Waves play a key part in our daily lives as they are widely used for communication. In senior high school, you need to be familiar with the situations in which waves are used to communicate and able to compare the use of different types of waves. Most of the waves used in this context are electromagnetic, however, we also use sound waves.
Sound waves are used for talking to each other and for getting attention (examples: shouting and the use of vehicle horns, sirens, bells and so on). You are expected to know the hazards associated with waves used in communications. Beginning with sound, the main one is too much exposure to loud sounds can damage the hearing. That is because loud sounds carry a lot of energy and have large amplitudes.
Radio waves are the lowest energy, longest wavelength and lowest frequency electromagnetic radiation. They are used to broadcast television and radio programs. The waves spread out from a large, powerful transmitter and can be received by anyone with a suitable receiver. The radio waves used to transmit television signals have higher frequencies and therefore shorter wavelengths than radio programs. You don't need to be able to see a transmitter to be able to receive radio signals as their wavelength is large enough to be diffracted by buildings and hills. Reception is better if you are in 'line of sight' (can directly see) a transmitter, so repeater stations are used. These boost the radio or television signals and improve reception. In the atmosphere, there is a layer of air that contains more charged particles than the rest of the air. This is called the ionosphere. Radio signals can be reflected off this layer which enables long range communications to be made round the curvature of the Earth. The layer is more strongly ionized during the night and it is possible to receive radio stations from other countries even on an ordinary radio receiver.
You probably associate microwave radiation with cooking, however, they are the basis of the cell phone industry. These have wavelengths that are too short to be diffracted and so if you are not in direct line with a mobile telephone mast, you will not have a signal. Certain wavelengths of microwave radiation can penetrate the atmosphere and are used to communicate with satellites and to beam satellite TV down to the surface. Some people believe that the microwave radiation used for cell phones could be a health risk. They think that the microwaves can cause cancer and brain damage. Not everyone accepts this because microwaves are a non-ionising form of radiation and used at intensities that are believed to be too low to damage living tissue.
Visible light has a much shorter wavelength and higher frequency than microwaves and radio waves and enables us to see writing, hand signals, watch TV and so on. Cameras can be used to record still and moving images and is an important aspect of communicating using light. The biggest hazard is that very intense light can cause temporary or permanent blindness. Looking at the Sun through any form of optical instrument (unless it is protected using special filters) will immediately burn the retina and damage the optic nerve, causing permanent blindness. Even looking directly at the Sun with no optical aid can severely damage or blind you.
Infrared radiation can be felt as heat on the skin and if it is sufficiently powerful, it can cause burns. The use of infrared for communications involves data transfer along optical fibers (e.g. fast broadband, communications between computers) and also in remote control devices for TVs, DVD players and so on. Infrared detectors can be used in security lights, burglar alarms, garden and indoor lighting to switch the lights on and off when someone is in the vicinity.
