IIT JEE , AIEEE Forum - Physics , Optics

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Let the angle made by the line joining the source to the circumference of the disc be q₁ with respect to the vertical. And the angle of refraction into the air be q₂. Then we have,

1.4 sin q₁ = sin q₂ --------------(1)

Also tan q₁ = r / 30 -------------(2)

From 1 and 2 we can calculate q₂ in terms of r

Now the shadow area on the roof will have a radius R = r + x, where x is given by,

Tan q₂= x / 100 -------------------(3)

From 3 we get the value of x in terms of r.

Now the maximum value of r is obtained by putting q₁ = C, where C is the critical angle for the water and air surface, where C is given by

Sin C = 1 / 1.4

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