Question

An object of size 7.5 cm is kept at a distance of 35 cm from a convex lens of
focal length 15 cm. Choose a suitable scale to locate the position, nature and size of
the image formed by the lens.

Answer 1

Given:

An object of size 7.5 cm is kept at a distance of 35 cm from a convex lens of focal length 15 cm.  

To find:

Choose a suitable scale to locate the position, nature and size of the image formed by the lens.

Solution:

From given, we have,

Object height = h = 7.5 cm

Object distance = u = -35 cm

Focal length = f = -15 cm

we use the mirror formula,

1/f = 1/u + 1/v

-1/15 = -1/35 + 1/v

1/v = 1/35 - 1/15

1/v = -4/105

v = -26.25 cm

Magnification, m = -v/u = - (-26.25)/(-35) = - 0.75

Image size = h' = -vh/u = - (-26.25)(7.5)/(-35) = - 5.625 cm

As v is negative, so a real, inverted image of height 5.625 cm is formed at a distance of 26.25 cm in front of the mirror.