Electromagnetic Waves · Question 27

Light with an energy flux of 25 104 W m-2 falls on a perfectly reflecting surface at normal incidence. If the surface area is 15 cm2, the average force exerted on the surface is

1. 1.25 10-6 N
2. 2.50 10-6 N
3. 1.20 10-6 N
4. 3.0 10-6 N

The condition under which a microwave oven heats up a food item containing water molecules most efficiently is

1. Microwaves are heat waves, so always produce heating.
2. Infra-red waves produce heating in a microwave oven.
3. The frequency of the microwaves must match the resonant frequency of the water molecules.
4. The frequency of the microwaves has no relation with natural frequency of water molecules.

An electromagnetic wave of frequency = 3.0 MHz passes from vacuum into a dielectric medium with relative permittivity r = 4.0. Then

1. Wavelength is doubled and frequency becomes half.
2. Wavelength is halved and frequency remains unchanged.
3. Wavelength and frequency both remain unchanged.
4. Wavelength is doubled and frequency unchanged.

The ratio of amplitude of magnetic field to the amplitude of electric field for an electromagnetic wave propagating in vacuum is equal to

1. the speed of light in vacuum
2. reciprocal of speed of light in vacuum
3. the ratio of magnetic permeability to the electric susceptibility of vacuum
4. unity

The electric field associated with an em wave in vacuum is given by E = i 40 (kz - 6 108 t) where E, z and t are in volt/m, meter and seconds respectively. The value of wave vector k is

1. 2 m-1
2. 0.5 m-1
3. 6 m-1
4. 3 m-1

The electric and the magnetic field, associated with an e.m. wave, propagating along the +z-axis, can be represented by

1. E = E0 i, B = B0 j
2. E = E0 k, B = B0 i
3. E = E0 j, B = B0 i
4. E = E0 j, B = B0 k

The decreasing order of wavelength of infrared, microwave, ultraviolet and gamma rays is

1. microwave, infrared, ultraviolet, gamma rays
2. gamma rays, ultraviolet, infrared, microwaves
3. microwaves, gamma rays, infrared, ultraviolet
4. infrared, microwave, ultraviolet, gamma rays

The electric field of an electromagnetic wave in free space is given by E = 10 (107t + kx ) j V/m, where t and x are in seconds and metres respectively. It can be inferred that Which one of the following pairs of statements is correct ?

1. C and D only
2. A and B only
3. B and C only
4. A and C only

Which of the following statements is false for the properties of electromagnetic waves?

1. Both electric and magnetic field vectors attain the maxima and minima at the same place and same time.
2. The energy in electromagnetic wave is divided equally between electric and magnetic vectors.
3. Both electric and magnetic field vectors are parallel to each other and perpendicular to the direction of propagation of wave.
4. These waves do not require any material medium for propagation.

The electric field part of an electromagnetic wave in a medium is represented by Ex = 0; Ey = 2.5 NC [ (2 106 rads )t - ( 10-2 radm )x ]; Ez = 0. The wave is

1. moving along x direction with frequency 106 Hz and wavelength 100 m.
2. moving along x direction with frequency 106 Hz and wavelength 200 m.
3. moving along -x direction with frequency 106 Hz and wavelength 200 m.
4. moving along y direction with frequency 2 106 Hz and wavelength 200 m.