Question

Object distance=30cm and image distance=45cm what is the focal length

Answer 1

Answer:

Object distance= 30cm

Image distance= 45cm

total distance= 30cm+45cm

=75 cm

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Answer 2

Given:

u=-30cm

v=45cm

To Find:

Focal length

Solution:

In case of concave mirror

$\sf{{\dfrac{1}{f}}={\dfrac{1}{v}}+{\dfrac{1}{u}}}$

$\sf{{\dfrac{1}{f}}={\dfrac{1}{45}}-{\dfrac{1}{30}}}$

$\sf{{\dfrac{1}{f}}={\dfrac{2-3}{90}}}$

$\sf{{\dfrac{1}{f}}=-{\dfrac{1}{90}}}$

$\sf{f=-90cm}$

In case of convex lens

$\sf{{\dfrac{1}{f}}={\dfrac{1}{v}}-{\dfrac{1}{u}}}$

$\sf{{\dfrac{1}{f}}={\dfrac{1}{45}}+{\dfrac{1}{30}}}$

$\sf{{\dfrac{1}{f}}={\dfrac{2+3}{90}}}$

$\sf{{\dfrac{1}{f}}={\dfrac{1}{18}}}$

$\sf{f=18cm}$

Thus, if the object is placed in front of a concave mirror, the focal length of mirror will be -90cm and if the object is placed in front of a concave lens, it's focal length will be 18cm.