An empty spherical flask of diameter `15 cm` is placed in water of `mu = 4//3`. A parallel beam of light strikes the flask. Where does it get focussed, when observed from within the flask ?

Explanation

Here, `R = PC = + 15//2 cm`
`mu_2 = 4//3, mu_1 = 1`
For a prallel beam of light, `u = oo`, Fig.
The beam diverges on refraction and appears to come from `I`.
`PI = v = ?`
As refraction ocurs from denser to rarer medium, therefore,
`-(mu_2)/(u)+(mu_1)/(v)=(mu_1 - mu_2)/( R)`
`-(4)/(3 xx oo)+(1)/(v)=(1 - 4//3)/(15//2) = -(2)/(45)`
`v = -(45)/(2) = -22.5 cm`.
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