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Physics Notes: Reflection of visible light 5. Uses of plane and curved mirrors

Reflected light: 5. Uses of curved mirrors and plane mirrors & other reflecting surfaces, total internal reflection in fibre optic cables

Doc Brown's Physics exam study revision notes

5. The uses of curved and plane mirrors and other reflecting surfaces, total internal reflection

(remember angle of reflection i always equals the angle of reflection r with respect to the 90o normal)

Reminder: The reflection rule (angle i = angle r) applies whatever the shape of the mirror.

However, the shape of the mirror surface is important for what you want to do with the mirror!

Mirrors can be all shapes and sizes depending on their uses, including distorting your shape at a fun-fair!

The most familiar use is a plane mirror in the home - you see a 'perfect' image of yourself, but it is laterally inverted - your left become right and right becomes left! Its called lateral inversion. However, 'top' and 'bottom' are still the same!

Note on the use of mirrors and safety equipment

Good reflectors used on bicycles and safety clothing because they are designed so that the light is directed back in one direction from where it came from e.g. the headlamps of a car.

The reflectors cover a wide range of angles so you cannot see a clear reflected image.

They are often made of red plastic and act as a colour filter, so you see the red warning colour.

A periscope is a simple method of observing something from a different height than that of your eye.

It is used to observe things when there is a barrier or other obstacle in the way.

You can buy a simple one using plane mirrors (left diagram above) for watching golf with spectators in front of you!

The periscopes of submarines require something a little more sophisticated.

The right-hand diagram shows how you use 45o triangular prisms instead of mirrors.

Prisms have a higher optical quality and note that the inside surfaces of solid '3D' prisms can act as a mirror.

This phenomena is called 'total internal reflection'.

You can use these 45o prisms to reverse the direction of a light beam - can you figure out how and sketch the ray diagram?

Comparing concave and convex mirrors

A concave mirror can focus light rays to a common point F in front of the mirror (F is called the principal focus). The distance from F to the centre of the mirror is called the focal length.

You come across the same terms when you study lenses.

This type of lens is used in reflecting telescopes (example further down).

A concave mirror is described as a converging mirror, for example it can converge the Sun's rays to a focus point to provide a workable solar heating system.

A shaving mirror is a concave mirror because it can produce an upright magnified image.

A convex mirror disperses the rays and the focal point F is behind the mirror.

Convex mirrors give you a wide field of view and collect light over a wide angle.

Convex mirrors are used by the driver on a bus, shop security and side-mirrors on cars to give a wide view of the road behind and to the side of a vehicle.

Use of a concave parabolic mirror in head lamps of floodlights

A parabolic concave mirror is used to produce a powerful beam of light.

In car headlamps the light from the bulb (filament or LED) is collected by the mirror and reflected to produce an approximately parallel beam of rays to illuminate a narrow field of view ahead of the vehicle - the mirror acts with a small angle of divergence.

So, in reality the diagram isn't quite correct because you want the rays to diverge a little to produce a wider beam to illuminate more of the road ahead.

You can make small changes to the parabolic shape to change the dispersion of the beam.

A parabolic array of mirrors can be used to make a solar furnace, reaching temperatures of over 2000oC.

French scientists have been experimenting with solar furnaces since 1949.

Hot countries like Spain are doing increasing research, no doubted prompted lately by climate change, since solar energy is free and doesn't produce carbon dioxide!

See https://en.wikipedia.org/wiki/Solar_furnace

Uses of total internal reflection

The internal surface of optical fibres acts as a mirror - another case of total internal reflection, this time in fine glass strands which allow the transmission of visible light rays and information signals over long distances with minimum loss of intensity (amplitude).

Keywords, phrases and learning objectives for the reflection of visible light rays

Be able to describe and explain the uses of curved parabolic concave mirrors, curved convex parabolic mirrors and plane mirrors and other reflecting surfaces e.g. car headlamps, shaving mirror.

Know , describe and explain total internal reflection and its use in fibre optic cables.

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