Ray Optics and Optical Instruments - Result Question 21
21. One face of a rectangular glass plate $6 cm$ thick is silvered. An object held $8 cm$ in front of the first face forms an image $12 cm$ behind the silvered face. The refractive index of the glass is
[1996]
(a) 0.4
(b) 0.8
(c) 1.2
(d) 1.6
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Answer:
Correct Answer: 21. (c)
Solution:
- (c) Thickness of glass plate $(t)=6 cm$;
Distance of the object $(u)=8 cm$.
And distance of the image $(v)=12 cm$.
Let $x=$ Apparent position of the silvered surface in $cm$.
Since the image is formed due to reflection at the silvered face and by the property of mirror image Distance of object from the mirror $=$ Distance of image from themirror or, $x+8=12+6-x \Rightarrow x=5 cm$.
Therefore, refractive index of glass
$ =\frac{\text{ Real depth }}{\text{ Apparent depth }}=\frac{6}{5}=1.2 \text{. } $
