Find the power of a convex lens if the image formed is at a distance of 20 cm from the lens when the object is placed on the other side of the lens at 25 cm from the optical centre?

Find the power of a convex lens if the image formed is at a distance of 20 cm from the lens when the object is placed on the other side of the lens at 25 cm from the optical centre? Correct Answer 9 dioptre

• The power of the convex lens is 9 dioptres.
• It is the reciprocal of the focal length(f) of the lens i.e. P=1/f 
• Power of lens=1/focal length of lens(f),
• To find focal length,
• Use lens formula  1/f = 1/v-1/u
• Where 'v' is the image distance and 'u' is the object distance from the optical centre of lens.
• P= 1/f = 1/20 - 1/(-25) = 1/20 + 1/25 = 9/100
• So the Focal length, f = 100 / 9
• The distance is in cm, so we have to convert it to dioptres.
• 1 Dioptre = 100 cm
• 100/9 cm = 100 × 9/100 Dioptres
• Therefore Power of the lens is 9 Dioptres.
• So, The formula for Dioptre = 100 / f, when the focal length is given in cms.