PHYSICS –PHY101 VU
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110
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Summary of Lecture 35 – GEOMETRICAL OPTICS
1. In the previous lecture we learned that light is waves,
and that waves spread out from every point. But in
many circumstances we can ignore the spreading
(diffraction and interference), and light can then be
assumed to travel along straight lines as rays. This
is hown by the existence of sharp shadows, as for
case of the eclipse illustrated here.
2. When light falls on a flat surface, the angle of
incidence equals the angle of reflection. You
can verify this by using a torch and a mirror,
or just by sticking pins on a piece of paper in
front of a mirror. But what if the surface is not
perfectly flat? In that case, as shown in fig. (b),
the angle of incidence and reflection are equal
at every point, but the normal direction differs
from point to point. This is called "diffuse
reflection". Polishing a surface reduces the
diffusiveness.
3. If you look at an object in the mirror, you
will see its image. It is not the real thing, and
that is why it is called a "virtual" image. You
can see how the virtual image of a candle is
formed in this diagram. At each point on the
surface, there is an incident and reflected ray.
If we extend each reflected ray backwards,
it appears as if they are all coming from the
same point. This point is the image of the tip
of the flame. If we take other points on the
candle, we will get their images in just the
same way. This way we will have the image
of the whole candle. The candle and its
image are at equal distance from the mirror.
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