CHAPTER 6
Scientists’ Ideas
LIGHT INTERACTIONS
In the 17th century, Sir Isaac Newton introduced a theory to explain the behavior of
light. Newton (and many other scientists) theorized light consisted of streams of some
sort of particles (called corpuscles) that were thought to act like tennis balls bouncing off
a wall at a certain angle. The corpuscle theory was effective in explaining most
observable phenomena, but it became very complicated when Newton had to start
introducing more and more different types of corpuscles in order to explain his
observations involving different colors of light. Christian Huygens criticized Newton’s
corpuscular theory of light because of its complexity and in 1678 argued in favor of a
different theory of light. He argued that light is not the transference of substance made
of corpuscles but instead it was the transference of energy.
Both theories were effective in explaining most observations but it wasn’t until the early
19th century that evidence that light was energy instead of a substance grew more
persuasive. The experiments of Augustin Jean Fresnel, Thomas Young, and others
revealed many phenomena that can be understood on the basis of an energy
explanation but not with the corpuscular model.
Some of the scientists’ ideas about light energy are listed below. Read through these
ideas with your team and, below each idea, make a note of the evidence or examples
you have seen in your investigations that support each idea.
Ideas about Light and Vision
Idea L1 - A light interaction occurs when a source of light illuminates an object:
During a light interaction, light energy is transferred from the source to the receiver,
there is a decrease in energy in the source, and there is an increase in energy in the
receiver.
Idea L2 - Light travels in straight lines:
We can represent this idea in diagrams by drawing light rays (straight lines with arrow
heads). [Note: In class we did not obtain direct evidence for this idea. We just state it
here because it is important and is the basis for constructing light ray diagrams.]
Idea L3 - When you look at a light source, light interacts with (enters) your eye.
Evidence/examples:
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Light ray diagram (seeing flashlight)
Flashlight
In terms of energy, when
you see a light source, light
energy is transferred from
the flashlight to your eye.
There is a decrease in
chemical potential energy
inside the source and an
increase in “eye-brain
system” energy inside your
eye/brain.
Eye
S/R Energy diagram (seeing light source)
Idea L4 - When light interacts with a shiny object, the light is reflected in a particular
direction:
The angle at which the light reflects from the surface equals the angle at which the light
strikes the surface.
When you see the mirror image of a source, light reflecting from the shiny surface
interacts with (enters) your eye.
Evidence/examples:
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Scientists’ Ideas: Light Interactions
Idea L5 - When light interacts with a white, non-shiny object, the light is reflected in
all directions away from its surface:
When seeing a white object, light reflecting from the object interacts with (enters) your
eye.
Evidence/examples:
Light ray diagram (shiny object)
Light ray diagram (white, non–shiny
object)
Energy diagram (shiny or white, non–shiny object)
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Idea L6 - When light travels from air into a transparent solid or liquid, or from a
transparent solid or liquid into air, the light changes direction:
This process is called refraction.

For light going from air into the transparent material, the angle at which the light
strikes the surface is less than the angle that the transmitted light makes with the
surface.

For light going from a transparent material into air, the angle at which the light
strikes the surface is greater than the angle that the transmitted light makes with
the surface.
Light going from air into water
Light going from water into air
Evidence/examples:
Idea L7 - When light interacts with a pure black object, all the light is absorbed in the
object and none is reflected:
The temperature of the black object increases. (With real black objects there usually is a
small percentage of light that is reflected. We will ignore this small percentage of
reflected light in our discussion below.)
When you look at a black object, no light interacts with (enters) your eye. When you
look at any object, and no light from that object interacts with (enters) your eye, the
object will appear to be black. (The perception of black arises when no light comes into
your eye from the object, but light does come into your eye from the surroundings.)
Evidence/examples:
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Scientists’ Ideas: Light Interactions
Light ray diagram (black object, person looking at it)
Black Card
ROYGBV
Energy diagram (black object)
Ideas about Light and Color
Idea L8 - Our eye-brain system perceives different energies of visible light entering
our eye as different colors:
From lowest to highest energy, the light energies are perceived as red, orange, yellow,
green, blue and violet. Most sources of visible light emit the whole range of light
energies. Our eye-brain system perceives this mixture as white.
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A light ray diagram that explicitly shows the color components of the light is called a
color light ray diagram.
Color Light ray diagram (seeing white source)
White
light
Eye
sees
white
source
ROYGBV
Idea L9 - When light strikes a color object, the light energies absorbed cause the
thermal energy of the object to increase:
The light energies reflected from a color object (or transmitted through a color gel) that
enter our eye determine the actual color that we perceive the object to be.
Evidence/examples:
Color Light ray diagram (seeing colored object)
White light
source
ROYGBV
R
R
R
R
Red paper
Energy diagram (seeing colored object)
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Scientists’ Ideas: Light Interactions
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