Question

A single bi-concave lens (a lens with two concave surfaces) made of glass (index of refraction n = 1.55) has surfaces with radii of curvature r1 = -19.0 cm and r2 = 19.0 cm.

What is the focal length of the lens in air?

I got 17.32cm as the correct answer. Dont know the other two.

If an object is placed at p = 20.0 cm from the lens, where is the image? (Use a positive sign for a real image or a minus sign for a virtual image.)

If the object has a height of h = 2.10 cm, how large is the image? (Use a positive sign for an upright image or a minus sign for an inverted image.)

Answer 1

(b). Object distance u = 20 cm

From the relation ( 1/ v ) – ( 1/ u ) = ( 1/ f)

                            (1/ v ) = ( 1/ f) + ( 1/ u )

                                        = 0.10

     Image distance v = 9.28 cm

(c) height of object O = 2.10 cm

Height of image I = ( v / u ) * O

                             = 0.974 cm