Diffraction

Huygen's principle states that every point on a wavefront of a progressive wave can be considered to be a source of circular waves moving forward from that point. The resultant wavefront is found by the superposition of these circular waves.

The intesity of light passing through a polaroid filter will be reduced by 50%. The direction of polarisation of light is actually determined by a vector quantity. Unpolarised light oscillates in every direction but can be resolved into two perpendicular components, e.g. vertical and horizontal. 50% will be vertically polarised and 50% will be horizontally polarised. A vertical polarising filter will only let through the vertically polarised component, i.e. 50% of the light.

Vertically polarised light can pass through a filter rotated 45 degrees to the plane of polarisation. Light passing through the first filter will be plane-polarised in a direction of 45 degrees to the second filter. This light can be resolved into components both parallel and perpendicular to the second filter and the parallel component can be transmitted. 

Uses - to reduce glare, stress analysis, measuring concentration of sugar solutions and LCDs

At one time, there were competing theories: some scientists (for example, Huygens) considered that light propagated through space as a wave, whereas others (most notable Newton) believed that light propagated through space as a stream of particles. However, the earlier particle model of light could not explain phenomena such as diffraction. Quantum theory proposed that light. consisted of quanta of energy or photons, as suggested by the photoelectric effect. At the same time, it was also recognised that light must nevertheless still behave like a wave under some circumstances, leading to the idea that light is neither just a wave nor just a particle phenomenon, but has aspects of both.