Triple Physics Topic 5 - Further Applications of Light

When we want to reflect something, we immediately think of a plane mirror.  However plane mirrors have limitations.  An ordinary mirror, like one in the bathroom, is silvered at the rear of the glass.  This protects the silvering, but you can get multiple images.  In normal use this doesn't really matter, but if you are using such a mirror in a camera or a telescope, there will be some degradation of the image.  You can avoid this by front-silvering the mirror, but the mirror can easily be scratched.

A much better solution is to use a 45-45-90 prism.

Prisms use the important principle, that of total internal reflection.  The next section explains how total internal reflection works.

Critical Angle

If we shine a ray along the normal through a semi-circular glass block, there is no refraction at all.  It is important that the ray is shone along the radius, otherwise there will be refraction at the air-glass boundary.

A ray of light passing from glass to air bends away from the normal.

If we increase the angle of incidence, the angle of refraction increases more:

At a particular value of angle of incidence, the angle of refraction is 90o.  This particular angle of incidence is called the critical angle.

Above the critical angle we get total internal reflection.  There is no transmission of light at all.

All light is reflected.  The angle of incidence = angle of reflection.  In glass, the critical angle is about 42o.  So if we use a 45-45-90 prism, the incident angle is 45o.  This is above the critical angle, so total internal reflection is seen.

Working out the Critical Angle (HT only)

We can work out the critical angle using the relationship:

sin c = 1 ÷ refractive index

 Worked example What is the critical angle of water, refractive index 1.33? Answer sin c = 1 ÷ 1.33 = 0.75   c = sin-1 0.75 = 48.6o.

Optical Fibres

Optical fibres are used extensively for transmission of data. They consist of very fine strands of very pure glass. They look like this:

Light bounces down the optical fibre like this:

Optical fibres are also used in endoscopes, which doctors use to examine inside a patient.

Photo by Kalumet, Wikimedia Commons

In optical fibre cables, the signals travel at about 200 000 000 m/s. The pulses are produced by an infra red laser, since glass is particularly transparent to infra red light. Although the glass is very pure, the signals are reduced, so every 10 km or so there is a booster, to bring the signal back to its previous level.

The advantages of optical fibres over ordinary cables are many:

• Cables are much lighter;

• Many thousands of signals can be sent at the same time;

• The cables don't get hot and burn out;

• They are cheaper, too.

• They don't get nicked by metal thieves.

Lasers

Laser stands for light amplification by stimulated emission of radiation.  We do not need to know how a laser works at this level, other than that it is a box that gives out an intense beam of radiation.  This can be visible light, or infra-red.  You will have seen a visible light laser at school, usually with a red beam.

A school laser is a very low-powered device, but it is intense enough to blind you.  NEVER EVER look up a laser beam.  Nor should you ever point a laser at anyone, even a weak one.

Graphic by Chris Chen, Wikimedia Commons

Lasers can be mounted on microchips, taking a tiny fraction of a watt, up to huge devices which need megawatts to drive them.  In the cold war, the American Forces experimented with a laser to shoot down enemy planes.  The effectiveness of the massive device depended on:

• the enemy plane being painted black;

• and the pilot being a frightfully good sport and flying his plane straight and level so that the machine could track it.

Back to the drawing board...

More powerful lasers can cause intense heating effects, sufficient to melt metals.

In medicine, lasers are used for cauterising wounds.  This means that the wound is sealed immediately to prevent blood loss.  Other medical uses include:

• spot-welding the retina in eye treatment;

• removing cancerous tissue, cauterising the blood supply;

• breaking up kidney stones.

Photograph by CMRF Crumlin, Wikimedia Commons

The typical power of a surgical laser is about 30 - 100 watts.