Question

In Myopia object distance is infinity and image distance is equal to far point= D. Find focal length and which lens is used?

Answer 1

Final Answer:f = -D units, Concave Lens

Steps:
1) We have,
$u = -\infty \\ \\ v = -D$
Using Lens Formula,
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f} \\ \\ => \frac{1}{-D}-\frac{1}{-\infty} = \frac{1}{f} \\ \\ => f = -D units$ .

Since, f < 0 ,
=> Lens used is concave.

Answer 2

Hey dear,

◆ Answer-
a) f = -25 cm
b) concave lens

◆ Explaination-
As object is at infinity.
u = ∞

As image is at far point i.e. distance of distinct vision.
v = -D = -0.25 m

Using lens makers formula-
1/f = -1/v - 1/u
1/f = -1/0.25 - 1/∞
1/f = 4
f = -1/4
f = -0.25 m
f = -25 cm

Focal length of given lens is -25 cm.
As f is negative, the lens used is concave lens.

Hope this helps you...