LIGHT
Electromagnetic Emissions
THEORIES OF LIGHT

Light used to be thought of as a stream of
particles.
This means light would not be able to diffract.
 This means light can travel through a vacuum
(doesn’t need a medium to travel in).


Light also shows properties of waves.

Light is able to diffract, and can move around
obstacles.
THE FACTS OF LIGHT



Light is the range of frequencies of
electromagnetic waves that stimulate the retina
of the eye (λ between 400 and 700nm).
Light can be defined using the Ray Model of
light.
Light has a finite speed.
THE ELECTROMAGNETIC SPECTRUM
The visible spectrum is a small portion of larger
picture.
 The wavelengths are measured in meters.
 Larger wavelengths are related to smaller
frequencies.

ELECTROMAGNETIC SPECTRUM
THE RAY MODEL OF LIGHT

The Ray Model of light
states that a ray is a
narrow beam of light that
travels in a straight path.
Assumes light travels in a
straight line.
 Ignores the wave-like
properties of light.
 Used to describe how light is
reflected and refracted.

THE SPEED OF LIGHT


Galileo first hypothesized that the
speed of light was an actual number –
not instantaneous. He could not prove
this though.
Danish astronomer Ole Roemer
determined that it took light 22
minutes to travel Earth’s diameter
based on measurements of the orbital
period of one of Jupiter’s moons.
THE SPEED OF LIGHT
Albert Michelson measured the speed of
light to be 2.997996 x 108 m/s.
 This earned him a Nobel prize.
 Accepted speed of light is:

c = 3x108m/s
LIGHT AND MATTER



Transparent: transmit light without
distorting the image.
Translucent: A material that transmits
light but objects cannot be seen clearly
through them.
Opaque: Transmit no light; all light is
absorbed or reflected.
PRISMS AND COLOR
PRISMS AND COLOR

Isaac Newton noticed that light is made of up a
spectrum of colors, that combined is called white
light.
COLORS OF LIGHT



Red, Blue and Green are called the primary
colors of light.
Yellow, Cyan and Magenta are secondary.
Complimentary colors (1 primary and the
opposing secondary) create white light.
COLORS OF LIGHT
THIN FILMS – LIGHT AS A WAVE


In bubbles and oil films rainbows appear.
Not explained by the ray model of light – no
absorption or white light separation!
This is a result of constructive and destructive
interference of light waves!
POLARIZATION OF LIGHT – LIGHT AS A
WAVE



Polarization is the property of transverse waves
that describes the orientation of the waves. Light
waves move in all many directions.
The polarization is perpendicular to the direction
the light travels. Sound is NOT polarized…why?
Polarizers are machines or substances that take
the unpolarized incident waves and convert them
all to a single plane of polarization.
POLARIZATION OF LIGHT

Polarizers reduce the amount of light that enters
an area by cutting out ½ the amount of light. The
end result is light that is less intense.
POLARIZATION OF LIGHT

Reflected light can also be
polarized. This produces
clearer images.
REFLECTION

Light can be reflected. (Specular Reflection)
Regular reflected light results in the angle of incidence and
the angle of reflection to be the same. Regular reflection
requires a smooth surface with reflective properties!
 This is known as the Law of Reflection!

REFLECTION

Specular Reflection occurs
on flat, smooth surfaces.


Therefore to see the
reflected light the observer
must look at the reflected
right at the right angle.
Diffuse reflection results
in a scattering of the rays
into multiple directions.

The reflected rays can be
seen from multiple angles.
REFLECTION

When light strikes a smooth surface with no reflective
properties the result is diffuse reflection (scattered
light).
REFRACTION

Light can be refracted.
When light strikes a new medium some of the light is
reflected, some enters the medium.
 This light that enters the medium changes angles
and velocities.

REFRACTION



The speed of light in a vacuum: c =
3x108m/s.
This value is true for ALL forms of
electromagnetic waves in a vacuum.
In a different medium the waves
slow down. The rate at which they
slow down depends upon the
density of the medium.
INDEX OF REFRACTION

To determine the speed
of light in a particular
medium the following
equation can be used:
c/v = n

n is known as the index
of refraction. Each
substance has its own
index of refraction. The
values for n are never
less than 1!!!
Substance
Index of
Refraction
Vacuum
1.0000
Air
1.0003
Water
1.33
Glass
1.50
Diamond
2.42
Ethyl Alcohol
1.36
REFRACTION

When light passes from one medium to the next:
Some light is reflected at the boundary
 Some is transmitted into the new medium
 The light that enters the new medium will change
angles and speed!

REFRACTION – AIR TO WATER



Initially the light is in the
air.
The light that enters the
water will change angles and
velocity (based on n)
Depends upon 5 things:





Angle of the light entering
the new medium (incident
angle) θ1
Index of refraction of the
incident medium n1
Angle of the light in the new
medium θ2
Index of refraction of new
medium n2
Normal = imaginary line
drawn perpendicular to the
medium
normal
SNELL’S LAW

Snell’s Law puts all of this information together
to create the Law of Refraction.
n1sinθ1 = n2sinθ2
When the ray enters a medium where the speed
is slower, the angle always bends towards the
normal.
 When the ray enters a medium where the speed
is faster, the angle bends away from the normal.

SNELL’S LAW


n for air is lower (higher v)
than n for water  light
bends toward the normal!
n for water is greater than
the n for air  light bends
away from the normal!