Optics 7 Question 27

28. Which of the following form(s) a virtual and erect image for all positions of the object?

(1996, 2M)

(a) Convex lens

(b) Concave lens

(c) Convex mirror

(d) Concave mirror

Show Answer

Solution:

  1. For a lens 1v1u=1f, i.e. 1v=1f+1u

For a concave lens, f and u are negative, i.e. v will always be negative and image will always be virtual.

For a mirror :

1v+1u=1f i.e. 1v=1f1u

Here, f is positive and u is negative for a convex mirror. Therefore, v is always positive and image is always virtual.



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