Visible and Invisible Radiation:

Light  is very much a part of our existence. Without it we cannot see; It Iends colour to the world around us. Light is also termed as visible radiation. There are other kinds of  radiations in nature, that we cannot see.'~hese  are termed  invisible radiations. Some examples of  invisible radiations are the infrared and ultravioler radiations, radiowaves, X-rays and gamma rays. We may c6me across all these radiations in our lives. For example, infrared  (IR) radiation  is given out by warm objects, such as  our bodies,  room heaters, buildings and the Earth after a warm day. Rattlesnakes dctcct infrared radiation very well.  Ultraviolet  (UV) radiation  can kill germs. It  is invisible'to us but can be detected by bumblebees. Radiowaves are.emitted by TV and radio broadcasting stations and are received by our TV or  radio sets through  the antennas. Thus, they are useful in communication.

They can also be detected by bats. X-rays are used in medicine, gamma rays aie us in cancer treatment and are also emitted  in nuclear explosions. All these radiations-the  gamma ray, the X-rays, ultraviolet rays, light, infrared rays, and radiowaves-are useful in astronomy. Actually  they are different  forms of the sanlekind of  radiation called the electromagnetic radiation. Electromagnetic radiation  is a form of  energy. There are other forms of  energy with which you must be  familiar, like heat, sound or the energy stored in thespring of  a watch. We usually think of electromagnetic radiation as being made up of waves that  travel with  the speed of light in vacuum. Now,  the simplest examples of waves that you may know are waves of water  in a pond or sea, waves on a string. You may have seen waves on a curtain fluttering in  the air. Some people have wavy hair. We will not go here into the details owhat waves are, or the special nature of electromagnetic waves. For details, you may like to refer to  the books listed at the end of the block. But clearly, from their description given above, the various kinds of electromagnetic radiation do not seem to be alike. What  is the difference between each  of  them? 

(hills) or two successive troughs (valleys) is defined as its  wavelength. It is measured  inmetres. The curve marked OABCD is called one cycle. The frequency of a wave is defined as  the number of cycles it travels in a second. It is then measured in  terms of cycles per second (cps) or Hertz. The product of  the wavelength A  and the frequency f of an electromagnetic wave is equal to  its speed c: