A concave lens of focal length 20 cm placed in contact with a plane mirror acts as aA- Convex mirror of focal length 10 cmB- Concave mirror of focal length 40 cmC- Concave mirror of focal length 60 cmD- Concave mirror of focal length 10 cm

The correct option is A Convex mirror of focal length 10 cmBy using 1/F=2/f_1+1/f_m Since f_m=∞⇒ F=f_l/2=20/2=10 cm (After silvering concave lens behave as con ...

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Standard X

Physics

Lens Formula and Magnification

A concave len...

Question

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

Convex mirror of focal length 10 cm

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Concave mirror of focal length 40 cm

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Concave mirror of focal length 60 cm

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Concave mirror of focal length 10 cm

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Solution

The correct option is A Convex mirror of focal length 10 cm
By using $\frac{1}{F} = \frac{2}{f_{1}} + \frac{1}{f_{m}}$
Since $f_{m} = \infty \Rightarrow F = \frac{f_{l}}{2} = \frac{20}{2} = 10 c m$
(After silvering concave lens behave as convex mirror)

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Similar questions

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

Q. According to New Cartesian Sign Convention:

(a) focal length of concave mirror is positive and that of convex mirror is negative
(b) focal length of both concave and convex mirrors is positive
(c) focal length of both concave and convex mirrors is negative
(d) focal length of concave mirror is negative and that of convex mirror is positive

Q. A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. The focal length of this combination of lenses will be:
(a) +10 cm
(b) +20 cm
(c) −10 cm
(d) −20 cm

Why focal length of convex mirror and convex lens is positive while concave mirror and concave lens negative?

Q. Study the given ray diagrams and select the correct statement from the following :

(a) Device X is a concave mirror and device Y is a convex lens, whose focal lengths are 20 cm and 25 cm respectively.
(b) Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 10 cm and 25 cm respectively.
(c) Device X is a concave lens and device Y is a convex mirror, whose focal lengths are 20 cm and 25 cm respectively.
(d) Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 20 cm and 25 cm respectively.

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

The correct option is A Convex mirror of focal length 10 cmBy using 1F=2f1+1fm Since fm=∞⇒F=fl2=202=10cm (After silvering concave lens behave as convex mirror)

The correct option is A Convex mirror of focal length 10 cm
By using $\frac{1}{F} = \frac{2}{f_{1}} + \frac{1}{f_{m}}$
Since $f_{m} = \infty \Rightarrow F = \frac{f_{l}}{2} = \frac{20}{2} = 10 c m$
(After silvering concave lens behave as convex mirror)