Question

If a lens converged a parallel beam of light at the focal point, the lens is -

• A. Convex lens
• B. Cylindrical lens
• C. Parabolic lens
• D. Concave lens

Answer 1

Answer: (A)

Convex lens

Using lens formula:

$\frac{1}{imagedistance}$ $-$$\frac{1}{objectdistance}$$=$$\frac{1}{focallength}$

For parallel beam of light the object distance is infinite, therefore $\frac{1}{objectdistance}$$=0$

$\therefore$ $\frac{1}{imagedistance}-0$$=$$\frac{1}{focallength}$

$\therefore$ $imagedistance=focallength$

As image distance comes out to be positive because focal length is positive, thus image will be formed behind the lens.

This implies in convex lens parallel beam converges to focal point as required.