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.
See also Part 7.
How a reflecting
telescope works
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).
INDEX notes: Light reflection and
uses of plane & curved mirrors
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.
Use your
mobile phone in 'landscape'?
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INDEX notes: Light reflection and
uses of plane & curved mirrors
