An object is placed first at infinity and then at 20 ~cm from the… - Vidyadip

MCQ

An object is placed first at infinity and then at $20 \mathrm{~cm}$ from the object side focal plane of the convex lens. The two images thus formed are $5 \mathrm{~cm}$ apart. The focal length of the lens is

A
$5 \mathrm{~cm}$
✓
$10 \mathrm{~cm}$
C
$15 \mathrm{~cm}$
D
$20 \mathrm{~cm}$

✓

Answer

Correct option: B.

$10 \mathrm{~cm}$

For first case : $\frac{1}{f}=\frac{1}{v}-\frac{1}{\infty} \Rightarrow f=v$
For second case $\frac{1}{f}=\frac{1}{(f+5)}-\frac{1}{-(f+20)} \Rightarrow f=10 \mathrm{~cm}$
Alternative sol. $-f^2=x_1 x_2 \Rightarrow f=10 \mathrm{~cm}$.

Need a full question paper?

Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.

Start Generating Free

Explore more

More "MCQ" from Ray Optics→Ray Optics — all question types→JEE physics STD 12 Science question papers→CBSE Board STD 12 Science question papers→CBSE Board question paper generator→

Similar questions

Dimensional formula $M L^2 T^{-3}$ represents

For one complete cycle of a thermodynamic process on a gas as shown in the $P-V$ diagram, Which of following is correct

The figure shows variation of $R, X$ and $X$ with frequency $f$ in a series $L, C, R$ circuit. Then for what frequency point, the circuit is inductive

The tail of the comet is due to

An ammeter reads upto $1\, ampere$. Its internal resistance is $0.81\, ohm$. To increase the range to $10\, A$ the value of the required shunt is ............ $\Omega $

What will be the ratio of de-Broglie wavelengths of proton and $\alpha$-particle of same energy

The eye can detect $5 \times 10^{4}$ photons per square metre per sec of green light $(\lambda=5000 \mathring A)$ while the ear can detect $10^{-13}\left(\mathrm{~W} / \mathrm{m}^2\right)$. The factor by which the eye is more sensitive as a power detector than the ear is close to

An express train is moving with a velocity $v$. Its driver finds another train is moving on the same track in the same direction with velocity $v$. To escape collision, driver applies a retardation $a$ on the train. the minimum time of escaping collision will be

Radius of curvature of concave mirror is $40 \mathrm{~cm}$ and the size of image is twice as that of object, then the object distance is

A photon of $1.7 \times 10^{-13}$ Joules is absorbed by a material under special circumstances. The correct statement is