Question

A double concave lens with the refractive index (n) = 1.5 is kept in the air. Its two spherical
surfaces have radii R = 20 cm and R = 40 cm. Find the focal length of the lens. Write the
characteristics of the lens.​

Answer 1

Explanation:

Given,

refractive index (n)       = 1.5

radius of curvature(R1)= -20 cm (by applying sign convention)

radius of curvature(R2)=60 cm   (by applying sign convention)

by applying formula,

1/f=(n-1)(1/R1-1/R2)

1/f=(1.5-1)(1/-20-1/60)

1/f=(0.5)(3+1/-60)

1/f=(0.5)(4/-60)

1/f=(0.5)(1/-15)

1/f=1/-30

f=-30 cm

focal length of the concave lens is 30 cm.

Characteristics of the concave lens is 

1)it will produce smaller image of object

2)image of the object is closer to the optical centre

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