Polarisation of light
Young and Fresnel at first thought light was a longitudinal wave, but experiments on polarisation showed that it is transverse. In natural light the directions of oscillation of the E vector are random; a tourmaline or polaroid plate (the polariser) passes only the component of E along a certain direction and produces linearly (plane) polarised light. A second plate (the analyser) analyses this light: rotating it about the ray changes the transmitted intensity with the angle α: I = I₀cos²α (Malus's law), where α is the angle between the axes of the polariser and analyser. With parallel axes (α = 0°) the intensity is greatest and with perpendicular axes (α = 90°) it is zero. Natural light passing through one polariser loses about half its intensity. Longitudinal waves (e.g. sound) cannot be polarised, so polarisation proves that light is a transverse wave. Polarised light is used in technology and science: polaroid glasses reduce glare (reflected polarised light), liquid crystal displays work with polarisers, the sugar concentration in solutions can be measured and stress in materials can be checked.
A safe observation: look at a lamp or computer screen through two polaroid glasses lenses or polaroid films, rotate one slowly and watch the darkening. Do not look at the Sun. Explain that the light of an LCD screen is polarised.