ELECTROMAGNETIC WAVES

6 )

According to Maxwell, a changing electric field produces

emf

Electric current

magnetic field

radiation pressure

View Solution

Answer :

Changing electric field produces magnetic field.

7 )

An electromagnetic wave going through vacuum is described by E = E_o sin(kx – ωt). Which of the following is independent of the wavelength?

(k / ω)

kω

View Solution

Answer :

E = E_o sin(kx – ωt)

Here k = (ω/c) hence (k/ω) = (ω/c) × (1/ω) = (1/c)

hence (k/ω) is independent of wavelength.

8 )

Which of the following have zero average value in a plane electromagnetic wave?

Electric energy

Magnetic energy

Electric field

None of these

View Solution

Answer :

Electric field has zero average value in electromagnetic waves.

9 )

If the relative permeability and dielectric constant of a given medium are equal to μ_r and K respectively, then the refractive index of the medium is equal to __________

Select correct option from the following options.

(A) √(μ_rK) (B) √(μ_rE_o) (C) √(μ_oE_o) (D) [1 / √(μ_rK)]

View Solution

Answer :

Refractive index = n = (c/v) = √(μ_rE_r) = √(μ_rK)

10 )

Astronomers have found that electromagnetic waves of wavelength 21 cm are continuously reaching the Earth’s surface . Calculate the frequency of this radiation.(c = 3 × 10⁸m/s)

14.28 GHz

1.428 kHz

1.428 MHz

1.428 GHz

View Solution

Answer :

λ = 21 cm = 21 × 10^–2m

c = 3 × 10^–8m/s

velocity = wavelength × frequency

c = λf

∴ f = (c/λ)

= [(3 × 10⁸) / (21 × 10^–2)]

= (1/7) × 10¹⁰Hz

∴ f = 0.142 × 10¹⁰Hz

= 1.42 × 10⁹Hz

= 1.42 GHz

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