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Unlock This CourseIn The Electromagnetic Spectrum you met the seven regions and their order. Now you need to know what each one is used for. The spec lists the uses, and you must learn them all. But it also uses the word 'explain'. That means naming the use is not enough. You must be able to say why that radiation is a good choice for the job.
The reason always comes down to what the radiation does when it meets a material. Three things can happen, and you need the word for each.
Key terms:
Different regions are transmitted and absorbed by different materials. That is why each one has its own uses. Here is the whole list from the spec in one table.
| Radiation | Uses you must know |
|---|---|
| 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 |
Broadcasting means sending the same signal out to many people at once, such as radio and television programmes. Communications means sending messages between people, such as the two-way radios used by ships, aircraft and the emergency services.
Why radio waves? They are transmitted through the air and can travel long distances. Their long wavelengths let them spread around hills and buildings, so the signal can reach places that are not in a straight line from the transmitter.
Signals for satellite television and satellite phones are carried by microwaves. A dish on the ground sends them up to a satellite, and the satellite sends them back down to another place on Earth.
Why microwaves? They are transmitted straight through the Earth's atmosphere. Some radio waves are reflected back down by a layer high in the atmosphere, so they never reach the satellite. Mobile phones use microwaves to carry their signals too.
Cooking is the other use of microwaves. A microwave oven fills the space inside with microwaves. They are absorbed by the water molecules in the food. Energy is transferred to the water, the food heats up, and it cooks. The microwaves pass a few centimetres into the food, so it heats inside as well as at the surface. That is why a microwave oven is quick. Microwaves are transmitted through glass and plastic, so the dish itself is not heated directly.
The glowing bars of an electric heater and the element of a grill emit lots of infrared. When infrared is absorbed by your skin or by food, energy is transferred and the surface heats up.
Why infrared? It is easily absorbed by surfaces, and it heats what it lands on. You feel the warmth from a glowing heater at once, even from across a room.
All objects emit infrared, and the hotter they are, the more they emit. Night vision equipment has a detector that picks up infrared instead of light. It turns the pattern into a picture on a screen.
Why infrared? A warm person or animal emits more infrared than the cooler ground and trees around it. So it shows up clearly, even when it is too dark to see anything with your eyes.
An optical fibre is a very thin strand of glass. Information such as telephone calls and internet data is sent along it as pulses of light.
Why visible light? It is transmitted through glass, so the pulses can travel a long way along the fibre. Light also has a much higher frequency than radio waves, so it can carry far more information each second. You will see how the light stays inside the fibre in Optical Fibres and Prisms.
A camera lets light in through a lens. The lens brings the light together to form an image on a light-sensitive detector, or on photographic film in older cameras. The detector records the pattern of light as a picture.
Why visible light? It is the only region our eyes detect. A photograph taken with visible light shows the scene just as we see it.
Ultraviolet: fluorescent lamps. Some chemicals absorb ultraviolet and emit visible light. This is called fluorescence. A fluorescent lamp is a glass tube with a coating of one of these chemicals on the inside. When the lamp is on, ultraviolet is produced inside the tube. The coating absorbs the ultraviolet and emits visible light, which lights the room. Fluorescent lamps are used because they waste much less energy by heating than filament lamps.
X-rays: seeing inside things. X-rays are transmitted through soft materials but absorbed by dense ones. In a hospital, X-rays are sent through part of the body towards a detector. They pass through skin and muscle but are mostly absorbed by bone. The detector records where the X-rays got through. The bones show up as clear shadows, so a doctor can see a break. Dentists use the same idea to look inside teeth.
This is not only a medical use. The spec says 'objects and materials'. Airport security scanners use X-rays to see inside bags without opening them, because metal objects absorb more X-rays than clothes. Engineers use X-rays to look for hidden cracks inside metal parts.
Gamma rays: sterilising. To sterilise something means to kill the bacteria and other microorganisms on it. Gamma rays carry enough energy to kill them.
You will learn where gamma rays come from in Alpha, Beta and Gamma Radiation.
Question: Explain why infrared is used in night vision equipment. [2 marks]
Step 1: say what the radiation does. 'Warm objects such as people emit infrared.'
Step 2: link it to the job. 'The equipment detects the infrared and makes a picture, so a person can be seen in the dark.'
Use this pattern every time: what the radiation does (transmitted, absorbed or emitted, and by what), then why that makes it right for the job.
Electromagnetic radiations have many uses.
(a) State one use of ultraviolet radiation. [1 mark]
(b) Explain why microwaves are suitable for satellite transmissions. [2 marks]
(c) A hospital uses X-rays to check a patient's arm for a broken bone. Explain how X-rays produce a picture that shows the bone. [3 marks]
(d) Plastic syringes are sealed in packets and then sterilised using gamma rays. Explain why gamma rays are suitable for this. [2 marks]
(a) Fluorescent lamps (1).
(b) Microwaves are transmitted through the Earth's atmosphere (1), so the signal can travel between the ground and the satellite without being absorbed or reflected back (1).
(c) X-rays pass through soft tissue such as skin and muscle (1). They are absorbed by bone (1). A detector behind the arm records where the X-rays got through, so the bone shows up as a shadow (1).
(d) Gamma rays kill bacteria and other microorganisms (1). They pass through the packet, so the syringe can be sterilised after it has been sealed and stays sterile (1). (Also allowed for the second mark: gamma rays do not heat the syringe, so the plastic does not melt.)
Check the command word in each part. Part (a) says 'state', so two words are enough. Parts (b), (c) and (d) say 'explain', so each answer needs what the radiation does (transmitted or absorbed, and by what) joined to the job with a word like 'so'.