Transcript lars - ruthedradan

Light
By: Lara Joy Macarine
Other sources of Light
Many substance gain energy and emit
light without being heated very much. they do
this through a process called
luminescence.Some lumminescent materials
glow in the long after they have recieved extra
energy. They are said to be phosphorescent.
Their atoms stay excited for some time before
they de-excite and emit light. Certain
phospherescent materials are used in the
markings that glow on watch faces. Other
luminescent materials emit light only during their
exposure to exciting energy. They are said to be
fluorescent.
Fireflies and a few
other types of organisms
emit light by a process
called bioluminescence.
In this process, chemicals
within the organisms
combine to produce a
different chemical that
has excited atoms. When
the atoms de-excite, they
emit photons.
The sun shines because
nuclear reactions
between hydrogen atoms
within its core produce a
tremendous amount of
energy. Photons and
other kinds of particles
carry the energy wto the
suns surface. At the
surface, these particles
mexcite atoms that then
de-excite by emitting
light. The Earth recieve
part of the Light .
An aurora such as the northern
lights is an emission of light by molecules
of air.When high-speed particles arrive at
the Earth from large eruptions on the sun,
they crash into air molecules. These
collisions excite the molecules with extra
energy. The molecules then release the
energy by giving off light. When the
collisions occur at night the light emitted
may be bright enough to be seen.
A laser is device that produces a
powerful, narrow beam of light in which all
the photons have the same energy travel
in the same direction. Lasers serve as
tools so scientific research surgery, and
telephone communications. They also
have many industrial and military users
Scientists thought of light as a wave
that travels much like a water wave. This
idea of light as a wave was popular
because it explained experiments in which
light created a series of bright and dark
lines called an Interference pattern.
They assumed that light waves
must also travel through some kind of
material, just as water waves travel
through water. Although scientists had no
evidence of this material, they called it the
ether. By the late 1800's, scientists had
concluded that light waves consist of
regions of force known as electric feilds
and magnetic feilds.
A simple moel of light wave begin with a ray (is
straight line) that shows the direction of the light's travel.
Along the ray and perpendicular (at right angles) to it,
short arrows represent the electric feild. Some arrows
point upward fom the ray and other arrows point
downward from it. They vary lenght so that the overall
patternof the tips of the arrows looks like a wave. Arrows
representing the magnetic feild also resemble a wave,
but these arrrows make right angles to the arrows that
represent the elctric feild. These patterns move along the
ray. They are the light.
By the early 1900's, exxperiments had shown
that scientist finally had to give up the idea of an ether.
Many scientist realized that a wave of light, as a regular
varying pattern of electric and magnetic feild, can travel
through empty spaces.
Light waves resemble other types of waves in
some features, including wavelength, frequncy and
amplitude. The wavelength is the distance along a
straigt line from one crest (peak) of the wave to the next.
The frequency of a wave is the number of times each
second that crests a stationary checkpoint. The
amplitude of a wave is the greatest distance of a crest or
trough (low point) from the ray.
Remember! A simple relation exists betweeen a wave's frequency and
wavelength.
The Higher the frequency, the shorter the wavelength.A wave energy
corresponds to its amplitude. The greater the amplitude, the
more energy

the wave has. The energy of a light wave also corresponds to its frequency.
The wavelength determines the color of the light. Just like a wave.
c  f
where c is the speed of flight in m/s
f is the frequency in Hz
 is the wavelength in m
Is Light a Wave or a Particle?
The best answer is that light is strictly neither. In
some experiments light behaves like a wave, and in others it
behaves like a particle.
Unlike other kinds of waves, light waves in a
vacuum have one speed, and thats peed is the fastest that anything
can travel. Scientist do not understand why this is true. The fact that
light in a vacuum has only one speed forms one of the foundations
of Einstein's theory of relativity.
When light enters a material, it continually runs into
atoms that delay its travel. But between atoms, light travels at its
normal speed.
Light is a transvers wave, meaning that the
vibrations are perpendicular to the motion and speed of
the wave. This is similar to the Slinky held up in the air
between you and your friend, where you move the end
up and down vibrations in the slinky. If you do this with a
set of frequency, you can see standing waves in the
Slinky. If you just jerk the end of the Slinky up and down
once, you can watch the vibration move from your end to
your friends end. This is a moving or propagating
transverse wave. So, in this case, the vibration is up and
down and the motion are perpendicular to each other.
