Concepts Of Physics MCQ Edition [Volume 1]PhysicsGeometrical Optics
A point source is located at a depth h beneath the surface of water (refractive index = ). Identify the shape of the area on the water surface through which light escapes, along with the position of its centre.
Options
- AA circular area with its centre directly above the point source
- BA rectangular area with its centre directly above the point source
- CA circular area whose centre is shifted horizontally by a distance h
- DAn elliptical area with its centre directly above the point source
Correct answer
A. A circular area with its centre directly above the point source
Step-by-step solution
Light rays from the point source at depth h will escape the water surface if their angle of incidence at the water-air interface is less than or equal to the critical angle _c . The critical angle is given by _c = 1 . By symmetry, the locus of all points on the surface where the angle of incidence is exactly _c is a circle. The radius of this circle is r = h _c = h ^2 - 1 . Thus, the light escapes through a circular area on the water surface. The centre of this circular area lies directly above the point source.