Question

two convex lenses of focal length 20 cm each are placed coaxially with a separation of 60 cm between them the distance of image formed of a distant object by combination from rear lens is​

Answer 1

Answer:

40 cm

Explanation:

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Answer 2

Final image is formed 40 cm to the right of the second lens.

Explanation:

The question is incomplete. Please note the full question below and the solution.

Two convex lenses f focal length 20 cm each are placed coaxially with a separation of 60 cm between them. find the image of a distance object formed by the combination by a. using the lens formula separately for the two lenses and b. using the equivalent lens. Note that although the combination forms a real image of as distance object on the other side, it is equivalent to as diverging lens as far as the location of the final image is concerned.

Solution:

a)  The first image is formed at the focus of the first lens. That is at 20 cm from the first lens and hence at u = −40 cm from the second. Using the lens formula for the second lens:

1/v = 1/u + 1/f  = 1/40 + 1/20

v = 40 cm

The final image is formed 40cm to the right of the second lens.

b) Equivalent focal length:

1/F = 1/f1 + 1/f2 - d/f1f2

     = 1/20 + 1/20 - 60/20²

Solving, we get F = -20cm

So it is a divergent lens and hence should be kept at dF/f1 behind the second lens.

D = dF/f1 = 60 * -20 /20 = -60cm

The equivalent lens should be kept 60cm to the right of the second lens.

The final image will be formed at the focus of this divergent lens (20 cm to the left). So, it is 40 cm to the right of the second lens.