Light starts as a vibrating magnetic feild. The magnetic
feild in a perpendicular direction. The vibrating feild vibration
generates a changing electric feild in a perpendicular direction. The
vibrating electric feild - that is, in the direction of the roriginal
magnetic feild.
These interacting magnetic and electric feilds propagate in
the direction perpendicular to both the electric and magnetic feild
and spread through spaces as a transverse wave. The successive
generate of changing electric and magnetic feilds can only
propagate waves at the speed of light, c , which equals 2.998 x 10
m/s in vacuum.
Another term for light, electromagnetic radiation, refers to
its generation from alternately oscillating electric feilds. These are
the only waves that can travel in a vacuum - sound and water waves
need a medium like air or water to transmit the disturbance.
Light is a very small slice of the
electromagnetic spectrum, just the wavelengths
that our eyes can see, about 380 - 7600nm.
Longer wavelengths (lower energy waves) are
microwave, TV and radio waves , while shorter
wavelengths (higher energgyy waves) are
ultraviolet, X-rays and gamma rays.
Electromagnetic Waves
Because light consists of electric and magnetic feilds, it is
called an electromagnetic wave. The term light commonly refers to
just chose electromagnetic waves that we can see. For light to be
visible, it must have a wavelength within a certain narrow range of
values called the visible spectrum.
Violet light has the shortest wavelength that is visible. Red
light has the longest. Between them lie all the other colors of the
spectrum, each with its own wavelength. Seen together at the same
time, the colors appear as white light. Sunlight is white because it
has all it colors. however, when it passes through a specially
shaped transparent solid called a prism, the different colors
separate and can be seen.
Waves that have wavelengths slightly too short to be seen
are called Ultraviolet Rays. They cause suntan, sunburn, and skin
cancer. Waves with somewhat shorter, wavelengths than ultraviolet
rays called X-rays. These rays can penetraite a person's body.
Doctors and dentists use them to "see" inside the body. Gamma
Rays have even shorterwavelengths than X-rays. They result fomm
nuclear reactions, such as those in the sun.
Waves with wavelength slightly longer than those of red
light are called infrared rays. When you stand in bright sunlight or
in front of a fire, you feel warm largely because of the infrared light
shinng on you. Microwaves and radio waves have no longer
wavelengths than infrared waves.
Sunlight spread into its different colors by a prism and
creates a continous spectrum. From violet to red, the spectrum
blends smoothly from one color to the next. For example, the yellow
comes from sodium atoms. each type of atom can produce only
certain colors.
Scientists can learn what kinds of atoms up a light
source by observing what colors are present in the light. They direct
the light through an instrument called a spectrumeter to separate
the colors. The spectrometer may be simple prism or it may be a
more complicated device.Each type of atom in the sun's atmosphere
absobs certain colors. By nothing whiich colors are removed,
scientists are able to determine what kinds of atoms are in teh
atmosphere of the sun.
How Light Behaves
The study of light is called optics. By
understanding how light behaves, scientists have
learned to design a variety of optical instruments that aid
in the study of the universe. For example, microscopes
enables us to examine extremely small objects, such as
single-celled organisms. With telescopes, we can
observe distant but very large objects, such as galaxies
and planets. Optics also enables us to undersatnd
vision, the colors of the sky, the sparkle of a diamond,
and many other parts of the everyday world.
Illumination
A body that gives off light is called luminous, like all those glowin-the-dark toys, and a body that gives off light when it is heated is
called incandescent when you can see it reflects light towards your
eyes.
The amount of light can be described by giving the radiant flux,
the maount of energy radiated in a unit of time by an
electromagnetic wave source. The part of this radiation that
contributes to our being able to see (the electromagnetic radiation is
only the visible range of wavelength) is called the luminous flux, F ,
and is measured in lumen (lm).
Illuminance, E , is the luminous flux per ssquare meter and is
measured in lux.
