Electromagnetic spectrum questions

The electromagnetic spectrum syllabus

Questions on the electromagnetic spectrum can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 3.10 know that light is part of a continuous electromagnetic spectrum that includes radio, microwave, infrared, visible, ultraviolet, x-ray and gamma ray radiations, and that all these waves travel at the same speed in free space
• 3.11 know the order of the electromagnetic spectrum in terms of decreasing wavelength and increasing frequency, including the colours of the visible spectrum
• 3.12 explain some of the uses of electromagnetic radiations, including: radio waves: broadcasting and communications microwaves: cooking and satellite transmissions infrared: heaters and night vision equipment visible light: optical fibres and photography ultraviolet: fluorescent lamps x-rays: observing the internal structure of objects and materials, including for medical applications gamma rays: sterilising food and medical equipment
• 3.13 explain the detrimental effects of excessive exposure of the human body to electromagnetic waves, including: microwaves: internal heating of body tissue infrared: skin burns ultraviolet: damage to surface cells and blindness gamma rays: cancer, mutation and describe simple protective measures against the risks

How to answer the electromagnetic spectrum questions

1. Label wavelength, amplitude, frequency and normal lines before interpreting a diagram.
2. Use a ruler for ray diagrams and measure angles from the normal, not the surface.
3. Keep wave properties separate from applications and link each hazard to a suitable precaution.
4. For calculations, convert frequencies and distances before using wave speed relationships.

The electromagnetic spectrum example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Recognising a digital signal

June 2018 (R) · Paper 2P · Question 5(a)(iv)

Question 1

A space probe was launched in 2007.

Its mission was to visit Ceres, a distant dwarf planet in the asteroid belt.

The space probe sends data back to Earth as a radio wave.

The sketch graph shows how the radio wave changes as the radio wave travels in free space towards the Earth.

Which of these is a digital signal?

Final answer

B is the digital signal because it switches between two distinct levels: high and low.

Mark scheme points

1. M1 Identify B.

Explanation

A digital signal has a limited number of distinct levels. In B, the signal alternates between two levels, representing low and high states.

Common mistakes

• Do not choose A, C or D: each varies through more than two amplitude values rather than switching between two distinct levels.

Example 2: Relating LED colour to voltage

June 2013 · Paper 2P · Question 5(b)(iii)

Question 2

A student has some LEDs connected in a circuit. They emit light of different colours.

An LED needs a minimum voltage to make it emit light. The student investigates this minimum voltage using the circuit shown.

The student concludes:

Evaluate the student’s conclusion.

Final answer

The conclusion is correct. Moving through the visible spectrum from red towards blue, the wavelength becomes shorter and the minimum voltage increases: red has the longest wavelength, while blue needs the highest minimum voltage.

Mark scheme points

Any two of these three points are needed for full marks. Saying only that the student is right or wrong earns no mark.

1. M1 Identify the visible-colour sequence from red to blue, or in reverse (for example, ROYGBIV).
2. M2 Relate colour to wavelength: red has the longest wavelength.
3. M3 Relate colour to minimum voltage: blue needs the highest voltage.

Explanation

Red light is at the long-wavelength end of the visible spectrum, and blue light is at the shorter-wavelength end. Blue therefore has a higher frequency than red. Its photons carry more energy, so the LED needs a higher minimum voltage to emit blue light. This supports the conclusion that the minimum voltage depends on the wavelength emitted.

Common mistakes

• Writing only “the student is right” does not earn a mark; support the conclusion with a colour–wavelength or colour–voltage relationship.
• Do not reverse the trend: blue has a shorter wavelength and needs a higher minimum voltage than red.

Practise The electromagnetic spectrum questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.