•Light
Light, sounds, and odors can
identify the pleasing
environment of this garden.
But light provides the most
information. Sounds and
odors can be identified and
studied directly, but light can
only be studied indirectly,
that is, in terms of how it
behaves. As a result, the
behavior of light has
stimulated thinking scientific
investigations and debate for
hundreds of years. Perhaps
you have wondered about
light and its behaviors. What
is light?
• Sources of Light
• Luminous
– When something produces light it is said to be luminous
• Incandescent
– When light is given off as a result of high temperatures
an object is said to be incandescent.
– The electromagnetic wave model is used to explain
incandescent light.
• Black Body Radiation
– The radiation given off by a mass at any temperature.
Three different objects emitting blackbody radiation at
three different temperatures. The intensity of blackbody
radiation increases with increasing temperature and the
peak wavelength emitted shifts toward shorter
wavelengths.
Sunlight is about
9 percent
ultraviolet
radiation, 40
percent visible
light, and 51
percent infrared
radiation before it
travels through
the earth's
atmosphere.
• Properties of Light
Light rays are
perpendicular
to a wave front.
A wave front
that has
traveled a long
distance is a
plane wave
front, and its
rays are parallel
to each other.
The rays show
the direction of
the wave
motion.
• Light interacts with matter
– A ray of light travels in a straight line from a source until
it encounters some object or particles of matter.
• What happens to the light depends on:
– the smoothness of the surface
– the nature of the material
– the angle that the light strikes the surface.
– If a surface is perfectly smooth, rays of light undergo
reflection
– If a surface is not smooth the light rays are scattered in
many random directions as diffuse reflection takes place.
– Materials that allow transmission of light are called
transparent.
– Materials that do not allow the transmission of light are
called opaque.
– Vertical rays of light are mostly transmitted through a
transparent material with some reflection and some
absorption.
– If the rays strike the surface at some angle much more of
the light is reflected.
Light that interacts
with matter can be
reflected, absorbed
or transmitted
through transparent
materials. Any
combination of
these interaction
can take place, but
a particular
substance is
usually
characterized by
what it mostly does
to light.
• Reflection
– Most of the objects that we see are visible from diffuse
reflection
– The rate at which groups of rays are spreading or their
essentially parallel orientation carries information about
distances.
– Light rays that are diffusely reflected move in all
directions while light rays reflected from a smooth
surface move out in definite directions.
(A)Rays reflected
from a perfectly
smooth surface are
parallel to each
other.
(B) Diffuse
reflection from a
rough surface
causes ray to travel
in many random
directions.
– When an object is reflected in a mirror there are three
lines of importance.
• The incident ray which is a line representing the
original ray.
• A reflected ray, which represents the reflected ray.
• A normal line which is a reference line that is
perpendicular to the reflecting surface.
• The angle between thee incident ray and the normal is
the angle of incidence.
• The angle between the reflected ray and the normal is
the angle of reflection.
• The Law of Reflection states that the angle of
incidence equals the angle of reflection.
Bundles of light rays
are reflected
diffusely in all
directions from every
point on an object.
Only a few light rays
are shown from only
one point on a tree in
this illustration. The
lights that move to
your eyes enable you
to see a particular
point from which
they were reflected.
Adjacent light rays spread farther and farther apart after
reflecting from a point. Close to the point, the rate of
spreading is great. At a great distance, the rays are almost
parallel. The rate of ray spreading carries information
about distance.
The law of reflection states that the angle of incidence ((i)
is equal to the angle of reflection ((r). Both angles are
measured from the normal, a reference line drawn
perpendicular to the surface at the point of reflection.
Light rays leaving a point on the block are reflected
according to the law of reflection, and those reaching your
eyes are seen. After reflecting, the rays continue to spread
apart at the same rate. You interpret this to be a block the
same distance behind the mirror. You see a virtual image of
the block, because light rays do not actually move from the
image.
• Refraction
– When a light ray moves from one transparent material to
another, such as from water through air, the ray
undergoes a change in the direction of travel at the
boundary.
• This change in direction is called refraction
– Refraction results from a change in speed when light
passes from one transparent material into another.
– Different materials transmit light at different speeds.
• When light moves from one material to another with a slower
speed of light, the light is refracted toward the normal
• When light passes from one material into another with a faster
speed of light, the light is refracted away from the normal
– The magnitude of refraction depends on:
• The angle at which the light strikes
• The ratio of the speed of light in the two materials.
• A ray of light that is perpendicular to the surface (90O) is not
refracted at all.
A ray diagram shows refraction at the boundary as a ray
moves from air through water. Note that (i does not equal
(r in refraction.
(A) A light ray moving
to a new material with
a slower speed of light
is refracted toward the
normal ((i(r). (B) A
light ray moving to
anew material with a
faster speed is
refracted away from
normal ((i<(r).
When the angle of incidence results in an angle of refraction
of 90o, the refracted light ray is refracted along the water
surface. The angle of incidence for a material that results in an
angle of refraction of 90o is called the critical angle. When the
incident ray is at this critical angle or greater, the ray is
reflected internally. The critical angle or water is about 49o,
and for a diamond it is about 25o.
