Download Chapter 12: Light

Chapter 12: Light
• Section 1: The Behavior of Light
• Section 2: Light and Color
• Section 3: Producing Light
• Section 4: Using Light
Section 1: The Behavior of Light
The Nature of Light
A basic question: Is light a wave or a particle?
• General properties of waves:
 Propagation within a uniform medium is along straight
lines
 Reflection occurs at a surface or boundary
(known for at least 2,000 years)
 Refraction occurs where a change in the speed of the
wave occurs (studied and observed since 2nd century A.D.)
Light has these properties, so light must be a wave
In 1670, Isaac Newton demonstrated that particles travelled in
straight paths and would reflect and refract, so light could be
consist of particles.
The question on the nature of light was a bit of a controversy.
Since both waves and streams of particles behaved similarly,
what exactly was light?
• The key to answering the question was how the speed of
light would change as it was refracted.
 The wave guys said the speed would decrease, while the
particle guys said the speed would increase
 Once scientists were able to measure the speed of light it
was determined that light’s speed decreased when it was
refracted. It was concluded that light behaved as a
wave.
Section 1: The Behavior of Light
The Nature of Light
In the late 19th century, experiments started showing problems
with the wave model, and Max Planck was able to prove that
light was “bundled”, and he called these bundles of light quanta
• Max Planck demonstrated the particle nature of light by the
year 1901
• In 1905, Albert Einstein was the particle nature of light to
explain the photoelectric effect and further confirmed the
particle nature of light
Today, scientist say that light is both a wave and a particle
simultaneously depending on how it is observed and measured.
This is called the wave/particle duality of light.
Reflection
Reflection can be defined as the bouncing back of a wave or
particle when it strikes a surface or boundary.
• Law of Reflection – the angle the incident ray makes to the
normal is equal to
Incident
Reflecting
the angle the
Wave
Surface
reflected ray make
to the normal line
i
Normal
Line
r
Reflected
Wave
Law of Reflection:
r=
i
Section 1: The Behavior of Light
Refraction
Refraction – the bending of the path of a wave or particle as it
travels from one medium to another
Incident ray
i
As light travels from a less
dense medium to a more
dense medium, the wave
bends toward the normal
medium 1
medium 2
i>
r
r Refracted ray
normal line
Incident ray
i
As light travels from a more
dense medium to a less
dense medium, the wave
bends away from the normal
medium 1
medium 2
i<
r
r
Refracted ray
normal line
Section 1: The Behavior of Light
Refraction
Refraction occurs because the speed of light changes when the
light goes from one medium to another
• We know that the speed of light in a vacuum is 3.0 x 108 m/s.
However, the speed of light in any other medium is always
less than that value
• The ration of the speed of light in a vacuum and the speed of
light in a medium is called the Index of Refraction. That is:
Where: IR = Index of Refraction
c = the speed of light in a vacuum
(3.0 x 108 m/s)
Vm = speed of light in the new
medium (in m/s)
Example 1: The speed of light in a certain substance is 2.21 x
108 m/s. What is the IR of that substance?
c
Solution c = 3.0 x 108 m
IR =
IR =
c
Vm
Vm
s
Vm = 2.21 x 108
IR = ?
m
s
m
s
IR =
m
2.21 x 108
s
IR = 1.36
3.0 x 108
Notice that IR has no units; it is just a number
Section 1: The Behavior of Light
Refraction
The IR for many transparent materials is known. If you know the
IR of a medium you can determine the speed of light in that
medium
Example 2: A transparent solid has an Index of Refraction (IR)
of 2.4195. What is the speed of light in the solid?
Solution
c
IR = c
Vm =
IR = 2.4195
Vm
IR
8
c = 3.0 x 10 m/s
c
(Vm)(IR) =
( Vm )
8m
3.0
x
10
V
m
Vm = ?
s
(Vm) (IR)
IR
Vm =
c
IR
c

IR
Vm =
2.419
Vm = 1.24 x 108
m
s
Snell’s Law
Suppose we have a light traveling from one medium to another.
