Light is a periodic travelling wave, in which the direction of vibration may, at one instance, be vertical, then horizontal, or any angle in between. That is to say, it vibrates equally in all planes perpendicular to the direction of propagation as illustrated in Fig 14.27(a).
(a)
(b)
Fig 14.27: Polarisation of light (a) unpolarised light passing through one piece of Polaroid (b) vertically polarised light cannot pass through a second polaroid material orintated horizontally
This light is said to be unpolarised. Polarisation of light is when the direction of vibration is restricted to one plane. When light passed through the Polaroid material, only the incident light that is vibrating parallel to the polarisation direction of the material is allowed to continue propagating unimpeded. If a second Polaroid is positioned at an orientation of 90° relative to the first, no light can pass through the second Polaroid, as illustrated in Fig 14.27 (b).
Only transverse waves can be polarised Demonstrating that light can be polarised shows that it is a transverse wave motion.
DEMONSTRATION 14.7
To Show Light is a Transverse Wave Motion Apparatus Two pieces of Polaroid, light source
Procedure Shine light through both pieces of Polaroid. Rotate Polaroid relative to the other through 90 as illustrated in Fig 14.28.
Observation There is a change in intensity of the light passing through the pieces of Polaroid decreasing to zero as the rotation reaches 90.
Fig 14.28
Conclusion As only transverse waves undergo the phenomena of polarisation, it can be concluded that light is a transverse wave motion.
