Question

List the sign conventions that are followed in case of refraction of light through spherical lenses. Draw a diagram and apply these conventions in determining the nature and focal length of a spherical lens which forms a four times magnified, real image of an object placed 20 cm from the lens.(All India - 2013)

Answer 1

The sign conventions followed in case of refraction of light through spherical lenses is that distances to the left side of the lens are negative and distances to the right side of the lens are positive.

Focal length of the lens is 16cm and it is convex in nature.

Given object distance, u = 20cm

• Image formed is real. Hence the lens is convex in nature
• Magnification = 4 = -v/u
• u = - 20cm
• Therefore, v = 80cm
• By lens formula, 1/f = 1/v - 1/u ==>
• 1/f = 1/80 - 1/-20  = -20 - 80 /-1600 ==>
• f =  1600/100 = 16cm

Answer 2

Answer:

upper one is the correct explanation because it is totally copied