Mirages are caused by hot air near the ground refracting,
or bending light rays upward into the eyes of a distant
observer. The observer believes he is seeing and upside
down image reflected from water on the highway.
• Dispersion and color
– Electromagnetic waves travel at the speed of light with a
whole spectrum of waves of various frequencies and
wavelengths.
– The speed of an electromagnetic wave (c) is related to the
wavelength () and frequency (f) by
• c=f
– Visible light is the part of the electromagnetic spectrum
visible to humans
– Six colors
• Red.
• Orange.
• Yellow.
• Green.
• Blue.
• Violet.
Light travels in a straight line, and the color of an object
depends in which wavelengths of light the object reflects.
Each of these flowers absorbs the colors of white light and
reflects the color that you see.
It also transmits some light
to a person behind the
mirror in a darkened room.

A one-way mirror reflects
most of the light that
strikes. 
The flowers appear to be red because the reflect light in
the 7.9 x 10 -7 m to 6.2 x 10-7 range of wavelengths.
– Light is interpreted to be white if it has the same mixture
as the solar spectrum.
– Light that is composed of several colors is called
polychromatic light
– Light that is composed of only one wavelength is called
monochromatic light.
– A glass prism separates light into a spectrum of colors
because the index of refraction is different for different
wavelengths of light.
– An transparent material in which the index of refraction
varies with wavelength has the property of dispersion.
• Evidence for waves
• Diffraction
– Diffraction is the bending of light around an opaque
object.
– The determining factor is the size of the opening as
compared to wavelength.
– A wave front passing through a large opening will
continue to generate wavelets, so the original shape of
the wave front moves straight through the opening.
– If the opening is very small a single wavelet will move
out in all directions as an expanding arc from the
opening.
(A) Huygens' wave theory
described wavelets that
formed from an older
wave front from a light
source. The wavelets then
moved out to form a new
wave front. (B) As the
wavelets spread, the wave
front eventually becomes
a plane wave, or straight
wave, at some distance.
This continuous process
explained how waves
could travel in a straight
line.
Water waves are
observed to bend
around an obstacle
such as a wall in
the water. Light
appears to move
straight past an
obstacle, forming
a shadow. This is
one reason that
Newton thought
light must be
particles, not
waves.
(A) When an opening
is large as compared to
the wavelength, waves
appear to move mostly
straight through the
opening. (B) When the
opening is about the
same size as the
wavelengths, the
waves diffract,
spreading into an arc.
Huygens' wave theory
explains diffraction of
light. When the opening
is large as compared to
the wavelength, the
wavelets form a new
wave front as usual, and
the front continues to
move in a straight line.
When the opening is the
size of the wavelength, a
single wavelet moves
through expanding in an
arc.
Young's double-slit experiment produced a pattern of bright
lines and dark zones when light from a single source passed
through the slits.
An interference
pattern of bright
lines and dark zones
is produced because
of the different
distances that waves
must travel from the
two slits. (A) When
they arrive together,
a bright line is
produced. (B) Light
from slit 2 must
travel further than
the light from slit 1,
so they arrive out of
phase.
• Polarization
– Unpolarized light consists of transverse waves vibrating
in all random directions.
– Light is polarized if it vibrates in only one direction.
– There are three ways to produce polarized light
• Selective absorption is the process that takes place in certain
crystals where light in one plane is transmitted and all the other
planes are absorbed
• Reflected light with an angle of incidence between 1O and 89O
is partially polarized as the waves parallel to the reflecting
surface are reflected more that other waves.
• Scattering occurs when light is absorbed and reradiated by
particles about the size of gas molecules.
(A)Unpolarized light
has transverse waves
vibrating on all
possible directions
perpendicular to the
direction of travel.
(B) Polarized light
vibrates only in one
plane. In this
illustration, the wave
is vibrating in a
vertical direction
only
(A) Two crystals
that are aligned
both transmit
vertically
polarized light
that looks like
any other light.
(B) When the
crystals are
crossed, no light
is transmitted
Light that is reflected becomes partially or fully polarized
in a horizontal direction, depending on the incident angle
and other variables.
• Evidence for particles
• Photoelectric effect
– Light is a form of energy and it gives off its energy to
matter when it is absorbed.
– Some materials acquire the energy in electrons which can
sometime acquire enough energy to jump out of the
material
• This is called the photoelectric effect.
A setup for observing the
photoelectric effect. Light
strikes the negatively
charged plate, and
electrons are ejected. The
ejected electrons move to
the positively charged plate
and can be measured as a
current in the circuit.
• Quantization of energy
– Vibrating molecules can have energy only in multiples of
certain amounts canned quanta.
– When a photon interacts with matter, it is absorbed and
gives up all of its energy.
– The energy given up by each photon is a function of the
frequency of the light.
• The present theory
• Light has a dual nature.
– Wave properties.
– Particle properties
• The actual nature of the photon is not actually
describable in terms that are very descriptive.
– The duality model does provide a conceptual framework
to at least begin talking about light.
It would seem very strange if there were not a sharp
distinction between objects and waves in our everyday
world. Yet this appears to be the nature of light.