Q. A sphere of glass (refractive index= 1.5) is of 20 cm diameter. A parallel beam enters it from one side. Where will it get focussed on the other side?

Explain it with the help of diagram.

Refer to the following figure...

we know that

here,

μ1 = refractive index of air = 1

μ1 = refractive index of glass sphere =  1.5

u =  image distance (image at infinity) = -∞

R = 20cm/2= 10cm 

so,

(-1/-∞) + (1.5/v) = (1.5-1) / 10

thus,

v = 10cm

now,

this image formed will be object for the second case

so,

u = 10cm

R = -10cm

using this in the above relation, we get

v' = 5cm 

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