Question

A convex lens of focal length 20cm and a concave lens of focal length 10cm are placed in contact in such a way that they have the common principal axis. What will be the nature of the lens so formed and the focal length of this combination? *​

Answer 1

Answer:

yes ryt answer

I,m not given the answer of u

Answer 2

Given:

A convex lens of focal length 20cm

A concave lens of focal length 10cm

To find:

Focal length of the combination

Solution:

When any of the two lenses are in the combination then the focal length of the combination is given by reciprocal of the resultant is the sum of the reciprocal of the focal lengths of the lenses used

$\frac{1}{fe} = \frac{1}{f1} + \frac{1}{f2}$

f1 = 20 cm

f2 = 10 cm

• $\frac{1}{fe} = \frac{1}{20} + \frac{1}{-10}$
• $\frac{1}{fe} = \frac{1-2}{20}$
• $\frac{1}{fe} = \frac{-1}{20}$
• fe = -20 cm

effective focal length is negative

so, the nature of the combination will be concave

Effective foacl length of the combination is 20 cm