We know that the speed of the light will changes, and because
of the change in speed, the light will be refracted. Two
questions:
1) At what angle will the light be refracted?
2) Does the angle at which a light travels through one medium
influence the angle at which it refracts upon entering the
second medium?
Section 1: The Behavior of Light
Snell’s Law
This diagram illustrates the variables we need to consider when
trying to answer those questions:
Θ1
medium 1
medium 2
N1
N2
Θ2
Where:
N1 = the IR of medium 1
N2 = the IR medium 2
Θ1 = angle the incident ray makes
to the normal
Θ2 = angle the refracted ray
makes to the normal
A Dutch scientist named Snell determined the relationship to
be: N1 sin 1 = N2sin2
• We can use this equation to determine the angle at which
the light is traveling through either medium, or what of
material either of the medium is by solving the equation for
either N1 or N2
Section 1: The Behavior of Light
Example: A light wave is traveling through crown glass at 45o,
when passes into a mass of sapphire. What is the angle of
refraction?
Solution
N1 sin 1 = N2sin2
N1 sin 1
N2 sin 2
=
N2
N2
N1 sin 1
sin2 =
N2
o
1.5172(sin45 )
sin2 =
1.7604
(1.5172)(0.7071)
sin2 =
1.7604
sin2 = 0.6094
 2 = sin-10.6094
 2 = 37.5o
When solving a Snell’s Law problem your solution must include a
drawing similar to the one in the example labeling the variables
you know and the variable you are solving for.
Section 2: Light and Color
Color and Wavelength
• The colors of the visible spectrum differ from one another in
terms of wavelength
• Each combination of wavelengths has its own special sense
of color
• When white light strikes most objects certain colors are
absorbed and certain colors are reflected, the reflected light
is the color you see
• The human eye is capable of distinguishing among 17,000
colors
How You See Color
• In the retina of the eye there two types of cells that respond
to light
• Rods – are light sensitive and allow you to see in dim light
• Cones – detect color and control the sharpness of the image
you see. They do not work in dim light.
• There are three types of cone cells, each type is sensitive to a
different set of wavelengths
• Red receptors receive only the longer wavelengths
• Blue receptors respond only to the shorter wavelengths
• Green receptors react to light in the middle of the visible
spectrum
• Colorblind – the inability to distinguish certain colors
Section 2: Light and Color
A Model for Color
• Primary colors – red, green, and blue—when combined
these colors produce any other color
• Secondary colors – any color formed by a combination of
two primary colors
• Pigments – colors used to tint other materials
 pigments are used to give paints their colors
Section 3: Producing Light
• Incandescent Light - light produced by heating a piece of
•
•
•
•
metal until it glows
Fluorescent Light - uses phosphors to convert ultraviolet
radiation to visible light
Lasers - produce light waves that have the same wavelength
 To create a laser a number of identical atoms are given a
certain amount of energy. Once each atom has received
that energy the energy is released as a light wave. Each
light wave is identical to the other light waves, that is,
they have the same frequency and wavelength
Coherent light - light of only one wavelength that travels
with its crests and troughs aligned
 Lasers produce coherent light
 The beam from a laser is a constant diameter, the light
does not spread out
Incoherent light - can contain more than one wavelength,
and the electromagnetic waves are not aligned
 Light from ordinary light bulbs is incoherent
Section 4: Using Light
• Linearly polarized light - light with a magnetic field that
•
•
•
•
•
vibrates in only one direction
 A polarizing filter reflects all the waves except those
vibrating in a certain direction
Sunglasses have a vertical polarizing filter, only light waves
vibrating straight up and down pass through the lens. This
vertical polarizer blocks the glare from the sun
Holography - a technique that produces a hologram, a
completer three-dimensional photographic image of an
object
Fiber Optics - transparent glass fibers that transmit light from
one place to another
Total internal reflection - light traveling from one medium to
another is completely reflected at the boundary of the two
mediums
Optical scanner – a device that reads intensities of reflected
light and converts the information to digital signals