Question

What is meant by power of a lens? You have three lenses L1, L2 and L3 of powers +10D, +5D and -10D respectively.
State the nature and focal length of each lens. Explain which of the three lenses will form a virtual and magnified image of an object placed at 15 cm from the lens. Draw the ray diagram in support of your answer.

Answer 1

Power of lens is the degree of convergence or divergence of light rays .

power of lens is defined as reciprocal of its focal length .

P=1/f [in meters]

Given L1 =+10 D

P1=100/f

10=100/f

f1=100/10=10cm

L2=+5D

f2=100/5=20cm

L3= - 10D

f3= -100/10=-10cm

So first and second lens are convex lens and third lens is concave lens because focal length is negative.

Lens 2 that is with focal length 20cm will produce magnified  and virtual image of an object placed at 15cm

Because the position of object is between P and F.