Optics Ques 187
- A container is filled with water $(\mu=1.33)$ up to a height of $33.25 $ $cm$. A concave mirror is placed $15 $ $cm$ above the water level and the image of an object placed at the bottom is formed $25 $ $cm$ below the water level. The focal length of the mirror is
(2005, 2M)
(a) $10 $ $cm$
(b) $15 $ $cm$
(c) $20 $ $cm$
(d) $25 $ $cm$
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Answer:
Correct Answer: 187.(c)
Solution:
Formula:
Refraction at Spherical Thin Lens:
- Distance of object from mirror
$ 15+\frac{33.25}{1.33}=40\ \text{cm} $
Distance of image from mirror $=15+\frac{25}{1.33}$
$ =33.8 $ $cm $
For the mirror, $\quad \frac{1}{v}+\frac{1}{u}=\frac{1}{f}$
$ \begin{aligned} & \therefore & \frac{1}{-33.8}+\frac{1}{-40} & =\frac{1}{f} \\ & \therefore & f =-18.3 cm \end{aligned} $
$\therefore \quad $ Most suitable answer is (c).
BETA
