CBSE Class 12 Physics 2026 All Sets Paper

Question : 11 of 16

Marks: +1, -0

The magnetic field in a plane electromagnetic wave travelling in glass (n = 1.5) is given by

B_y = (2 \times 10^{-7} \sin(\alpha x+1.5 \times 10^{11} t)

where x is in metres and t is in seconds. The value of α is:

Solution:

From the given expression:

\omega = 1.5 \times 10^{11}

rad/s and

k = \alpha

. The speed of light in glass with refractive index n = 1.5 is:

v = \;\frac{c}{n} = \;\frac{3 \times 10^8}{1.5} = 2 \times 10^8\,\text{m/s}

Using the relation

v = \;\frac{\omega}{k}:

2 \times 10^8 = \;\frac{1.5 \times 10^{11}}{\alpha}

\alpha = \;\frac{1.5 \times 10^{11}}{2 \times 10^8} = 0.75 \times 10^3 = 7.5 \times 10^2\,\text{m}^{-1}

The value of α is

7.5 \times 10^2\,\text{m}^{-1}

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