Electromagnetic Waves

Transcription

1 Chapter 13 Electromagnetic Waves 13.1 Gamma Rays Gamma rays have a very short wavelength and are very penetrating. They are produced by radioactive substances and are very dangerous to humans unless used very carefully. These rays can be used to kill cancer cells and to sterilise food X-Rays X-rays are very like gamma rays, but are produced by an X-ray tube. Doctors and dentists use X-rays to check bones and teeth. As with gamma rays, X-ray photos are used by engineers to check welds and metal joints. In factories, X-rays can be used to check that food is free from metal and stones. Mr. C.Busuttil Ch 13 Page 1 of 7

2 13.3 Ultraviolet Rays Ultra-violet rays can also be dangerous to us. Hot objects like the sun produce ultra-violet rays, and so do the electric arcs used in electric welding. Small amount of UV rays are good for us, producing vitamin D in our skin. Large amounts of UV rays are bad for our eyes and cause skin cancer! These rays also cause suntan. Luckily, most of the sun s UV is absorbed high in the atmosphere by the ozone layer. Some chemicals fluoresce when they absorb UV rays and convert the energy to visible light rays. Washing powders include these chemicals so that your shirts look brighter in sunlight Visible Light Visible light is the only part of the full spectrum that we can see. Our eyes are more sensitive to some wavelengths than to others, as the graph shows. Your eye is most sensitive to green-yellow light Infra-Red Infra-red rays are given out by warm objects. In fact, every object that has a temperature above absolute zero gives out IR waves. Due this fire-fighters can use IR viewers to search for unconscious people in smokefilled buildings, and to search for survivors trapped alive beneath buildings by earthquakes. Mr. C.Busuttil Ch 13 Page 2 of 7

3 Special photographs taken with IR rays are called thermographs. They can help doctors to detect circulation problems, arthritis and cancer. Astronomers take IR photos to get data about the temperatures of planets and stars. Burglar alarms are designed to detect the IR rays from an intruder. Infra-Red is unconsciously continuously used whenever we are in front of a TV set. A remote control transmits IR, with a receiver in the TV. Consequently, the IR wave gives a signal to the set to switch on to another channel Radio Waves Radio waves have a longer wavelength. There are several kinds of radio waves. Microwaves are the shortest of the radio waves. They are used in microwave ovens. They are also used for communicating with satellites and for radar. UHF waves are used to transmit TV programmes to your home. VHF waves are used to transmit local radio programmes, and local police and ambulance messages. Medium wave and long wave radio are used to transmit over long distances. Because they have long wavelengths, they can diffract round the curve of the Earth and round hills. Mr. C.Busuttil Ch 13 Page 3 of 7

4 13.7 Properties of Electromagnetic Waves They transfer energy from one place to another. They are all transverse waves. They can all be reflected, refracted, diffracted. They can all travel through a vacuum. They all travel at the speed of light in vacuum. The shorter the wavelength, the higher the frequency and so the more dangerous. This is a list of types of wave: gamma, infra-red, light, microwaves, radio, ultraviolet, X-rays. Choose from the list the type of wave which best fits each of these descriptions. (d) (e) Stimulates the sensitive cells on the retina. Used inside fluorescent lamps. Used for rapid cooking in an oven. Used to take a photograph of the bones in a broken leg. Emitted by a remote control unit. The electromagnetic spectrum consists of a family of waves. Give three properties which are the same for all these waves. Which parts of the electromagnetic spectrum: (i) will cause a sun tan? (ii) can be used both to cook and in communications? (iii) can be used to sterilise fruit and vegetables? (iv) can be detected from warm objects? Mr. C.Busuttil Ch 13 Page 4 of 7

5 13.8 The Electromagnetic Spectrum Mr. C.Busuttil Ch 13 Page 5 of 7

6 Which region of the spectrum: (d) has the longest wavelength? has the highest frequency? causes a suntan? is used in burglar alarms? Light is a type of electromagnetic radiation. Write down one type of electromagnetic radiation which has a wavelength: (i) longer than that of light. (ii) shorter than that of light. Complete the table with different types of electromagnetic radiation. Ultraviolet, gamma rays, radio waves are parts of the electromagnetic spectrum. Write down: (d) which has the longest wavelength. which has the highest frequency. which is emitted by the nucleus of an atom. two properties they have in common. Mr. C.Busuttil Ch 13 Page 6 of 7

7 Use the list below to copy and complete the electromagnetic spectrum chart above; gamma rays, infra-red, red light, ultraviolet, X-rays, radio waves. Name the part of the spectrum that: (i) has the longest wavelength, (ii) is emitted by all hot objects. State two practical uses of microwaves. Mr. C.Busuttil Ch 13 Page 7 of 7