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  4. A horizontal ray is incident on one of the refracting faces of a prism of angle 8°. The ray strikes a vertical plane mirror after refraction as shown in figure. Find the angle (in degree) by which the mirror should be rotated in order to make the reflected ray antiparallel to incident ray. (Take R.I. of material of prism =5 / 4 )

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Q. A horizontal ray is incident on one of the refracting faces of a prism of angle $8^{\circ}$. The ray strikes a vertical plane mirror after refraction as shown in figure. Find the angle (in degree) by which the mirror should be rotated in order to make the reflected ray antiparallel to incident ray. (Take R.I. of material of prism $=5 / 4$ )Physics Question Image

Ray Optics and Optical Instruments

Solution:

For thin prism,
$\delta=(\mu-1) A =\left(\frac{5}{4}-1\right) \times 8=\frac{8}{4}=2^{\circ}$
image
By geometry of the figure,
angle through which the mirror should be rotated
in order to make the ray horizontal is $\theta=1^{\circ}$