Question

(a) An object 3 cm high is placed 24 cm away from a convex lens of focal length 8 cm. Find by calculations, the position, height and nature of the image, and also draw the ray diagram for above condition. (b) If the object is moved to a point only 3 cm away from the lens, what is the new position, height and nature of the image?

Answer 1

Answer

(a) h1 = 3 cm

u = -24 cm

f = 8 cm

Lens formula : 1/v – 1/u = 1/f

1/v – 1/-24 = 1/8

1/v = 1/12

V = 12 cm

Image is formed 12 cm behind the lens.

m = v/u = h2/h1

12/-24 = h2/3

h2 = -1.5 cm

Image is 1.5 cm high, real and inverted.

(b) u = - 3 cm

h1 = 3 cm

f = 8 cm

Lens formula : 1/v – 1/u = 1/f

1/v – 1/-3 = 1/8

1/v = - (5/24)

V = -4.8 cm

Image is formed 4.8 cm in front of the lens.

m = v/u = h2/h1

-4.8/-3 = h2/3

h2 = + 4.8 cm

Image is 4.8 cm high, virtual and erect.

hope this will help you...