15.3Fundamentals of Light

Color and the Electromagnetic Spectrum — American System Physics Revision Notes & Topical Questions

10 min read 7 parts Lesson 5 of 5 1 quiz

📚 American System Physics Hub / Topical Revision

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Lesson summary

List the regions of the EM spectrum in order and give uses and hazards.Wave equation for all EM wavesc = fλ, c = 3.00 × 108 m/s in vacuum.Order by increasing frequencyradio, microwave, infrared, visible, UV, X-ray, gamma.Visible order by increasing frequencyred, orange, yellow, g…

Subtractive
Additive mixing of light
Uses of EM waves depend on their properties
A microwave oven uses waves of frequency 2.45 GHz. Find the wavelength
An X-ray has wavelength 1.0 × 10−10 m. Find its frequency
Electromagnetic spectrum

Section 1 of 7Color and the Electromagnetic Spectrum

Essential knowledge

15.3 Color and the Electromagnetic Spectr… concept map: Wave equation for all EM waves, Order by increasing frequency, Visible order by increasing f, Additive, Subtractive — Visual guide to the key ideas explained in this lesson.

The electromagnetic spectrum is continuous. All its members are transverse waves of electric and magnetic fields, transfer energy, can travel through a vacuum at the same speed c, and can be reflected, refracted and diffracted.

Moving from radio waves towards gamma rays, frequency increases and wavelength decreases; energy carried per photon increases, so hazards increase.

White light is a mixture of all visible wavelengths. A prism disperses it because the refractive index of glass is slightly different for each colour: violet slows the most and bends the most; red bends the least. A rainbow is dispersion in raindrops.

The colour of an opaque object is the colour of the light it reflects; other colours are absorbed. A red book in white light reflects red and absorbs the rest. Under blue light, the red book reflects nothing and appears black.

A white object reflects all colours; a black object absorbs all colours (and so warms up more in sunlight).

A coloured filter transmits only its own colour and absorbs the rest. Red light passing through a green filter is absorbed, so nothing comes through.

Additive mixing of light

Additive mixing of light red, green and blue are the primaries. Screens make every colour by varying the brightness of tiny red, green and blue pixels. Complementary colours add to white.

Subtractive mixing of pigments

Subtractive mixing of pigments each primary pigment (cyan, magenta, yellow) absorbs one primary light colour. Mixing yellow and cyan paint gives green, because yellow absorbs blue and cyan absorbs red, leaving only green reflected.

Uses of EM waves depend on their properties

Uses of EM waves depend on their properties radio waves diffract around hills for long-distance broadcasting; microwaves pass through the atmosphere for satellites and are absorbed by water in food; infrared is emitted by all warm objects; X-rays pass through soft tissue but are absorbed by bone.

Ionising radiation (high-energy UV, X-rays, gamma rays) can damage DNA and cause cancer, so exposure is limited using lead shielding, distance and short exposure times.

Section 2 of 7Process & sequence

Direct a narrow beam of white light from a ray box onto one face of a triangular glass prism.

Rotate the prism until a band of colours appears on a white screen beyond it.

Observe that violet is deviated (refracted) the most and red the least.

Place a second, inverted prism in the path to recombine the colours into white light, showing white light is a mixture.

Place a red filter in the beam: only red passes, because the filter absorbs other colours.

Place a thermometer with a blackened bulb just beyond the red end: its reading rises, showing invisible infrared is present.

How a prism disperses white lightwrite these steps in this order; each step is a marking point.

White light, a mixture of all visible wavelengths, strikes the first face of the prism.

Each colour slows down on entering glass by a different amount (violet most, red least).

Colours are refracted through different angles and begin to separate.

At the second face they refract again away from the normal, increasing the separation.

A spectrum from red (least deviated) to violet (most deviated) appears on the screen.

Section 3 of 7Compare

The electromagnetic spectrum (increasing frequency, decreasing wavelength)
Region	Typical λ	Uses	Hazards
Radio	> 1 m	Radio and TV broadcasting, communication	Very low risk
Microwaves	1 mm – 1 m	Satellite TV, mobile phones, cooking	Heating of body tissue
Infrared	700 nm – 1 mm	Heaters, remote controls, thermal imaging, optical fibres	Skin burns
Visible light	400 – 700 nm	Vision, photography, optical fibres	Eye damage from intense light (e.g. lasers)
Ultraviolet	10 – 400 nm	Security marking, sterilising water, sun tanning	Sunburn, skin cancer, eye damage
X-rays	0.01 – 10 nm	Medical imaging, airport security	Ionising: cell mutation, cancer
Gamma rays	< 0.01 nm	Sterilising equipment, cancer treatment	Ionising: cell mutation, cancer

All EM waves are transverse, travel at c in a vacuum and transfer energy; higher frequency means higher energy.

Colour mixing
Mixing type	Primaries	All three combined	Example
Additive (light)	Red, green, blue	White	TV and phone screens
Subtractive (pigment)	Cyan, magenta, yellow	Black (ideally)	Printer inks, paints

Section 4 of 7Worked examples

Worked examples

A microwave oven uses waves of frequency 2.45 GHz. Find the wavelength.

λ = c/f = 3.00 × 10⁸ ÷ 2.45 × 10⁹ = 0.122 m (12.2 cm).

An X-ray has wavelength 1.0 × 10−10 m. Find its frequency.

An X-ray has wavelength 1.0 × 10−10 m. Find its frequency.

f = c/λ = 3.00 × 10⁸ ÷ 1.0 × 10⁻¹⁰ = 3.0 × 10¹⁸ Hz.

Red

A white T-shirt with a blue logo is viewed under pure red light. Describe what is seen, with reasons.

The white fabric reflects all colours, so it reflects the red light and appears red.
- The blue logo reflects only blue and absorbs red, so it reflects nothing and appears black.

Section 5 of 7Key terms

Section 6 of 7Command terms

Command terms

Identify / Describe (FRQ)

A short, specific statement; describing requires relevant characteristics, not a single word.

Explain / Justify (FRQ)

Give the cause-and-effect reasoning or evidence; a restated fact does not score.

Calculate

Show setup, substitution and the answer with units.

Predict

State the expected outcome; add a reason only if asked to justify.

MCQ technique

Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Section 7 of 7Exam technique

Keywords the mark scheme looks for in this lesson

Electromagnetic spectrum · Dispersion · Spectrum (visible) · Primary colours of light · Secondary colours of light · Complementary colours · Pigment · Primary pigments · Dye · Ionising radiation · Infrared (IR) · Ultraviolet (UV)

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).

Use precise vocabulary, never “it/stuff/things”.

Show units and working in calculations.

In MCQs, predict the answer before reading the options, then eliminate.

Words to avoid

a red object gives out red lighta red object reflects red light and absorbs other colours.

gamma rays travel faster than radio wavesall EM waves travel at the same speed in a vacuum.

red is refracted the mostviolet is refracted the most, red the least.

mixing red, green and blue paint gives whitemixing red, green and blue light gives white; pigments mix subtractively.

microwaves are ionisingonly high-frequency UV, X-rays and gamma rays are ionising.

Last updated: 11 October 2026