Light of wavelength 5000 å propagating in air gets partly reflected from the surface of water. how will the wavelengths and frequencies of the reflected and refracted light be affected ?
Answers
According to Huygen’s wave theory, the light propagates in the form of waves. The phenomenon of interference and diffraction verify that the light travels in the form of waves, but they do not tell about the nature of light waves, i.e., whether the light waves are transverse or longitudinal. Only the phenomenon of polarisation gives the information that the light waves are transverse. We know that in a transverse wave the particles of medium vibrate perpendicular to the direction of wave propagation. When Huygen proposed wave-theory, it was assumed that light waves propagate through the medium known as ‘ether’. Ether was assumed to be a transparent medium filled in all space. Therefore the transverse nature of light waves meant that the ether-particles vibrate perpendicular to the direction of wave-propagation. But it has been proved by modern experiments that there is no existence of medium-ether. According to electromagnetic theory of light, the electric and magnetic field vectors vibrate perpendicular to the direction of wave-propagation and are mutually perpendicular too. Therefore in place of medium particles, we now assume that in a light wave the field vector (or electric field vector) vibrates perpendicular to the direction of wave-propagation.
In ordinary light, the electric field vector vibrates in all possible directions perpendicular to the direction of wave propagation, i.e., the vibrations of electric field vector are symmetrical about the direction of wave-propagation. Such a wave is said to be unpolarised. For example if a wave is propagating along Z-axis, chosen perpendicular to plane of paper and the field vector vibrates along all possible directions in XY plane, the wave is unpolarised. But if by any device the vibrations of electric field vector are restricted only in one direction perpendicular to the direction of wave-propagation (i.e., the vibrations of field vector be made asymmetric about the direction of wave propagation); the light will be polarised and the phenomenon is called polarisation.
Clearly, in a polarised wave, the vibrations of field vector are asymmetric about the direction of propagation of the wave. For example, if the wave be propagating along Z-axis, chosen perpendicular to the plane of paper, and the vibrations of field vector are along X or along Y-axis, the wave is polarised.
Thus it is clear that the polarisation is possible only in transverse waves.