A diver in a swimming pool wants to signal his distress to a person lying on the edge of the pool by flashing his water proof flash light
He must direct the beam vertically upwards
He has to direct the beam at an angle to the vertical which is slightly less than the critical angle of incidence for total internal reflection
When incident angle is greater than critical angle, then total internal reflection takes place and will come back in same medium. That's how he will be able to flash the light signal to the person at the edge of the pool.