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(a) Describe briefly, with the help of suitable diagram, how the transverse nature of light can be demonstrated by the phenomenon of polarisation. (b) When unpolarised light passes from air to a transparent medium, under what condition does the reflected light is plane polarised ? |
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Answer» Solution :(a) Light waves are transverse in nature. This can be demonstrated by the phenomenon of polarisation, by using two polaroids. When ordinary light from a monochromatic light source S passes through a POLAROID sheet `P_(1)`, known as polariser, the light becomes polarised in which electric vector is oscillating only along the pass axis of polaroid `P_(1)`. if the light is allowed to pass through a second polaroid `P_(2)`, known as analyser, whose pass axis is exactly parallel to that of `P_(1)`, then the light passes through it also and we see maximum light. HOWEVER, if `P_(2)` is rotated about its axis, there is decrease in INTENSITY of transmitted light. when pass axis of polaroid `P_(2)` is rotated about its axis, there is decrease in intensity of transmitted light. when pass axis of polaroid `P_(2)` is making an angle of `90^(@)` from that of `P_(1)`, no light passes through `P_(2)` and there is complete darkness on other side of it. however, on making pass axis of `P_(2)` parallel to `P_(1)` there is again maximum light passing through it. the experiment proves that light passing through polaroid `P_(1)` gets polarised about its pass axis. if the pass axis of `P_(2)` makes an angle `theta` with the pass axis of `P_(1)`, then only a component of electric vector passes through `P_(2)` and intensity of light decreases. when pass axis of `P_(2)` is making an angle of `90^@` with `P_(1)`, no vibrations of electric vectors can be pass through `P_(2)` and intensity of emergent light is ZERO.  (b) Plane polarised light can be obtained by reflection from a transparent surface (say a plane glass plate) of refractive index n when the light is incident on the surface at the polarising angle `i_(p)`. Value of polarising angle is given by Brewster.s law, according to which `tani_(p)=n or i_(p)=tan^(-1)(n)`. |
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