Question

The image placed at a distance of 36cm from a spherical lens is formed on a screen placed at 72cm from the lens. Identify the type of lens and calculate its focal length. If the height of the object is 2.5cm find the height of its image.

Answer 1

u= -36cmv = 72 cmh = 2.5cm

using lens formula ,

1/f = 1/v - 1/u

1/f = 1/72 - 1/(-36)

= 1/72 + 1/36 = 1+2/72

= 3/60 = 1/20

f = +20cm

postive focal length represents cnvx lens

so ,

m= v/u = h'/h

= 72/-36 = h'/2.5

h' = -72/36 x 2.5

h' = -5.0cm