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Electromagnetic Waves MCQs for NEET — Physics Questions with Answers

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In a plane E.M. wave, the electric field oscillates sinusoidally at a frequency of 2.5 $\times 10^{10}$ Hz and amplitude 480 V/m. The amplitude of the oscillating magnetic field will be:

$1.52 \times 10^{- 8} W b / m^{2}$

$1.52 \times 10^{- 7} W b / m^{2}$

$1.6 \times 10^{- 6} W b / m^{2}$

$1.6 \times 10^{- 7} W b / m^{2}$

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Explanation

$(3) A m p l i t u d e o f M a g n e t i c f i e l d = \frac{E_{0}}{c} = \frac{480}{3 \times 10^{8}} = 1.6 \times 10^{- 6} W b / m^{2}$

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The energy density of the electromagnetic wave in vacuum is given by the relation

$\frac{1}{2} . \frac{E^{2}}{ε_{0}} + \frac{B^{2}}{2 μ_{0}}$

$\frac{1}{2} ε_{0} E^{2} + \frac{1}{2} μ_{0} B^{2}$

$\frac{E^{2} + B^{2}}{C}$

$\frac{1}{2} ε_{0} E^{2} + \frac{B^{2}}{2 μ_{0}}$

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Explanation

$E l e c t r i c e n e r g y d e n s i t y = \frac{1}{2} ε_{0} E^{2} M a g n e t i c e n e r g y d e n s i t y = \frac{B^{2}}{2 μ_{0}} S o, t o t a l e n e r g y = \frac{1}{2} ε_{0} E^{2} + \frac{B^{2}}{2 μ_{0}}$

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A lamp radiates power $P_{0}$ uniformly in all directions, the amplitude of electric field strength $E_{0}$ at a distance r from it is:

$E_{0} = \frac{P_{0}}{2 {πε}_{0} {cr}^{2}}$

$E_{0} = \sqrt{\{\frac{P_{0}}{2 {πε}_{0} c r^{2}}\}}$

$E_{0} = \sqrt{\{\frac{P_{0}}{4 {πε}_{0} {cr}^{2}}\}}$

$E_{0} = \sqrt{\{\frac{P_{0}}{8 {πε}_{0} {cr}^{}}\}}$

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Explanation

$S_{a v} = \frac{P}{4 {πr}^{2}} = \frac{1}{2} ε_{0} {E_{0}}^{2} c E_{0} = \sqrt{\frac{P}{2 π ε_{0} c r^{2}}}$

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The intensity of visible radiation at a distance of 1 m from a bulb of 100 W which converts only 5% its power into light is :

0.4 $W / m^{2}$

0.5 $W / m^{2}$

0.1 $W / m^{2}$

0.01 $W / m^{2}$

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Explanation

$I n e n s i t y = \frac{P o w e r}{A r e a} = \frac{0.05 \times 100}{4 {πr}^{2}} = \frac{5}{4 \times 3.14} = 0.4 W$ $/ m^{2}$

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On an EM wave, the amplitude of electric and magnetic fields are 100 v/m and 0.265 A/m. The maximum energy flow is

26.5 $W / m^{2}$

46.7 $W / m^{2}$

66.5 $W / m^{2}$

86.5 $W / m^{2}$

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Explanation

$M a x i m u m E n e r g y F l o w, S = E_{0} B_{0} = 100 \times 0.265 = 26.5 W / m^{2}$

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The most penetrating radiation out of the following is:

X-rays

$β$ -rays

$α$ -rays

$γ$ -rays

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Explanation

$γ - r a y s$

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Which of the following statements about electromagnetic waves is/are correct:

(A) X-rays in vacuum travel faster than light waves in vacuum.

(B) The energy of an X-ray photon is greater than that of a light photon.

A and B

B and C

A, B and C

B only

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Explanation

The energy of an X-ray photon is greater than that of a light photon.

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The magnetic field in a plane EM wave traveling in the positive x-direction is given by

$B = (100 μ T) \sin [(2 \times 10^{15} s^{- 1}) (t - x / c)] \hat{j}$

The equation for electric field is :

E=100sin [(2 $\times 10^{15} s^{- 1})$ (t-x/c)] $(- \hat{k})$

E=3 $\times 10^{10}$ sin [(2 $\times 10^{15} s^{- 1})$ (t-x/c)] $(- \hat{k})$

E=3 $\times 10^{10}$ sin [(2 $\times 10^{15} s^{- 1})$ (t-x/c)] $\hat{k}$

E=100sin [(2 $\times 10^{15} s^{- 1})$ (t-x/c)] $\hat{k}$

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Explanation

$E = c B = 3 \times 10^{8} \times (100 μ T) \sin [(2 \times 10^{15} s^{- 1}) (t - x / c)] = 3 \times 10^{10} \times \sin [(2 \times 10^{15} s^{- 1}) (t - x / c)] A s t h e w a v e i s p r o p a g a t i n g a l o n g t h e + x - d i r e c t i o n, \vec{E} \times \vec{B} s h o u l d b e a l o n g + x - d i r e c t i o n . \vec{E} = (3 \times 10^{10} μ V / m) \times \sin [(2 \times 10^{15} s^{- 1}) (t - x / c)] (- \hat{k})$

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Statement I: A changing electric field produces a magnetic field.

Statement II: A changing magnetic field produces an electric field.

If both statement-I and Statement-II are true, and Statement-II is the correct explanation of Statement-I.

If both Statement-I and Statement-II are true but Statement-II is not the correct explanation of Statement-I.

If Statement-I is true but Statement-II is false.

If Statement-I is false but Statement-II is true.

If both the assertion and the reason are true and the reason is a correct explanation of the assertion
If both the assertion and reason are true but the reason is not a correct explanation of the assertion
If the assertion is true but the reason is false
If both the assertion and reason are false

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Explanation

A changing electric field produces a magnetic field. It is accounted for by Ampere-Maxwell 's law.
Here, Assertion and Reason both are correct but the Reason is correct not the explanation of Assertion.

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Statement-I: Gamma rays are more energetic than X-rays.

Statement-II: Gamma rays are of nuclear origin but X-rays are produced due to sudden deceleration of high energy electrons while falling on a metal of high atomic number.

If both statement-I and Statement-II are true, and Statement-II is the correct explanation of Statement-I.

If both Statement-I and Statement-II are true but Statement-II is not the correct explanation of Statement-I.

If Statement-I is true but Statement-II is false.

If Statement-I is false but Statement-II is true.

If both the assertion and the reason are true and the reason is a correct explanation of the assertion
If both the assertion and reason are true but the reason is not a correct explanation of the assertion
If the assertion is true but the reason is false
If both the assertion and reason are false

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Explanation

The Statement-1 and Statement-2 are true but the Statement-2 is not the correct explanation of
Statement-1. In fact, the energy of gamma-ray is more than X-rays because the frequency of gamma-ray is higher than that of X-rays as $E = h ν$ .

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