Question

the focal length of a concave lens is 25cm express its power with signs​

Answer 1

$\LARGE{\bf{\underline{\underline{GIVEN:-}}}}$

• Focal length :- +25 cm . (the focal length is always positive for concave lens)

$\LARGE{\bf{\underline{\underline{TO:FIND:-}}}}$

• We need to find the power.

$\LARGE{\bf{\underline{\underline{SOLUTION:-}}}}$

We know that:

$\mapsto \sf{\red{P=\dfrac{100}{f} }}$ [This formula applies only when f is in cm]

Where:

• P - power.
• f - Focal length.

Now, just substitute.

$\to \sf P=\dfrac{100}{25}$

$\sf \to P= 4 D$

∴Power is 4 Dioptre.

Note:

• If focal length is in metres, then use this formula:

$\boxed{\sf{\green{P=\dfrac{1}{f} }}}$

• If focal length is in centimetres, use this formula:

$\boxed{\sf{\green{P=\dfrac{100}{f} }}}$