DO COLOURS BEND ? Light traveling through a medium , propagates in a straight line and at a relatively constant speed , unless it is disturbed in some manner . This depends on two key concepts : » the optical density of the medium » the frequency of the light wave
☼ The optical density of a medium is the medium ’ s ability to slow down the light which passes through it . More optically dense materials will cause light to travel slower , and less dense materials will cause it to travel faster .
The optical density of a medium is not the same as its physical density . The physical density of a medium refers to the mass / volume ratio . The optical density of a material relates to the sluggish tendency of the atoms of a medium to transfer energy between each other . In every medium , other than a vacuum ( a perfect particle free medium ), there is a relative time delay as this energy is passed along .
This relative speed is quantified in what is called the index of refraction ( n ), ie ., the number of times slower that a wavelength will travel in a given medium in comparison to a vacuum ( which has the lowest possible optical density ). For example , the refractive index of water is 1.3 , meaning that light will travel 1.3x slower in water than in a vacuum .