Question

An image (I) is formed in front of a concave lens of power 6.66 D as shown in the figure. Where would the object be?

[1 mark]

• A. At $F_2$
• B. Between $F_1$ and optical centre $C$
• C. Beyond $F_2$
• D. At infinity

Answer 1

The correct option is D At infinity

Given,
Power of lens (P) = 6.66 D

As it is a concave lens, according to sign convention, the power will be negative.
∴ P = -6.66 D
Power of lens P = $\frac{1}{focallength\left(f\right)inmetres}$

∴ Focal length of the lens (f) in metres = $\frac{1}{Poweroflens\left(P\right)}$

$f=\frac{1}{-6.66D}$

∴ f = -0.15 m = -15 cm

So, the focus is at a distance of -15 cm from the lens.

For a concave lens, the image forms on the same side of the lens as the object. From the figure, as we can see, the image is formed at the focus. So, the object must be at infinity.