Electromagnetic Waves Questions — JEE Physics

Q1

For an electromagnetic wave travelling in free space, the relation between average energy densities due to electric (Ue) and magnetic (Um) fields is :

A Ue = Um

B Ue

\ne

Um

C Ue < Um

D Ue > Um

Correct Answer

Option A

Solution

In EMW, average energy density due to electric field (Ue) and magnetic field (Um) is same.

Q2

Electromagnetic waves are transverse in nature is evident by

A polarization

B interference

C reflection

D diffraction

Correct Answer

Option A

Solution

The phenomenon of polarisation is shown only by transverse waves.

Q3

Light is incident normally on a completely absorbing surface with an energy flux of 25 W cm–2. If the surface has an area of 25 cm2, the momentum transferred to the surface in 40 min time duration will be :

A 6.3 × 10–4 Ns

B 5.0 × 10–3 Ns

C 1.4 × 10–6 Ns

D 3.5 × 10–6 Ns

Correct Answer

Option B

Solution

P = {{\Delta E} \over C}

= {{\left( {25 \times 25} \right) \times 40 \times 60} \over {3 \times {{10}^8}}}N - s

= 5 \times {10^{ - 2}}N - s

Q4

Choose the correct option relating wave lengths of different parts of electromagnetic wave spectrum:

A

\lambda

radio waves >

\lambda

micro waves >

\lambda

visible >

\lambda

x-rays

B

\lambda

visible >

\lambda

x-rays >

\lambda

radio waves >

\lambda

micro waves

C

\lambda

visible <

\lambda

micro waves <

\lambda

radio waves <

\lambda

x-rays

D

\lambda

x-rays <

\lambda

micro waves <

\lambda

radio waves <

\lambda

visible

Correct Answer

Option A

Solution

Decreasing order of wavelength, Radio wave > Microwave > Infrared > ROYGBIV(Visiable Resion) > Ultraviolet > X rays > $\gamma$ rays So the correct order - $\lambda$ radio waves > $\lambda$ micro waves > $\lambda$ visible > $\lambda$ x-rays

Q5

A plane electromagnetic wave propagating along y-direction can have the following pair of electric field

\left( {\overrightarrow E } \right)

and magnetic field

\left( {\overrightarrow B } \right)

components.

A Ex, Bz or Ez, Bx

B Ex, By or Ey, Bx

C Ey, By or Ez, Bz

D Ey, Bx or Ex, By

Correct Answer

Option A

Solution

$\because$

\widehat E \times \widehat B = \widehat C

$\therefore$

\widehat E \times \widehat B

should point in the direction of propagation of wave (y-direction here) $\therefore$ possible combinations are (Ex, Bz) or (Ez, Bx)

Q6

In an electromagnetic wave the electric field vector and magnetic field vector are given as

\overrightarrow E = {E_0}\widehat i

and

\overrightarrow B = {B_0}\widehat k

respectively. The direction of propagation of electromagnetic wave is along :

A

\left( {\widehat k} \right)

B

\widehat j

C

\left( { - \widehat k} \right)

D

\left( { - \widehat j} \right)

Correct Answer

Option D

Solution

Direction of propagation =

\overrightarrow E \times \overrightarrow B = \widehat i \times \widehat k = - \widehat j

Q7

Sun light falls normally on a surface of area

36 \mathrm{~cm}^{2}

and exerts an average force of

7.2 \times 10^{-9} \mathrm{~N}

within a time period of 20 minutes. Considering a case of complete absorption, the energy flux of incident light is

A

25.92 \times 10^{2} \mathrm{~W} / \mathrm{cm}^{2}

B

8.64 \times 10^{-6} \mathrm{~W} / \mathrm{cm}^{2}

C

6.0 \mathrm{~W} / \mathrm{cm}^{2}

D

0.06\mathrm{~W} / \mathrm{cm}^{2}

Correct Answer

Option D

Solution

Pressure

= {l \over c}

\Rightarrow {F \over A} = {l \over c}

\Rightarrow l = {{7.2 \times {{10}^{ - 9}} \times 3 \times {{10}^8}} \over {36 \times {{10}^{ - 4}}}}

W/m2

= 600

W/m2

\Rightarrow l = 0.06

W/cm2

Q8

An electromagnetic wave is transporting energy in the negative

z

direction. At a certain point and certain time the direction of electric field of the wave is along positive

y

direction. What will be the direction of the magnetic field of the wave at that point and instant?

A Negative direction of

y

B Positive direction of

z

C Positive direction of

x

D Negative direction

x

Correct Answer

Option C

Solution

As, poynting vector

\overrightarrow{\mathrm{S}}=\overrightarrow{\mathrm{E}} \times \overrightarrow{\mathrm{H}}

Given energy transport $=$ negative $\mathrm{z}$ direction Electric field $=$ positive $\mathrm{y}$ direction $(-\hat{\mathrm{k}})=(+\hat{\mathrm{j}}) \times[\hat{\mathrm{i}}]$ Hence according to vector cross product magnetic field should be positive $\mathrm{x}$ direction.

Q9

A plane electromagnetic wave propagating in

\mathrm{x}

-direction is described by

E_y=\left(200 \mathrm{Vm}^{-1}\right) \sin \left[1.5 \times 10^7 t-0.05 x\right] \text{; }

The intensity of the wave is : (Use

\epsilon_0=8.85 \times 10^{-12} \mathrm{C}^2 \mathrm{~N}^{-1} \mathrm{~m}^{-2}

)

A

35.4 \mathrm{~Wm}^{-2}

B

53.1 \mathrm{~Wm}^{-2}

C

26.6 \mathrm{~Wm}^{-2}

D

106.2 \mathrm{~Wm}^{-2}

Correct Answer

Option B

Solution

\begin{aligned} & \mathrm{I}=\frac{1}{2} \varepsilon_0 \mathrm{E}_0^2 \times \mathrm{c} \\ & \mathrm{I}=\frac{1}{2} \times 8.85 \times 10^{-12} \times 4 \times 10^4 \times 3 \times 10^8 \\ & \mathrm{I}=53.1 \mathrm{~W} / \mathrm{m}^2 \end{aligned}

Q10

Electromagnetic waves travel in a medium with speed of

1.5 \times 10^8 \mathrm{~m} \mathrm{~s}^{-1}

. The relative permeability of the medium is 2.0. The relative permittivity will be:

A 4

B 1

C 2

D 5

Correct Answer

Option C

Solution

\begin{aligned} & v=\frac{1}{\sqrt{\mu_0 \mu_r \varepsilon_0 \cdot \varepsilon_r}} \\ & \Rightarrow 1.5 \times 10^8=\frac{3 \times 10^8}{\sqrt{2 \cdot \varepsilon_r}} \Rightarrow \varepsilon_r=2 \end{aligned}