Matching a wave to a job
- Each group of the electromagnetic spectrum has a different wavelength and frequency, so each group behaves differently when it meets matter.
- A material may let a group pass through, reflect it or absorb it, and the same material can treat two groups in completely different ways.
- A use is chosen to match these properties, so a good answer links each use to the property that makes it work.
- Waves at the radio end of the spectrum are good at carrying signals over long distances, while waves at the gamma end pass easily through the body and can kill cells.
Radio waves and microwaves
- Radio waves
- Radio waves are used to broadcast radio and television programmes and for two-way radio, such as between aircraft and air traffic control.
- They travel long distances, pass through the walls of buildings and, at long wavelengths, spread around hills, so a receiver does not need to be in sight of the transmitter.
- Microwaves
- Microwaves carry signals for mobile phones, Wi-Fi and satellites, including satellite television and satellite navigation.
- They pass easily through the atmosphere, so signals can travel between the ground and satellites in orbit.
- Microwaves are also used to cook food, because they are strongly absorbed by the water in food and transfer energy to its thermal store.
Practical
Absorbing microwaves
- Aim: to show that some materials absorb microwaves, some reflect them and some let them pass through.
- Apparatus: a school microwave transmitter with a wavelength of about 3 cm3\ \text{cm}3 cm, a microwave receiver probe connected to a meter, a metre rule, and sheets of dry card, glass, hardboard and metal, a damp cloth and a shallow plastic tray of water.
- Method:
- Place the receiver about 40 cm40\ \text{cm}40 cm from the transmitter, with both at the same height and pointing straight at each other.
- Switch on and record the meter reading with nothing between them.
- Hold the first material midway between them, keeping the distance and alignment exactly the same, and record the new reading.
- Repeat for each material in turn, taking each reading three times and finding the mean.
- Finally, move the receiver round to the same side as the transmitter and angle it towards the metal sheet to detect any microwaves reflected from it.
- Results: the reading falls only a little for dry card, glass and hardboard, falls sharply for the damp cloth and the tray of water, and drops almost to zero for the metal sheet, which instead gives a strong reading when the receiver is placed to pick up reflected waves.
- Conclusion: water absorbs microwaves strongly, which is why a microwave oven heats the food, while metal reflects them, which is why the oven's metal walls keep the microwaves inside.
- Watch out: the readings only compare fairly if the distance and alignment stay fixed, because moving the receiver changes the reading even with nothing in the way.
- Safety: school transmitters have a very low power, but do not put your eye close to the transmitter horn, and keep the tray of water well away from mains sockets.
Infrared and visible light
- Infrared
- All warm objects emit infrared, and thermal imaging cameras detect it to make images that show temperature differences, for example to find a person in the dark or heat escaping from a house.
- Grills, toasters and electric heaters use infrared, because it is absorbed by the surface of food or skin and heats it.
- Remote controls send coded pulses of infrared to a sensor on a television.
- Infrared also carries telephone and broadband data along optical fibres, which are thin glass strands that guide the waves.
- Visible light
- Visible light lets us see, and it is used for lighting and in photography, where a camera sensor records the light.
- Visible light sent along optical fibres in an endoscope lets doctors see inside the body without major surgery.
Practical
Seeing infrared from a remote control
- Aim: to show that a remote control emits a signal that your eyes cannot detect but a camera sensor can.
- Apparatus: a television remote control and a phone camera or webcam.
- Method:
- Look straight at the small bulb on the front of the remote control and press a button, and note that you see nothing.
- Open the camera app and point the camera at the bulb from about 20 cm20\ \text{cm}20 cm away.
- Hold down a button and watch the screen.
- Repeat with the room lights dimmed, where the flashes are easier to see.
- Results: the screen shows a flickering white or purple light at the bulb while the button is held down, because the camera sensor detects the infrared and displays it as visible light.
- Watch out: many rear phone cameras have a filter that blocks infrared, so if nothing appears, switch to the front camera.
- Safety: this activity is low risk, because the infrared from a remote control is very weak.
Ultraviolet, X-rays and gamma rays
Definition
Fluorescence
The process in which a substance absorbs ultraviolet and then emits visible light.
Definition
Sterilisation
The killing of all bacteria and other microorganisms on an object or in a substance.
- Ultraviolet
- Some substances absorb ultraviolet and give out visible light, a process called fluorescence.
- Security marks written in special ink are invisible in daylight but glow under an ultraviolet lamp, which is used to check banknotes and to mark valuable property.
- Fluorescent tubes produce ultraviolet inside the glass, and a coating on the inside of the tube converts it into visible light.
- Ultraviolet is used to sterilise drinking water and surfaces, because it kills bacteria.
- Sunbeds use ultraviolet to give the skin a tan.
- X-rays
- X-rays pass easily through soft tissue but are absorbed by bone, so they are used to make medical images of broken bones and teeth.
- Airport security scanners use X-rays to show the contents of luggage without opening it.
- Gamma rays
- Gamma rays are used to sterilise medical equipment, such as syringes already sealed in their packaging.
- Gamma rays are also used to treat some food, killing microorganisms so that the food keeps for longer.
- Carefully aimed gamma rays can treat cancer by killing the cells of a tumour, which is called radiotherapy.
Common Mistake
- Do not say that remote controls use radio waves or visible light, because a television remote uses infrared.
- Do not say that microwaves cook food by heating the air in the oven, because the energy is absorbed by the water in the food itself.
- Do not give "communication" as the use for every group, because each group needs a specific use, such as satellite television for microwaves.
Exam technique
Giving uses of electromagnetic waves
- For "give a use", name a specific device or job together with the correct group, such as "X-rays are used to image broken bones".
- For "explain why", add the property that makes it work, for example X-rays pass through soft tissue but are absorbed by bone.
- Learn at least two uses for each group, because a question often asks for a use different from the one already given in the question.
Self review
- Which group of the electromagnetic spectrum do thermal imaging cameras detect?
- Why are microwaves suitable for communicating with satellites?
- What is fluorescence, and how does it help to check banknotes?
- Why do X-rays produce an image of a broken bone?
- Give two uses of gamma rays.
