Question

A thin equiconvex lens has focal length $10$cm and refractive index $1.5$. One of its faces in now slivered and for an object placed at a distance u in front of it, the image coincides with the object. The value of u is?

• A. $10$cm
• B. $5$cm
• C. $20$cm
• D. $15$cm

Answer 1

The correct option is A $10$cm

For, image to coincide with the object , rays of light reflected back from mirror must retrace its path.

That is the the light rays must be normally incident on the mirror.

That is light after refraction from lens should go parallel .

Hence. object must be at focus of lens.

$u=10cm$

Answer-(A)