# Electromagnetic Waves

### Question:

An EM wave of intensity I falls on a surface kept in vacuum and exerts radiation pressure p on it. Which of the following are true?
(a) Radiation pressure is I/c if the wave is totally absorbed
(b) Radiation pressure is â€”I/c if the wave is totally reflected
(c) Radiation pressure is 2I/c if the wave is totally reflected
(d) Radiation pressure is in the range I/c < p < 2I/c for real surfaces

(a, c, d)
Key concept: Radiation pressure (p) is the force exerted by electromagnetic wave on unit area of the surface, i.e., rate of change of momentum per unit area of the surface.
Let us consider a surface exposed to electromagnetic radiation as shown in figure. The radiation is falling normally on the surface. Further, intensity of radiation is I and area of surface exposed to radiation is A.

### Electromagnetic Waves

#### Q 1.

A variable frequency AC source is connected to a capacitor. How will the displacement current change with decrease in frequency?

#### Q 2.

A charged particle oscillates about its mean equilibrium position with a frequency of 109 Hz. The electromagnetic waves produced
(a) will have frequency of 109 Hz
(b) will have frequency of 2 x 109 Hz
(c) will have wavelength of 0.3 m
(d) fall in the region of radio waves

#### Q 3.

What happens to the intensity of light from a bulb if the distance from the bulb is doubled? As a laser beam travels across the length of room, its intensity essentially remains constant.
What geometrical characteristic of LASER beam is responsible for the constant intensity which is missing in the case of light from the bulb?

#### Q 4.

An EM wave radiates outwards from a dipole antenna, with E0 as the amplitude of its electric field vector. The electric field E0 which transports significant energy from the source falls off as

#### Q 5.

Why does microwave oven heats up a food item containing water molecules most efficiently?

#### Q 6.

Even though an electric field E exerts a force qE on a charged particle yet electric field of an EM wave does not contribute to the radiation pressure (but transfers energy). Explain.

#### Q 8.

Professor CV Raman surprised his students by suspending freely a tiny light ball in a transparent vacuum chamber by shining a laser beam on it. Which property of EM waves was he exhibiting? Give one more example of this property.

#### Q 9.

A plane electromagnetic wave propagating along x-direction can have the following pairs of E and B.
(a) Ex, By (b) Ey, Bz
(c) Bx,Ey (d) Ez,By

#### Q 10.

Why is the orientation of the portable radio with respect to broadcasting station important?

#### Q 11.

The charge on a parallel plate capacitor varies as q=q0   cos 2Ï€vt. The plates are very large and close together (area = A, separation = d). Neglecting the edge effects, find the displacement current through the capacitor.

#### Q 12.

The source of electromagnetic waves can be a charge
(a) moving with a constant velocity
(b) moving in a circular orbit
(c) at rest
(d) falling in an electric field

#### Q 13.

The magnetic field of a beam emerging from a fitter facing a flood light is given by  B0= 12 x 10-8 sin (1.20 x 107z- 3.60 x 1015 t) T What is the average intensity of the beam?

#### Q 15.

The electric field intensity produced by the radiations coming from a 100 W bulb at a 3 m distance is E. The electric field intensity produced by the radiations coming from 50 W bulb at the same distance is

#### Q 16.

You are given a 2 Î¼F parallel plate capacitor. How would you establish an instantaneous displacement current of 1 mA in the space between its plates?

#### Q 17.

Show that the radiation pressure exerted by an EM wave of intensity I on a surface kept in vacuum is I/C.

#### Q 20.

The ratio of contributions made by the electric field and magnetic field components to the intensity of an EM wave is

#### Q 21.

An EM wave of intensity I falls on a surface kept in vacuum and exerts radiation pressure p on it. Which of the following are true?
(a) Radiation pressure is I/c if the wave is totally absorbed
(b) Radiation pressure is â€”I/c if the wave is totally reflected
(c) Radiation pressure is 2I/c if the wave is totally reflected
(d) Radiation pressure is in the range I/c < p < 2I/c for real surfaces

#### Q 22.

Electromagnetic waves with wavelength
(i) Î»1, is used in satellite communication.
(ii) Î»2, is used to kill germs in water purifier.
(iii) Î»3, is used to detect leakage of oil in underground pipelines.
(iv) Î»4, is used to improve visibility in runways during fog and mist conditions.
(a) Identify and name the part of electromagnetic spectrum to which these radiations belong.
(b) Arrange these wavelengths in ascending order of their magnitude.
(c) Write one more application of each.