Draw a ray diagram of an image formed when an object is placed at a distance of 10 cm from a convex lens having focal length 20 cm. Write any two nature of image. Calculate the power of lens.

Solution
Given
Focal length, f = 20 cm = 0.2 m
Object distance, u = 10 cm
Let, image distance be v

From lens formula

⅟𝖿 = ⅟𝗎 + ⅟𝗏

⅟₂₀  = ⅟₁₀ + ⅟𝗏

0.05 cm⁻¹ = 0.1 cm⁻¹ + ⅟𝗏

⅟𝗏  = 0.05 cm⁻¹ - 0.1 cm⁻¹ 

 ⅟𝗏  = - 0.05 cm⁻¹

v = - 20 cm

Here "-" sign indicates that image is in the same side of the object and at 20 cm distance from the optical center.

Ray diagram is drawn below:-

Magnification of the image, m = v/u = -20/10 = -2

Nature of the image:-

(I)     Virtual

(II)    Erect

(III)  Twice magnified from the object size.

Power of the lens, P = ⅟𝖿 = 1/0.2 = 5 Dioptre.