequency of a green laser

A laser gives green light of wavelength 520 nm. Speed of light c = 3.0 × 10⁸ m/s.
λ = 520 nm = 520 × 10⁻⁹ m = 5.2 × 10⁻⁷ m
f = v ÷ λ = 3.0 × 10⁸ ÷ 5.2 × 10⁻⁷ = 5.8 × 10¹⁴ Hz

Answer: 5.8 × 10¹⁴ Hz

Colour filters

A colour filter transmits (lets through) light of its own colour and absorbs the other wavelengths. A filter cannot add colour to light: it can only remove colours.

Red, green and blue are the primary colours of light. Mixing them in different amounts gives every other colour, which is how screens make their pictures. Red + green = yellow, red + blue = magenta, green + blue = cyan, and all three together = white.

Key fact: Filters subtract colours from light. Two filters of different primary colours, one after the other, let no light through.

Why opaque objects have colours

The surface of an opaque object contains pigments. A pigment absorbs some wavelengths and strongly reflects others. The colour you see is the colour of the light the surface reflects into your eye, so it depends on the pigment and on the colours in the light falling on it.

• White surfaces reflect all wavelengths, so they look the colour of whatever light falls on them.
• Black surfaces absorb all wavelengths and reflect (almost) none.
• A red surface reflects red and absorbs the other colours. In blue light there is no red to reflect, so it looks black.

• white T-shirt; in white light white; in red light red; in green light green
• red football shirt; in white light red; in red light red; in green light black
• green leaf; in white light green; in red light black; in green light green
• black shoe; in white light black; in red light black; in green light black

Exam tip: Ask two questions: (1) which colours are in the light falling on it? (2) which of those does the surface reflect? If the answer to (2) is “none”, it looks black.

Transparent, translucent and opaque

• transparent; What happens to the light transmitted straight through with very little absorbed or scattered: you see clear images through it; Examples clear glass, still clean water, a clear plastic bottle
• translucent; What happens to the light transmitted, but scattered or refracted many times at boundaries inside the material: light gets through but images do not; Examples tracing paper, a frosted shower screen, thin cloth
• opaque; What happens to the light none is transmitted: it is reflected or absorbed at the surface; Examples wood, brick, metal, card

Transparent and translucent objects can also be coloured. A piece of red glass is transparent but transmits only red light: it is acting as a filter.

Worked example: choosing materials

A designer wants a lampshade that spreads light softly around a room and a panel for a hospital door that lets staff see who is inside.
Lampshade: translucent (e.g. thin fabric or frosted plastic): light gets through but is scattered, so there is no glare.
Door panel: transparent (clear glass or clear plastic): light is not scattered, so a clear image is seen.

Answer: translucent for soft, private lighting; transparent for seeing through

Common mistakes

• Getting the spectrum the wrong way round. Red has the longest wavelength (lowest frequency) and violet the shortest (highest frequency).
• Saying a filter “turns light red” or “adds” colour. It absorbs the other colours and transmits only its own.
• Saying a red object reflects red light in any lighting. It can only reflect red if red is in the light falling on it.
• Saying black objects reflect black light. Black is what you see when no light is reflected.
• Mixing up translucent and transparent. Translucent materials let light through but scatter it, so no clear image is seen.

Revision questions

Try these, then check your answers below each one.

Question 1. Which colour of visible light has the longest wavelength? (Choose from: red, violet, green, blue.)

Answer: red

Working: Red has the longest wavelength (about 700 nm) and lowest frequency; violet has the shortest (about 400 nm).

Question 2. A red book is lit only by blue light. What colour does it appear? (Choose from: black, red, blue, purple.)

Answer: black

Working: The red surface reflects only red light. Blue light contains no red, so the blue is absorbed and nothing is reflected: it looks black.

Question 3. True or false? A green filter turns white light green by adding green to it.

Answer: False

Working: False. The filter transmits the green part of the white light and absorbs all the other colours.

Related lessons

• Previous lesson: 14.2 Refraction of Light
• Next lesson: 14.4 Lenses
• In this chapter: 14.1 Reflection of Light
• In this chapter: 14.5 Using Lenses
• In this chapter: 14.6 Required Practical: Reflection and Refraction

Continue in the full course

This lesson is section 14.3 (AGCPH1403) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.