Physics
Double Award
Double Award
Cambridge IGCSE
TOPIC 3: WAVES
In this section we will learn about how the colours that we see in light from the Sun are not the only waves radiating all around us. There is a huge spectrum of other waves - similar to light - that our eyes cannot detect:
As we met in section 3.3, a prism can refract white light and disperse the beam into the colours of the spectrum. The spectrum is traditionally described as consisting of 7 different colours. The red end of the spectrum is the light wave with the longest wavelength and lowest frequency. Violet light is the part of the visible spectrum with the shortest wavelength and the highest frequency. However, scientists have found that some animals can see light with a shorter or longer wavelength than visible light:
For example, bees can see light with a much higher frequency than violet light. This light is called ultraviolet light. Snakes, on the other hand, can detect light with a longer wavelength than red light, called infrared light.
Light can pass through a vacuum - completely empty space. The light wave is actually an electric and magnetic vibration in space. As visible light, ultraviolet and infrared light all have identical properties, they are all called electromagnetic waves. They form parts of the electromagnetic spectrum. This family of waves extends far beyond that which any animal eyes can detect, as figure 1 shows:
Figure 1. The electromagnetic spectrum
You may well recognise some of the types of waves in the figure above. Even though the behaviour of these waves are quite different, they are actually all in the same family.
Note that is some books this diagram is reversed.
All of these waves have some key identical properties:
Learn these key properties that show they are all in the same family. You will also need to know a bit more about each of the types of waves shown in figure 1, as described here:
Radio waves are low energy waves that travel through many objects quite easily. They are used for communications because of these properties. They can pass though thin concrete, glass, and our own bodies. FM and AM radio stations that we often listen to in cars and at home are 'broadcasted' using standard radio waves to send the signals to us.
To produce a radio wave we need a transmitter - these produce radio waves from an alternating current passed through them. We need an aerial to receive the radio waves, converting them to an electrical signal. and then a circuit to decode the signals. Two-way radios communicate with radio signals, as the name suggests. Television signals around the world are transmitted using radio waves, although this technology is rapidly being replaced by high speed internet transmission.
Radio frequency identification tags ('RFID' tags) are small devices that can be fitted to cars, parcels and a range of other items. When a radio wave is emitted by a nearby transmitter, the tags respond with an identification code. This means the tag can be tracked, really useful for cars going through tolls and for parcels being tracked as they are transported. In figure 2, the transmitter is above the cars as they drive past the RFID check point.
In addition, radio waves can also be used in astronomy, as many objects in space emit radio waves. This can be useful as radio waves can penetrate through dust clouds more effectively than visible light. Figure 3 gives a radio image of the centre of our own Milky Way galaxy, showing details not seen with visible light telescopes.
Radio waves are all around us and we do not notice them at all. They are not dangerous at typical strengths because of their low energy.
These are similar to radio waves, but - as the name suggests - they have much smaller wavelengths. They are also used for communication, and are used in all mobile phones. (Cell phones). They can carry more information in the signal than radio waves, but need a more direct line of sight to link with your phone. Microwaves are used to communicate with satellites orbiting the Earth, - allowing us to receive T.V. signals, and also G.P.S. signals to show our location on our phones.
There is one specific frequency of microwaves that makes water molecules vibrate. This means that it will heat up water in food, and this is the technology used in microwave ovens. Note that it is only a specific frequency that is utilised - there are many fake videos available that show mobile phones used to cook food! This cannot happen, as mobile phones use a different frequency from microwave ovens.
If you stand in front of a large fire, you can feel the heat radiation on your skin coming from the fire. All heat radiation is in fact infrared radiation. This heating effect is used in appliances like electric grills, toasters and electric fires.
Some camera lenses can detect infrared, and this can be used to great effect in security cameras. An area can be lit with a strong infrared light source, but still still look dark to us as our eyes cannot see these long wavelength waves. As our bodies are at 370C, we radiate a small amount of infrared heat all the time. This can be detected with very sensitive cameras - and other 'night vision' thermal imaging technology that uses infrared light. The heat from our bodies can be detected by infrared intruder alarm systems, looking for unwanted visitors in secure places like banks and jewellry shops.
All of our remote controls use infrared light to send signals to our electrical devices like televisions. In the YouTube clip, you can see several features of infrared light being displayed. These include the fact that the camera can 'see' the infra red signal, and display it on our screens as visible light. We can also see the infrared light being reflected.
YouTube - infrared light in remote controls
We should all be familiar with some basic applications of visible light. We can use light rays for photography, and obviously for our own vision. Creating visible light for our own use at night is a global industry, and the development of highly efficient LED lighting has led to considerable recent developments in the illumination industry.