If you do not have constant illumination in all directions, you can
specify the luminous intensity of a source in a particular direction, I
, using the unit candela (cd)
Reflection
When ray of light
reaches a surface between two
types of materials, such as air
and glass, several things can
happen. Some of the light may
reflect from the surface, while
some may pass through the
surface. The light that enters
the second material may
refract (cange its direction). In
addition, some light may be
absorbed by molecules on the
surface or within the seconf
material.
A transparent material lets light rays pass through it without
mixing them up. You can see through such material. A translucent
material also allows rays to pass through it, but it mixes them up so
that you cannot see clearly through the material. An opaque
material blocks all light.
The reflection of the ray from a surface resembles the bounce
a pool ball takes at the edge of a pool table. Imagine a line
perpendicular to the reflecting surface. Such line is usually called
the normal. The angle between the path of an incoming ray and the
normal is called the angle of incidence.
The reflected makes the same angle to the normal ray as the
incoming ray, but on the other side of the normal. Reflection works
this way even when it involves rough surfaces. Wherever a ray
reflects from a surface, it has an equal angle to the normal at that
spot as it had before.
Waves behave according to the Law of Reflection, which
states that the angle of reflection equals the angle of incidence.
Angle of incidence is usually defined as the angle of the incoming
wave relative to the normal angle, which is perpendicular to the
reflecting surface. The angle of reflection is also relative to "normal".
A wave Reflects when some otrthe entire wave bounces
back from a boundary between two different media.If you shine a
flasjlight directly at a flat miror, the light bounces back to travel in
exactly the opposite direction.
Is you have a nice, clean, flat, shiny mirror, the light will still
be recognizable as the beam from the flashlight. This is called a
specular reflection. The diffuce reflection the reflected light goes
in many different directions and is so much more spread out.
Refraction
Waves that do not reflect back from a boundary but travel
into the new medium instead are said to refract.
When light passes through a surface, its speed changes.
This happens because the light must travel through a different kind
of molecule than it passedcthrough before. For example, if light
passes from air into glass, it slows because the glass molecules are
more densely packed than the air molecules. If the light enters at
any angle except a right angle, the change in the lights speed
changes its direction of travel. In other words, the light refracts.
Snell's Law
Snell's Law gives a relationship between indices of
refraction, the angles of incidence and refraction, and
speeds in this problem.
We have a relation
v
This result is known as Snell's law after its discoverer, the
seventeenth-century Dutch astronomer WillebrordSnell.To observe
refraction, place a pencil in a glass of water and then look at the
pencil from top and one side. The pencil appears bent at the water
surface. The light from the top part of the pencil comes directly to
your eyes. The rays from the bottom part pass through the surface
between the water and the air. There the rays refract, and so they
seem to have come from a pencil bottom bent from the pencil top.
Index of Refraction
The ratio between the the speed of
light c and it speed vin a particular
medium. The greater the index of
refraction, the greater the extent to which
a light beam is reflected on entering or
leaving the medium. The symbol for the
index of refraction is , so that
n
n  c/v
List of values of n for a number of substance
Substance
n
Air
1.0003
Benzene
1.5
Carbon Disulfide
1.63
Diamond
2.42
Ethyl Alcohol
1.36
Glass, crown
1.52
Glass, flint
1.63
Ice
1.31
Lucite and plexiglass
1.51
Quartz
1.46
Water
1.33
Zircon
1.92
The index of refraction of any material is always compared to a
vacuum, which is given as n=1. (Air's index is 1.0003 and for most
purposes can be rounded to 1.0). Water has an index equal tp 1.333,
and most optical glass has an index of about 1.5 to 1.6.
it is easy to write Snell's Law in terms of the indexes of
refraction n 1 and n 2 of two successive media. This is usually written
in the form
n1
sin i 
n2 sin r
Absorption
Opaque materials absorb certain colors of lights. For instance,
a red book cover exposed to white light looks red because
molecules on its surface absorb all other colors in the light.
Transparent materials also absorb certain colors if they contain dyes
or pigments.
Scattering describes what happens when light rays strike
atoms, molecules, or other individually, tiny particles. These particles
send the rays of light off in new directions that is, they cause the
rays to scatter.
On a clear day, the ocean appears blue because of two processes:
1. the ocean's surface reflects some of the blue light from the
sky toward the observer.
2. light coming directly from the sun enters the water. The
water molecules then scatter more blue rays toward the observer
than they do the other colors in sunlight.