Question

a person with myopia can see objects placed at a distance of 4m if he wants to see objects at a distance of 20m, what should be the focal length of the power concave lens he must wear​

Answer 1

QUESTION :-

A person with myopia can see objects placed at a distance of 4 m. If he wants to see objects at a distance of 20 m, what should be the focal length of the power concave lens he must wear​.

SOLUTION :-

It is given that,

• x = 4 m.

• y = 20 m.

We know, for a myopic eye,

$\boxed{\sf{\dag\ Focal\ length=\frac{xy}{x-y} }}$

Putting the values :-

$\sf{f=\dfrac{20 \times 4}{4-20} }\\\\\sf{\implies f=\dfrac{80}{-16} }\\\\\boxed{\sf{\implies f=-5\ m }}$

We also know,

Power = reciprocal of the focal length = 1/f

⇒Power = 1/-5

⇒Power = -0.2 D

Answer 2

Answer:

It is given that,

x = 4 m.

y = 20 m.

We know, for a myopic eye,

\boxed{\sf{\dag\ Focal\ length=\frac{xy}{x-y} }}† Focal length=x−yxy

Putting the values :-

\begin{gathered}\sf{f=\dfrac{20 \times 4}{4-20} }\\\\\sf{\implies f=\dfrac{80}{-16} }\\\\\boxed{\sf{\implies f=-5\ m }}\end{gathered}f=4−2020×4⟹f=−1680⟹f=−5 m

We also know,

Power = reciprocal of the focal length = 1/f

⇒Power = 1/-5

⇒Power = -0.2 D