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When light travels from an optically denser medium to a rarer medium at the interface, it is partly reflected back into the same medium and partly refracted to the second medium. This reflection is called the internal reflection.
If we gradually increase the incident angle we find there is also increase in angle of refraction. The incident angle corresponding to which the angle of refraction is ; called critical angle ic.
When incident angle is greater than ic the ray will reflect back in the same medium following the law of reflection, called total internal reflection.
The Doppler Effect for light
If there is no medium and the source moves away from the observer, then later wavefronts have to travel a greater distance to reach the observer and hence take a longer time. The time taken between the arrival of two successive wavefronts is hence longer at the observer than it is at the source. Thus, when the source moves away from the observer the frequency as measured by the source will be smaller. This is known as the Doppler effect. Astronomers call the increase in wavelength due to doppler effect as red shift since a wavelength in the middle of the visible region of the spectrum moves towards the red end of the spectrum. When waves are received from a source moving towards the observer, there is an apparent decrease in wavelength, this is referred to as blue shift. Thus,
( symbols has usual meaning.)
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Waves and Optics