Question

 Two Plano concave lenses of glass of refractive index 1.5 have radii of curvature of 20 & 30 cm. They are placed in contact with curved surfaces towards each other & the space between them is filled with a liquid of refractive index 4/3. Find the focal length of the system 

(a) divergent lens of focal length 72cm 

(b) divergent lens of focal length 60cm

(c) convergent lens of focal length 72cm 

(d) convergent lens of focal length 60cm 

Answer 1

The system is equivalent to the combination of three thin lenses. 

f = -72 cm 

i.e. the system will behave as a divergent lens of focal length 72 cm