Question

a child is using a spectecle with power -2.5 d what is focal length

Answer 1

Answer:

$\boxed{\sf Focal \ length = 0.4 \ m \ \ \ or \ \ \ 40 \ cm}$

Given:

Power of the spectacle (P) = -2.5 D

To Find:

Focal length of the spectacle (f)

Explanation:

$\sf Focal \ length \ (f) = \frac{1}{Power \ (P)}$

$\sf f = \frac{1}{ - 2.5}$

$\sf f = - 0.4 \: m$

$\sf f = - 0.4 \times 100 \: cm$

$\sf f = - 40 \: cm$

Additional information:

♠ -ve sigh of focal length and power of the lens denotes that the lens is concave lens.

♠ Concave lens is used to correct Myopia (Nearsightedness/short-sightedness)