Edexcel IGCSE Physics · Spec 3.11-3.13
Visible Colours, Electromagnetic Uses and Hazards
Covers order visible-light colours, Match named waves to uses and Explain hazards and protection.
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Visible Colours, Electromagnetic Uses and Hazards
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Explained
The spectrum, its uses, and why the dangerous end is dangerous
Visible light runs red, orange, yellow, green, blue, indigo, violet. Red has the longest wavelength and the lowest frequency, violet the shortest and the highest.
That order continues in both directions. Beyond red come infrared, microwaves and radio waves, with ever longer wavelengths. Beyond violet come ultraviolet, X-rays and gamma rays, with ever shorter wavelengths and higher frequencies.
All of them travel at the same speed in a vacuum, three times ten to the eight metres per second, and all are transverse waves. What changes across the spectrum is frequency, and with it the energy each wave carries.
The uses, in order
- Radio waves: broadcasting and communications.
- Microwaves: cooking, satellite communication and mobile phones.
- Infrared: heaters, remote controls, night vision and thermal imaging.
- Visible light: seeing, photography and optical fibres.
- Ultraviolet: security marking, fluorescent lamps and sterilising water.
- X-rays: seeing inside the body and inside luggage.
- Gamma rays: sterilising equipment and food, and treating cancer.
The hazards follow the energy
Higher frequency means more energy per wave, and that is what makes the short wavelength end dangerous.
Ultraviolet, X-rays and gamma rays are ionising, which means they carry enough energy to remove electrons from atoms. Doing that inside a cell can damage or mutate the DNA, which can lead to cancer.
The longer wavelengths are not ionising, so their hazards are different in kind. Microwaves and infrared heat the tissue they are absorbed by, causing internal heating or burns. Radio waves are the least harmful of all.
Protection follows the mechanism. Sunscreen and sunglasses absorb ultraviolet; lead aprons and screens absorb X-rays; distance and shielding reduce exposure to gamma.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme for a question on the dangers of a broken fluorescent tube shows how these answers are marked. It credits the idea that mercury or ultraviolet would escape from the tube, that ultraviolet is ionising radiation, a named harmful effect of ultraviolet, and the idea that mercury vapour is poisonous.
The phrase named harmful effect is the one to notice, and the mark scheme gives its own examples: skin cancer, damage or mutation to skin cells, skin burns, damage to eyes, blindness. Writing that ultraviolet is harmful or dangerous names nothing and earns nothing. Writing that it can cause skin cancer does.
Elsewhere on the same paper, explaining why a fluorescent tube has a coating, the mark scheme wants two things: that humans cannot see ultraviolet, and that the coating emits visible light. Both halves are needed, and the first is the one people assume rather than state.
Answering a uses or hazards question
Name the wave, name the use or hazard, and give the property that explains it. Microwaves are used for satellite communication because they pass through the atmosphere. Infrared is used in remote controls because it is absorbed by a detector and does not need line of sight through a wall. X-rays produce an image of bone because bone absorbs them while soft tissue lets them through.
The property is what turns a list into an explanation, and in a two mark question it is usually the second mark.
Spec 3.11-3.13
What you need to know
- Order visible-light colours
- Match named waves to uses
- Explain hazards and protection
Active recall
Quick check
Answer each question before opening the answer.
List visible colours from longest wavelength
Red, orange, yellow, green, blue, indigo, violet
Name one ultraviolet use
Fluorescent lamps or security marking
Why are gamma rays especially hazardous?
They are penetrating ionising radiation that can damage DNA and cause mutations
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