Check sibling questions

A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

 

Answer

We know that, 

Power of a lens = 1/Focal Length

Therefore, 

Focal Length = 1/ Power of the lens

 

Note : Here power is in m

 

(i) distant vision

For distant vision,

Power of the lens = −5.5 D

Focal length(m) = 1/ Power of the lens

                      = 1/ (−5.5)

                      = −10/55

                      = −2/11

                      = -0.1818 m

                      = −0.1818 × 100 cm

                      = −18.18 cm

                      = −18.2 cm

 

Focal length of the lens required for correction for distant vision is −18.2 cm

Since the focal length is negative, the lens used is concave lens.

 

(ii) near vision

For near vision,

Power of the lens = 1.5 D

Focal length(m) = 1/ Power of the lens

                      = 1/ (1.5)

                      = 10/15

                      = 2/3

                      = 0.667 m

                      = 0.667 × 100 cm

                      = 66.7 cm

 

Focal length of the lens required for correction for near vision is 66.7 cm

Since the focal length is positive, the lens used is convex lens.

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CA Maninder Singh is a Chartered Accountant for the past 12 years and a teacher from the past 16 years. He teaches Science, Accounts and English at Teachoo