(O Level Phy) Uses of EM waves

EM waves are waves that are made up of an electric field component and a magnetic field component oscillating at 90º to each other and propagating at 3 x 10⁸ m/s in vacuo.

EM waves have various uses depending on their properties.

Part of EM waves	Uses	Danger
Radio (longest wavelength, lowest frequency, lowest energy)	Used for FM and AM radio broadcasts Used for communication by sending information to and from satellites in space Reason: Radio waves are able to pass through the atmosphere. Also, having very long wavelengths, they are able to diffract or bend around buildings and hills so that places in the shadow of large buildings or hills can receive the radio waves. Note: The radio wave and MW frequencies used in commercial, scientific and military satellites are different so that they do not interfere with one another.
Microwave	Used for cooking Reason: The MW has a certain frequency that can be absorbed by water molecules. The water molecules then vibrate faster. This translates to heat and cooks the cells in food. Used for communication by sending information to and from satellites in space Reason: MW, especially the low frequency MW that is near the radio waves region, can travel through the Earth's atmosphere and through clouds.	May kill healthy cells Reason: Water in cells can be heated up by MW.
Infrared	Used in cameras to detect hot objects in the dark Reason: The surface of hot objects emit IR radiation faster than the surface of cold objects. Used in remote controls for TV and videos Used in optical light fibre	Can cause sunburn Reason: IR radiations are heat waves that can damage cells.
Visible from 400 nm (red) to 700 nm (violet)	Used in optical light fibre e.g. In endoscopes for seeing into human body How it works: Light travels through the glass fibre without emerging as they undergo total internal reflection on the glass-air boundary.	Light of high intensity such as lasers can damage the retina of the eye.
Ultraviolet	Used in sun beds to give people sun tans Reason: The skin absorbs UV rays causing melanin to undergo a chemical reaction. The skin then looks tanned.	Can cause skin cancer Reason: UV rays are ionizing.
X-way	To produce X-ray images of bones Reason: X-rays can pass through muscle tissue but cannot pass through bones.	Can cause cancer Reason: X-rays are ionizing.
Gamma (shortest wavelength, highest frequency, highest energy)	Used to kill cancer cells in chemotherapy Used to kill bacteria in foods and sterilize surgical equipment Reason: Gamma rays have high frequency and hence high energy. They are ionizing radiation, capable of knocking electron off from atoms and breaking chemical bonds in molecules such as DNA.	Can damage healthy cells Reason: Gamma rays are highly ionizing

Attempt these questions before you refer to the answers that follow.

1. What properties of EM radiations show that they are waves?
   
2. State two differences between EM waves and sound.
   
3. Ultraviolet, X-ray, infrared radiation, microwave radiation
   

(a) Arrange the EM radiations in order of increasing frequency.
(b) (i) What are ionizing radiations?
(ii) Which of the above four are ionizing radiations?
(iii) State one use for each of the four radiations.


Answers:

1. EM waves can undergo reflection, refraction and diffraction which are general properties of waves.
   
2. EM waves travel at 3 x 10⁸ m/s in air and sound travels at 330 m/s in air. / EM waves can travel through vacuo but sound needs a material medium to travel. / EM waves are transverse waves but sound waves are longitudinal waves. (any two)
   
3. (a) microwave (lowest frequency), infrared, ultraviolet, X-ray (highest frequency) (b) (i) Ionizing radiation is a radiation that is able to “knock off” electrons from atoms. (iii) Microwave – transmission of signals via satellites, infrared – in remote controls, ultraviolet – in sun bed, X-ray – to take X-ray images of bones in medicine