Chapter2

CO: Imagine a magic trick. A “magician” in front of your eyes breaks a glass vile with a
hammer and places the broken glass pieces in a beaker full of oil that has nothing but oil in it.
Then she says “abracadabra” and places her hand in the beaker and takes out an intact vile.
Would you believe that the magic words made the broken pieces combine in the vile again, as the
magician wants you to believe? You probably would not. Maybe the magician hid another vile in
the oil prior to the experiment and just picked it out at the right time? But how could the magician
hide an unbroken vile in transparent oil through which you can see? In this chapter you will learn
how to do this trick!
Lead: So far we have studied mechanical, thermal and electro-magnetic phenomena. These
phenomena seem different on the surface, but can be understood using physical quantities such as
acceleration, force and electric charge and fundamental principles such as momentum, energy,
and charge conservation. We also found that many tools that we invented in mechanics, such as
motion and force diagrams and momentum and energy bar charts, help us understand and build
microscopic models of processes involving fluids, gases and electrically charged objects. One
phenomenon we have not investigated so far is LIGHT. Is light different from everything we
studied before or can we use the principles and tools we mastered before to understand its nature
and apply these ideas for useful purposes? This chapter and the rest of the book will help you see
the connections between the nature of light and many physical phenomena already familiar to us
and the ones that are awaiting our attention.
21.1. Sources and propagation of light, shadows
Light is a big part of everyday life but we rarely question how it works. A long time ago
people thought that humans saw by using special invisible rays emitted by their own eyes. Their
eyes emitted these rays, which then reached an object and wrapped around it to collect
Etkina/Gentile/Van Heuvelen Process Physics 1/e, Chapter 21 21-2
information about it. The rays then returned to the person’s eyes with this information. According
to this model, we can see in total darkness. That idea can be easily checked if you sit for a while
in a completely closed room with no lights or windows. It is difficult to find a room like this so if
you try this experiment on your own make sure that the room is “light-sealed”. You will find that
if the room is totally isolated from the outside, then no matter how long you wait you see nothing.
This experiment rejects the ancient model. There must be some other explanation for how we see
things. Consider the experiments in the Observational Experiment Table 21.1.
Observational Experiment Table 21.1 How do we see objects?
Observational experiment Analysis
(a) In a dark room shine a laser pointer so you see a spot
of light on the wall. You do not see
light going from the laser pointer to
the wall. If you place your hand
(like a screen) along the straight
line connecting the laser to the
bright spot on the wall – the bright
spot disappears. If you place your
hand along a straight line connecting the spot on the wall
and your eye, you do not see the spot.
The light has to travel to the wall along a
straight line. When the light reaches the wall,
it somehow bounces to your eyes so that you
see the bright spot on the wall).
(b) Repeat experiment (a). This time
sprinkle chalk dust along a line from
the laser pointer to the wall. You now
see the path of the light from the laser
to the wall. The path is a straight line
Evidently, you see
the path because
light reflects off the
tiny pieces of chalk
dust into your eyes.
Pattern
We can see surfaces of objects (even the tiny ones) illuminated by light by not light itself. The path of light
is a straight line – from the source of light to the object and from the object to our eyes.
From the pattern in the table above we can conclude that light from a light source
propagates in a straight line until it hits an obstacle from which it reflects. Some of the reflected
light reaches your eyes and some goes in other directions, as others see the same spot on the wall.
Summarizing the findings, three things are needed for us to see something: we need a source of
light, an object from which the light bounces off and our eyes, which record light entering them.
Experiments in the table above indicate that light travels from a source in a straight line
to an object in its path. We can indicate this path with a straight line and arrow showing the
direction of travel. This line with an arrow can be a simple model of the process of light
transmission – a ray of light. A light ray is not a real thing—just a way for us to represent the
direction it travels. Light rays seem real when we see light traveling in a dusty room