One of the most useful recent developments is the use of light rays for communications using optical fibres. The light ray can be switched on and off extremely quickly, and therefore used to send a high rates of data in a very short time. This application is speeding up communication through the internet and allowing all of our digital devices at home to receive information down one fibre. Optical fibres are made of high quality glass, and can be used for cable television and high-speed broadband because glass is transparent to visible light and some infrared. See section 3.2b for more about optical fibres.
In terms of harmful effects, very strong visible light can potentially cause skin burns.
Ultraviolet (sometimes shortened to U.V. light) is shorter wavelength than visible light and has more energy. This can be dangerous as it can damage our eyes and cause blindness, and also cause burns to our surface skin cells. The Sun produces U.V. light, and it is this that can give us sun burn if we stay out in the Sun for too long.
However, U.V. light also has some useful applications. Some chemicals absorb U.V. light, and then re-radiate it as visible light. This means that they chemicals 'glow' under a U.V. source, an effect called fluorescence. (Figure 2). Fluorescent chemicals are used in bank notes making them harder to forge. To check the note is genuine, the note is held under a U.V. lamp to see the hidden images or text from the fluorescent inks. This can be applied to a wide range of security markings including expensive sports or concert tickets.
Figure 4. Fluorescent chemicals
Erin Rod CC by SA 4.0
If water flows past a strong source of U.V. light, microbes are killed, and therefore U.V. light can also be used to sterilise water for safe drinking.
These rays are much higher energy waves than U.V. light. They can therefore penetrate through our skin. This, though, is one of the key applications of x-rays in that they can pass through us and out of the other side, only being stopped by dense bones. By taking a picture of the x-rays that emerge, we can 'see' our skeletons and other features inside the body.
This technology is also used for security in airports, to take a picture of what is inside our luggage in an X-ray scanner. We can also detect internal breaks or defects inside materials using this scanning technology, for example to check that an aircraft wing is still strong and has no cracks or other flaws.
X-rays can be dangerous and we must minimise exposure to them. Doctors, nurses and technicians in hospitals must not be exposed to too much radiation over a lifetime. X-rays can cause internal cell damage, organ damage and can mutate cells, causing cancer.
These are the waves with the highest frequency in the electromagnetic spectrum, and are therefore the most dangerous. Gamma rays can damage cells, killing them, or producing mutations which lead to cancer.
Gamma rays can be used in medical tracers that can detect cancers in the body. See section 5.2d for more information on how this works. In addition, by directing gamma rays at a cancer growth, the cancer cells can be killed, whilst more resilient healthy cells around the tumor can recover. This treatment is called radiotherapy.
You will need to be able to explain some uses and some dangers for each of the sections of the electromagnetic spectrum. Table 1 below gives a quick checklist to help you remember the key points required for this course:
| Region of E.M. spectrum | Uses | Dangers* |
|---|---|---|
| radio | broadcasting TV and radio, communication | none/insignificant |
| microwaves | mobile (cell) phones, satellite communication, microwave ovens | none/insignificant |
| infrared | electric grills, thermal imaging, optical fibres, | none/insignificant |
| visible | photography, vision, illumination, optical fibres | none/insignificant |
| ultraviolet | fluorescent lamps and inks for security marking and bank notes | damage to surface cells and eyes, leading to skin cancer and eye conditions |
| x-rays | observing internal structures, for medicine and materials |
mutations of cells, damage to internal cells and organs, cancer |
| gamma rays | detection of cancer as well as treatment (radiotherapy) | mutation of cells, damage to internal cells and organs, cancer |
Table 1. Uses and dangers of the electromagnetic spectrum
*exposure to intense microwaves, infrared and visible light can be dangerous, but not required for this course.
Questions:
1. Fluorescence can be used to protect bank notes from being forged.
a) Fluorescence uses ultraviolet light.
b) U.V. light from a lamp is directed on the notes. Fluorescent inks /chemicals in the note absorb the U.V. and re-radiate it as visible light. This causes the inks / chemical to glow, known as fluorescence, showing images or text that was previously hidden.
2. An X-ray source produces a wave that has a wavelength of 2 x 10-9 m.
a) X-rays can be used for observing internal structures, for example in medicine or for scanning luggage at an airport.
b) X-rays can cause mutations of cells, damage to internal cells and organs, cancer.
c) The speed of all electromagnetic waves should be learned: It is 3 x 108 m/s.
We know that v = ƒ x λ, so the frequency ƒ is:
| ƒ = | v |
| λ |
| ƒ = | 3 x 108 |
| 2 x 10-9 |
ƒ = 1.5 x 1017 Hz
Now test your understanding using this quick quiz on the EM Spectrum:
