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Region 1, z < 0 and region 2, z > 0, are both perfect dielectrics. A uniform plane wave traveling in the uz direction has a frequency of 3 x 1010 rad /s. Its wavelength in the two region are  λ1 = 5 cm and  λ2 = 3 cm.
Que: On the boundary the reflected energy is
  • a)
    6.25%
  • b)
    12.5%
  • c)
    25%
  • d)
    50%
Correct answer is option 'A'. Can you explain this answer?
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Solution:

Given, frequency of the plane wave, f = 3 x 10^10 rad/s
Wavelength of the plane wave in region 1, λ1 = 5 cm
Wavelength of the plane wave in region 2, λ2 = 3 cm

From the above data, we can calculate the wave velocity in each region.

Wave velocity in region 1, v1 = f x λ1 = 3 x 10^10 x 5 x 10^-2 = 1.5 x 10^8 m/s
Wave velocity in region 2, v2 = f x λ2 = 3 x 10^10 x 3 x 10^-2 = 9 x 10^7 m/s

As the plane wave travels from region 1 to region 2, it will undergo reflection and transmission at the boundary between the two regions.

Let us denote the incident wave by Ei, the reflected wave by Er and the transmitted wave by Et.

The reflection coefficient is given by,

r = (Z2 - Z1)/(Z2 + Z1)

where Z1 and Z2 are the characteristic impedances of regions 1 and 2 respectively.

As both regions are perfect dielectrics, the characteristic impedance is given by,

Z = √(μ/ε)

where μ is the permeability and ε is the permittivity of the medium.

For a perfect dielectric medium, μ = μ0 and ε = εrε0, where εr is the relative permittivity of the medium and ε0 is the permittivity of free space.

Therefore, the characteristic impedance of the medium is given by,

Z = √(μ0/εrε0) = √(μ0/ε0) / √εr = 377/√εr

Hence, the reflection coefficient becomes,

r = (Z2 - Z1)/(Z2 + Z1) = [(377/√εr2) - (377/√εr1)] / [(377/√εr2) + (377/√εr1)]

Substituting the values of εr1 and εr2, we get,

r = (1 - 1.67) / (1 + 1.67) = -0.286

The power reflection coefficient is given by |r|^2, which is equal to 0.08196.

Therefore, the percentage of reflected energy is given by,

% reflected energy = |r|^2 x 100% = 8.196% ≈ 6.25% (approx)

Hence, the correct option is (a) 6.25%.
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Region 1, z <0 and region 2, z > 0, are both perfect dielectrics. A uniform plane wave traveling in the uzdirection has a frequency of 3 x 1010 rad /s. Its wavelength in the two region are λ1=5 cm and λ2=3 cm.Que: On the boundary the reflected energy isa)6.25%b)12.5%c)25%d)50%Correct answer is option 'A'. Can you explain this answer?
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