Total internal reflection requires light travelling from higher to lower refractive index with incidence angle above the critical angle. At the critical angle the transmitted ray is along the boundary: sinθc= nlow/nhigh. Below that angle, some light can transmit. Optical fibres use repeated total internal reflection, with an appropriately higher-index core than cladding.
Diffraction is spreading around edges or through apertures and is strongest when aperture width is comparable to wavelength. It does not require a speed change. Polarisation restricts the direction of electric-field oscillation and demonstrates light’s transverse character. Ideal crossed linear polarisers transmit no light through both; aligned ones transmit the component selected by the first. Keep these mechanisms separate from reflection and refraction in classification questions.
Example 1
Find the critical angle from glass n=1.50 to air n=1.00.
sinθc=1.501.00=32, giving θc=41.8∘. Incidence above this value produces total internal reflection.
Example 2
Which spreads more through the same narrow gap: wavelength 2cm or 10cm?
The 10cm wave has the larger wavelength-to-gap ratio, so it diffracts more strongly under otherwise equal conditions.
Watch out:It must also have incidence angle above the critical angle. At smaller angles a transmitted ray is possible.
Total internal reflection, diffraction and polarisation practice | Year 11 Physics | Ascent Arcade