Question

An object is placed 20 cm in front of a concave mirror, whose focal length is 25 cm. Where will the image be located?" (a) 100 cm in front of the mirror (b) 100 cm behind the mirror (0) 50 cm in front of the mirror (d) 500 cm behind the mirror​

Answer 1

Answer:

Given : Object distance, u=−20 cm

A concave mirror forms a real magnified image only when the image to be formed is inverted i.e. m=−3

Magnification m=

u

−v

∴ −3=

−20

−v

⟹v=−60 cm

Using mirror formula :

v

1

+

u

1

=

f

1

∴

−60

1

+

−20

1

=

f

1

⟹ f=

4

−60

=−15 cm

Thus focal length of the mirror is 15 cm

Explanation:

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Answer 2

Given : Object distance, u=−20 cm

A concave mirror forms a real magnified image only when the image to be formed is inverted i.e. m=−3

Magnification m=

u

−v

∴ −3=

−20

−v

⟹v=−60 cm

Using mirror formula :

v

1

+

u

1

=

f

1

∴

−60

1

+

−20

1

=

f

1

⟹ f=

4

−60

=−15 cm

Thus focal length of the mirror is 15 cm.

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