Prove that the average energy density of the oscillating electric field is equal to that of the oscillating magnetic field.

The energy density of the electric field is given by, U E = 1 2 ε 0 E 2 — equation i where, ε 0 is the permittivity of free space, E is the electric field. The energy density of the magnetic field is given by, U B = 1 2 B 2 μ 0 — equation ii where μ 0 is permeability of free space and B is the magnetic field. Now, we know that E = cB , where c is the speed of the light and c = 1 μ 0 ε 0 , therefore E = 1 μ 0 ε 0 B Substituting this value in equation i , we get U E = 1 2 ε 0 1 μ 0 ε 0 2 = 1 2 B 2 μ 0 from equation ii , U E = U B So the average energy density of the oscillating electric field is equal to that of the oscillating magnetic field.

Why are infra-red waves often called heat waves ? Explain.

The frequencies of the infrared waves are lower than those of visible light and these waves have the ability to vibrate not only the electrons but the entire atoms or molecules of a body. Due to these vibrations, the internal energy and hence, the temperature of the body increases. Since these waves cause a heating effect on the object that they fall on, the infrared waves are often called heat waves.

Name the electromagnetic radiations used for eye surgery.

The ultraviolet radiation, that according to electromagnetic spectrum lies between X-rays and visible light in the increasing order of wavelength is used for water purification and for Laser eye treatment (LASIK) or eye surgery.

Which mode of propagation is used by short wave broadcast services ?

The broadcast services use the mode of Skywave propagation. These are short waves having frequencies within the range of a few MHz up to 30 MHz. Skywaves can propagate through the atmosphere and are reflected back by the ionosphere of the earth's atmosphere. These can cover very long distances and can even travel around the earth. It is a type of radio wave propagation.

Name the electromagnetic radiations used for (a) water purification, and (b) eye surgery.

The ultraviolet radiation, that according to electromagnetic spectrum lies between X-rays and visible light in the increasing order of wavelength is used for water purification and for eye surgery.