Question

In an experiment, students need to get an upright image of an object using a lens of focal length 25 cm. The experiment was set up by putting the lens and the object at a distance of 10 cm. Students of group A used a convex lens and group B used a concave lens for their experiment. The ratio of magnification obtained by group A and group B is:

A. 3/7
B. 7/3
C. 3
D. 1​

Answer 1

Answer:

A is the right answer

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Explanation:

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Answer 2

Given: In an experiment, students need to get an upright image of an object using a lens of focal length 25 cm. The experiment was set up by putting the lens and the object at a distance of 10 cm. Students of group A used a convex lens and group B used a concave lens for their experiment.

To find: The ratio of magnification obtained by group A and group B

Explanation: Let the focal length be f, image distance be v and object distance be u.

For group A, the lens used was a convex lens which has a positive focal length.

f = + 25 cm

u= -10 cm

Using lens formula:

$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$

$\frac{1}{v } = \frac{1}{f} + \frac{1}{u}$

$\frac{1}{v} = \frac{1}{25} + \frac{1}{ - 10}$

$\frac{1}{v} = \frac{2 - 5}{50}$

$\frac{1}{v} = \frac{ - 3}{50}$

$v = \frac{ - 50}{3}$

Let magnification produced by group A be m1. The formula for magnification is:

$m 1= \frac{v}{u}$

$m 1= \frac{ \frac{ - 50}{3} }{ - 10}$

$m1 = \frac{5}{3}$

For group B, the lens used was a concave lens which has a negative focal length.

f = - 25 cm

u= -10 cm

Using lens formula:

$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$

$\frac{1}{v } = \frac{1}{f} + \frac{1}{u}$

$\frac{1}{v} = \frac{1}{ - 25} + \frac{1}{ - 10}$

$\frac{1}{v} = \frac{ - 2 - 5}{50}$

$\frac{1}{v} = \frac{ - 7}{50}$

$v = \frac{ - 50}{7}$

Let magnification produced by group B be m2. The formula for magnification is:

$m 2= \frac{v}{u}$

$m 2= \frac{ \frac{ - 50}{7} }{ - 10}$

$m2 = \frac{5}{7}$

Ratio of magnification of group A and B:

=$\frac{m1}{m2}$

=$\frac{ \frac{5}{3} }{ \frac{5}{7} }$

=$\frac{7}{3}$

Therefore, ratio of magnification is option (b) 7/3.