A beam of light from a monochromatic laser shines into a piece of glass. The glass has thickness L and index of refraction n =1.5. The wavelength of the laser light in vacuum is L/10 and its frequency is f = 1.5. How long does it take for a short pulse of light to travel from one end of the glass to the other?

Engines for propeller-driven aircraft are limited in their maximum rotational speed by the fact that the tip speed of the propeller must not approach the speed of sound in air (mach i). taking 6 ft as a typical diameter for a propeller of a light airplane and 1100 fils as the speed of sound, find the upper limit on the rpm (revolutions per minute) of the propeller shaft.