Total internal reflection, the critical angle, and the use of total internal reflection in optical fibres and prisms.

What this dot point is asking

WJEC Double Award Unit 3 wants you to explain total internal reflection and the critical angle, and describe its use in optical fibres and prisms.

What total internal reflection is

Normally, when light reaches a boundary, some refracts out and some reflects. But as the angle of incidence increases, more light reflects, until at the critical angle all of it reflects.

The critical angle

For glass, the critical angle is about 42 degrees.

Using total internal reflection

• Optical fibres: a thin glass fibre carries light (or infrared) by total internal reflection. The light hits the sides at more than the critical angle, so it keeps reflecting along the fibre, even around bends, carrying telephone and internet data quickly and with little loss.
• Prisms: right-angled glass prisms reflect light through 90 or 180 degrees by total internal reflection. They are used in periscopes and binoculars, giving sharper reflections than a mirror.

Why optical fibres are useful

Optical fibres can carry huge amounts of information as pulses of light, much more than electrical wires, and the signal loses little energy along the way. The light is kept inside the fibre by total internal reflection, so it can travel long distances and around corners. This is why optical fibres are used for high-speed internet and telephone networks, and in medical endoscopes to see inside the body. Linking the use back to total internal reflection is the key to these application questions.

The critical angle and density

The size of the critical angle depends on how much the two materials differ in optical density. A material in which light slows down a lot (a very dense material such as diamond) has a small critical angle, so total internal reflection happens easily, even at quite shallow angles. This is why cut diamonds sparkle: light entering them is totally internally reflected many times before coming back out. A material that bends light less has a larger critical angle. Knowing that a denser material has a smaller critical angle helps explain why some materials are better than others for optical fibres and prisms.

Total internal reflection versus a mirror

Prisms that use total internal reflection are often better than mirrors for reflecting light. A normal mirror reflects from a silvered surface behind the glass, which absorbs a little light and can give faint extra reflections, so the image is slightly dimmer and less sharp. With total internal reflection, all the light is reflected and there is no silvering to absorb it, giving a brighter, sharper reflection. This is why good binoculars and periscopes use prisms rather than mirrors, which is a common reason asked for in exams.

Try this

Q1. What happens to light at an angle greater than the critical angle? [1 mark]

• Cue. It is totally internally reflected (all reflected back inside).

Q2. Name one use of total internal reflection. [1 mark]

• Cue. Optical fibres (or prisms in periscopes and binoculars).