A convex lens of focal length 20 cm placed in contact with a plane mirror acts as a

Question

A convex lens of focal length 20 cm placed in contact with a plane mirror acts as a (A) convex mirror of focal length 10 cm (B) concave mirror of focal length 40 cm (C) concave mirror of focal length 60 cm (D) concave mirror of focal length

Tardigrade Admin · Accepted Answer

When two lenses are kept in contact, the combined focal length of the combination is

\frac{1}{F} = \frac{2}{f} + \frac{1}{F _{m}}

For a plane mirror,

f_{m} = \infty

∴ F = \frac{f}{2} = \frac{20}{2} = 10 c m

Hence, arrangement behaves as a convex mirror of focal length

10 c m

Q. A convex lens of focal length 20cm placed in contact with a plane mirror acts as a

convex mirror of focal length 10 cm

concave mirror of focal length 40 cm

concave mirror of focal length 60 cm

concave mirror of focal length

When two lenses are kept in contact, the combined focal length of the combination is F1​=f2​+Fm​1​ For a plane mirror, fm​=∞ ∴F=2f​=220​=10cm Hence, arrangement behaves as a convex mirror of focal length 10cm

Q. A convex lens of focal length $20 c m$ placed in contact with a plane mirror acts as a

When two lenses are kept in contact, the combined focal length of the combination is
$\frac{1}{F} = \frac{2}{f} + \frac{1}{F _{m}}$
For a plane mirror, $f_{m} = \infty$
$∴ F = \frac{f}{2} = \frac{20}{2} = 10 c m$
Hence, arrangement behaves as a convex mirror of focal length $10 c m$