Revision:Reflection, Refraction, Total internal Reflection

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Total internal reflection

Waves going from a slower to a faster medium speed up and bend at the boundary, e.g. light going from glass to air. Beyond a certain angle (the critical angle) all the waves bounce back into the glass - they are totally internally reflected.

All waves that hit the surface beyond this critical angle are effectively trapped. The critical angle for most glass is about 42^o.

An optical fibre is a thin rod of high-quality glass. Very little light is absorbed in the glass. Light getting in at one end is totally internally reflected, even when the fibre is bent.

Optical fibres can carry enormous amounts of information in light pulses trapped inside them.

Reflection

When waves reflect, they always do it regularly.

Remember

The angle of incidence = the angle of reflection

Rough surfaces

Each bit of the surface obeys this law, but the overall effect of the jagged surface is to scatter the light diffusely. The reflected waves head off in all directions, e.g. sunlight on a piece of paper.

Smooth surfaces

These act as mirrors. The rays are reflected uniformly and can form images. They can:

1. be focused to a point e.g. sunlight off a concave mirror telescope
2. appear to come from a point behind the mirror e.g. a looking glass

Check what your syllabus needs you to know in detail about mirrors and images.

The radiation from all these sources - whether they are part of planet Earth or caused by man - is called our background radiation.

Refraction

If a surface is transparent to waves, some or most of the waves hitting the surface will pass through. The rest get internally reflected.

The speed of waves usually changes when they cross a boundary. This also changes their direction. (Imagine a car driving off a smooth, hard road into a muddy field.)

The bending follows a regular pattern known as Snell's Law. Most syllabuses don't test this in detail, but check in case yours does.

In refraction, the bigger the change in speed, the bigger the change in direction.

Spectrum of light

In a vacuum, light of all colours travels at the same speed. In any transparent medium (e.g. glass), the different colours travel at different speeds, so they bend by different amounts. This is why light is split by a prism. Violet light is the most violently bent.

Diffraction

When waves meet a gap in a barrier, they carry on through the gap. This may seem obvious, but what happens on the far side of the gap isn't so straightforward. There is never a perfect cut-off between the waves that get through and the non-wavy surroundings. The waves always 'leak' to some extent into the shadow area beyond the gap. This is diffraction - the spreading-out of waves when they go through a gap.

The extent of the spreading depends on how the width of the gap' compares to the wavelength of the waves.

Remember: the bigger the width of the gap compared with the wavelength of the wave, the less the diffraction.

Some examples of diffraction

Water

Waves spreading into a harbour; narrower gap, wider spread

Sound

1. Sound through a doorway

   Lower pitched sounds travel better into the space behind the door than high-pitched sounds.

   Low sound - a long wavelength compared with the gap - large spread.

   High sound - a short wavelength compared with the gap - little spread.

2. Ultrasound

   Very short wavelength compared with most gaps - little spreading. This makes sharp focusing of #:ultrasound easier, which is good for medical scanning or surveillance.

Light

Very short wavelength compared with most everyday gaps - windows, pupil in eye, lens in camera etc. - so there is little obvious diffraction and sharp shadows. Try looking at a bright light through the narrow gap between your fingers and squeezing the gap smaller - strange diffraction patterns appear.

Radio

Long-wave radio signals are much less affected by buildings, tunnels, etc. than those of short-wave or vhf radio.

The detailed patterns of diffraction are complicated, especially when lots of different wavelengths (e.g. white light) go together through complex patterns of gaps (e.g. in a grid of fibres). Try looking at a bright white light at night through an umbrella. Diffraction lies behind the colourful reflected patterns from CDs and oil films, the colour of dragon-flies' wings, and helps us find out what stars are made from.

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