Electromagnetic Waves JEE Main Questions 2025: Important Practice Questions with PYQs

Questions

Solution

Q: A plane electromagnetic wave travels in free space along the x-direction. The electric field component of the wave at a particular point of space and time is E = 6 V m–1 along the y-direction. Its corresponding magnetic field component, B would be

1. 6 × 10–8 T along the x-direction
2. 2 × 10–8 T along the y-direction
3. 2 × 10–8 T along the z-direction
4. 6 × 10–8 T along the z-direction

The direction of electromagnetic wave travelling is given by

K- = E- X B-

As the wave travels along the x-direction and E along the y-direction,

So, B must point towards the z-direction.

The magnetic field, B = E/c = 6/(3 x 108) = 2 x 10-8 T

Q: A red LED emits light at 0.1 watts uniformly around it. The amplitude of the electric field of the light at a distance of 1 m from the diode is

1. 5.48 V/m
2. 7.75 V/m
3. 1.73 V/m
4. 2.45 V/m

The intensity of light, I = uav c

Also, I = P/4pi r2 and uav = 1/2 e0E02

∴ P/4pi r2 = (½)ε0E02c

Here, P = 0.1 W, r = 1 m, c = 3 × 108 m s–1

1/4pi E0 = 9 x 109 N C–2 m2

E0=2 X 0.1 X 9 X 10912 X 3 X 108=6

= 2.45 V m-1

Q: The mean intensity of radiation on the surface of the Sun is about 108 W/m2. The RMS value of the corresponding magnetic field is closest to

1. 10-2 T
2. 1 T
3. 10-4 T
4. 102 T

Mean intensity I= (B2rms/u0)c

B2rms = (108 x 4pix 10-7)/(3 x 108)

Brms ≈ 10-4 T

Q:  50 W/m2 energy density of sunlight is normally incident on the surface of a solar panel. Some part of incident energy (25%) is reflected from the surface and the rest is absorbed. The force exerted on 1m2 surface area will be close to (c = 3 × 108 m/s)

1. 20 × 10–8 N
2. 10 × 10–8 N
3. 35 × 10–8 N
4. 15 × 10–8 N

Given energy density = 50 W/m2

Here, change in momentum delta p = pf - pi

delta p = (-pi /4) – pi

delta p = -5pi /4, since pi = E/c = 50 W/s/(3 x 108)

delta p/delta t = F = |-5pi/4丨= (5/4) x (50/(3 x 108) = 20.8 x 10-8 N ≈ 20x 10-8 N

Q: Match List-I (Electromagnetic wave type) with List-II (Its association/application) and select the correct option from the choices given below the lists

P  Q   R    S

(a) (i) (ii) (iii) (iv)

(b) (iv) (iii) (ii) (i)

(c) (i) (ii) (iv) (iii)

(d) (iii) (ii) (i) (iv)

Infrared waves are used to treat muscular strain. Radio waves are used for broadcasting. X-rays are used to detect the fracture of bones. Ultraviolet rays are absorbed by the ozone layer of the atmosphere.

Answer: (a)

P   Q     R      S

(i) (ii) (iii) (iv)

Q: The magnetic field in a traveling electromagnetic wave has a peak value of 20 nT. The peak value of electric field strength is:

1. 12 V/m
2. 3 V/m
3. 6 V/m
4. 9 V/m

In an electromagnetic wave, the peak value of the electric field (E0) and peak value of magnetic field (B0) are related by

E0 = B0C

E0 = (20 × 10-9 T) (3 × 108 m s-1) = 6 V/m

Answer: (c) 6 V/m

Q: During the propagation of electromagnetic waves in a medium

1. both electric and magnetic energy densities are zero
2. electric energy density is double of the magnetic energy density
3. electric energy density is half of the magnetic energy density
4. electric energy density is equal to the magnetic energy density

In an electromagnetic wave, energy is equally divided between the electric and the magnetic fields.

Answer: (d) The electric energy density is equal to the magnetic energy density

Q: The energy associated with electric field is (UE) and with magnetic field is (UB) for an electromagnetic wave in free space. Then

1. UE > UB
2. UE = UB/2
3. UE = UB
4. UE < UB

Answer: (c) UE = UB

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

(a) wavelength is doubled and the frequency remains unchanged

(b) wavelength is doubled and frequency becomes half

(c) wavelength is halved and frequency remains unchanged

(d) wavelength and frequency both remain unchanged

Determination of frequency of the EM wave when passes from vacuum to dielectric medium:

1. The frequency of the electromagnetic wave does not change with the change in the medium.
2. ​The wavelength and speed of the wave change with the change in the medium.

Thus, the frequency of the EM wave when it passes from a vacuum to a dielectric medium will not change.