Slide 1
A change in light’s speed can bend its path
Light often changes direction when it enters a material in which it travels at a different speed. This bending is called refraction.
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At normal incidence, where the light meets the boundary straight on, its direction need not change even though its speed does. Bending therefore depends on both material properties and entry angle.
Slide 2
Refractive index compares wave speed with vacuum speed
Refractive index compares light’s speed in vacuum with its phase speed in a material: how fast a wave crest travels. A larger index usually means a lower phase speed.
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The index belongs to a material at a specified wavelength and conditions. It can vary with colour and temperature, so a single quoted number is often an approximation for a particular optical task.
Slide 3
A stationary boundary preserves frequency but changes wavelength
When light crosses a stationary boundary, its frequency stays the same. A change in speed therefore changes its wavelength inside the material.
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Wave speed equals frequency multiplied by wavelength. If speed decreases while frequency is fixed, the spacing between wave crests decreases; the light does not acquire a new colour merely by entering glass.
Slide 4
Different wavelength speeds can separate colours
Many transparent materials bend different wavelengths by different amounts. A prism can therefore spread white light into a spectrum.
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This dispersion also affects lenses, which may bring different colours to slightly different focal positions. Combining materials with suitable dispersions can reduce the resulting colour fringes in optical instruments.
Slide 5
Above a critical angle, light can be reflected back inside
Light travelling toward a lower-index material can undergo total internal reflection when its incidence angle exceeds a critical value.
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The condition requires travel from the higher-index side. It explains why light can remain guided inside an optical fibre, although practical fibres also face losses from absorption, scattering and bends.