Ray Optics and Optical Instruments — Practice

Question 1

A convex lens of focal length 20 cm forms a real image at 30 cm. Find the object distance.

Accepted Answer

1/v - 1/u = 1/f: 1/30 - 1/u = 1/20, so 1/u = 1/30 - 1/20 = -1/60 and u = -60 cm.

Question 2

Find the critical angle for a glass-air interface (mu = 1.5).

Accepted Answer

C = arcsin(1/1.5) = arcsin(0.667) = 41.8 degrees.

Question 3

A 60 degree prism has minimum deviation 40 degrees. Find the refractive index.

Accepted Answer

mu = sin((60 + 40)/2)/sin(60/2) = sin50/sin30 = 0.766/0.5 = 1.53.

Question 4

A compound microscope has f0 = 1 cm, fe = 2.5 cm, tube length 20 cm. Find the magnification.

Accepted Answer

M = (L/f0)(1 + D/fe) = (20/1)(1 + 25/2.5) = 20 x 11 = 220.

Question 5

An object is 5 cm from a concave mirror of focal length 10 cm. Find the image and magnification.

Accepted Answer

u = -5, f = -10. 1/v = 1/f - 1/u = -1/10 + 1/5 = 1/10, so v = +10 cm (virtual). m = -v/u = +2 (erect, enlarged).

Practice — Ray Optics and Optical Instruments