Reflection of light
The light that enters your eye allowing you to see has come either
directly from a source, like a light globe, or has come from a source,
been reflected off an object, and then to your eye. Look around you,
most things you can see are due to reflected light from an object to
your eyes. Your eyes then transmit this information to your brain via
the optic nerve which then interprets the electrical signal as light
of a particular colour and intensity. You can see an object only because
it either reflects light from a surface, or because the object itself
emits light.
Some surfaces, like gloss paint, are shiny, and reflect a fairly
good image back to your eye. Others, like matt paint or paper do not
reflect a clear image. They all reflect light but the reflected light
is spread unevenly and is called a diffuse reflection.
For an almost perfect reflection glass coated with a thin layer of
metal on the back is used. This is called a mirror and a totally flat
mirror is called a plane mirror.
Plane mirror reflection
From
the diagram you can see that the ray of light travelling towards the
mirror is the incident ray and the ray leaving the surface is the reflected
ray. A line drawn at right angles to the mirror is called the normal.
There are two basic laws of reflection for light from a plane mirror.
These laws are not new, in fact they were recognised by Plato around
400 BC.
They are:
- The reflected ray, the incident ray and the normal at the point
of incidence are all in the same plane.
- The angle of reflection equals the angle of incidence.
These laws apply to all the rays of light reflecting from the mirror.
Mirror images
When you look at yourself in a plane mirror you see a reflection or
an image of the same size, that moves closer and further away to "mirror"
what you do. You should notice two other effects that are important.
Firstly, the image appears to travel towards you or away from you at
twice your speed. Secondly, if you raise your left hand, the reflected
image raises its right hand.
From this simple ray diagram you can see that the image is as far behind
the mirror as the object is in front, and reversed.
Real or virtual?
The image in a plane mirror appears to be behind the mirror, if you
were to look behind the mirror you would see nothing. This type of image
is called a virtual image. Virtual images are only formed by mirrors.
The other type of image is one that can form on a screen, like in a
slide show. These images are called real images. A real image can be
shown on a screen, a virtual image cannot be shown on a screen.
Diffuse reflection
A piece of paper does not reflect light to produce image but the same
laws of reflection apply. Why doesn't the paper reflect light in a way
that produces a clear image if these laws of reflection apply? Because
the surface of the paper is uneven, light reflects from each section
but as the reflected rays emerge in many different directions, it appears
scattered. This is called diffuse reflection. Gloss paper does produce
a better image because the coating on the surface fills some of the
irregularities, so reducing scatter.
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