Test: Electromagnetic Waves - 2 - CUET Humanities MCQ

Microwaves are used in telecommunication.
So, option 4 is correct.
Radio waves - These are used for television and radio programmes.
Visible radiation - These are used in photography.
Ultraviolet rays - These can be used to kill microbes. Hospitals use UV lamps to sterilise surgical equipment and the air in operating theatres.

We know that thermal radiations consist of larger wavelength as compared to gamma rays and wavelength in visible regions. So, thermal radiations belong to infra-red region.

The refractive index (n) of a medium is defined as
n = c/v
where c is the speed of the electromagnetic wave in free space and v that in the medium. Thus, v = c/n. Since the value of n is different for different media, the value of v will also be different for different media. Hence choice (3) is false.

E is the electric field vector and B is the magnetic field vector of the EM wave. For electromagnetic waves, E and B are always perpendicular to each other and perpendicular to the direction of propagation. The direction of propagation is the direction of E x B.

Accelerating charges produce changing electric and magnetic fields. Changing electric fields produce magnetic fields and changing magnetic fields produce electric fields. This interplay between induced electric and magnetic fields leads to propagating electromagnetic waves.

As the poynting vector  gives the direction of energy flow, i.e. direction of propagation , direction of  is along z-axis. From the equation, we can conclude that the direction of the electric field is along x-axis and direction of the magnetic field is along y-axis.

Radio waves are transverse waves, whereas the sound waves are longitudinal waves. Transverse waves can be polarised, whereas longitudinal waves cannot be polarised but both of them can be reflected, refracted and diffracted.

Here, E = 6.3 Vm^-1, C = 3 × 10⁸ ms^-1
Using B = , we get
B =  = 2.1 × 10^-8 T
Since wave is travelling along x-direction and  is along y-direction, therefore  must be along z-direction.
  = 

In the presence of the atmosphere, heat radiations reflected by the earth are further reflected back to the earth by the atmosphere, which increases the temperature. In the absence of the atmosphere, no such reflection back to earth will take place. Hence the heat radiations will be reflected back and temperature of the earth will decrease.

Infrared radiations emitted by the sun are responsible for heating Earth's atmosphere due to greenhouse effect. The correct choice is (2).