Question

A diver in the swimming pool wants to signal his distress to a person lying on the edge of the pool by flashing his waterproof flashlight
(A) He must direct the beam vertically upward
(B) He has to direct the beam horizontally
(C) He has to direct the beam at an angle to the vertical which is slightly less than the critical angle of the incidence
(D) He has to direct the beam at an angle to the vertical which is slightly more than the critical angle of the incidence for total internal reflection

Answer 1

Hint: Use the concept of total internal reflection for the reflection of the light ray from the denser medium to the rarer medium. Also consider the different conditions when the angles of incidence are different as compared to the critical angle.

Complete step by step answer:
The angle for which the angle of refraction for the ray of light travelling from denser medium to rarer medium is \[90^\circ \] is known as the critical angle.The phenomenon in which the light travelling from the denser medium to rarer medium get reflected back in the same denser medium for an angle of incidence greater than the critical angle is known as the total internal reflection.

If the light is incident on the interface of the denser and rarer medium horizontally, then it
will travel along the same interface without any reflection or refraction.

If the light is incident on the interface of the denser and rarer medium making an angle equal to the critical angle, then the light will travel along the interface after refraction.

If the light is incident on the interface of the denser and rarer medium with an angle less than the critical angle then the refracted ray of the light will travel out of the water in the air medium with the refracted ray away from the normal.

-The diver is in the swimming pool and wants to signal his distress to the person at the edge of the swimming pool with his waterproof flashlight.If the beam is incident vertically on the surface of water (interface) then the beam of the flash light will travel in the straight vertical line when it emerges out of the water surface. Hence, the flash light will not reach to the eye of the person at the edge of the swimming pool.Hence, the option A. is not correct.
-If the diver makes the beam of the flash light incident horizontally, the flash light will travel in the straight line along the interface of the air and water and hence, will not reach to the eye of the person at the edge.Hence, the option B. is not correct.
-If the diver directs the incident beam of the flash light at angle to the vertical which is slightly more than the critical angle then the beam of the flash light will reflect back in the water due to the phenomenon of total internal reflection instead of reaching towards the eye of the person at the edge of the pool.Hence, the option D. is not correct.
-If the diver directs the beam of the flash light at an angle to the vertical which is slightly less than the critical angle then the beam of the flash light undergoes refraction from the water medium to the air medium and will reach to the eye of the person at the edge of the swimming pool.
Hence, the correct option is C.

Note:Use the proper conditions while checking for each option. The normal (vertical) is imaginary and the interface between the water and air medium is the water surface.And don’t confuse between reflection and total internal reflection because reflection occur when a ray of light travels in a medium and strikes to the surface of mirror and reflected back while the total internal reflection takes place when a ray of light travels from denser medium to rarer medium.