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Physics Laboratory 2
last update: 2009. 8. 31
Exp. 8. Properties of Light
- Reflection, Refraction, Internal Reflection and Diffraction
of Light -
Purpose of Experiment
The theory that light is kind of electromagnetic wave was predicted by
Maxwell’s electromagnetic theory, proved by Hertz’s electromagnetic
occurrence experiment. At now, there is no more objection of opinion that
light is electromagnetic wave. So in category of general electromagnetic
wave, light is also a propagating wave oscillating electric(magnetic) field.
But, light has a special feature that it can propagate without a medium. It
can be propagated through free space without matter. The sunlight
propagating us is a good example. Also, electromagnetic wave is a
transverse wave. Oscillating direction of electric field is vertical with
propagating direction of wave.
Light also can be propagated through a medium. But, the direction of
propagation is changed when it propagates interface of different medium.
This phenomenon is called ‘reflection and refraction of light.’. We can
apply the reflection and refractive law called ‘Snell’s law’ to reflection and
refraction of light, this phenomenon is also interpreted by Fermat’s
principle – Light propagates between two arbitrary points in shortest time
interval.
In this experiment, we experiment reflection, refraction, and polarization
of light. Also, we experiment total internal reflection and Brewster’s angle
which are highly applicable in an optical communication and laser.
Outline of Experiment

Observe origination, propagation and velocity of electromagnetic wave.

Using laser pointer as light source, observe reflection, refraction and
polarization of light.
- How do you experiment?
- What measure?

Study structure, a kind and function of diffraction grating
- What is the most important part of diffraction grating?
-
Study relationship between structure and character of diffraction
grating.

Experiment the interference of light from diffraction grating.
- Using light of laser pointer, observe character of propagation of light
from diffraction
grating.
- Is interval of diffraction calculated from angle of grating coincides with
given interval?
- Using given interval of diffraction grating, calculate the wavelength of
laser light.
Let know about kind of laser, structure of radiation, structure and characteristic of
diode used as light detector, and DC amplifier week amplifying electric signal.
Experimental Method
These equipments are prepared in the laboratory. (Parentheses mean the number of
them.)
Laser pointer(1)
Polarizer(1)
Transparency Acryl bar(1)
Semicircle plastic instrument(1)
Protactor(1)
Diffraction grating(3 including 600 line/mm, only 2 are given)
Goggle(1)
Spectrum analyzer(1)
Wastepapaer
If you need more stuff, inquire to your teaching assistant or experiment preparation
room (19-114), or prepare yourself.
1) Observe characteristic of light reflecting between interface of different medium.
1.
Expose the light of laser pointer in acryl bar with incident angle .[cf : It can be done with
semicircle instrument filling with water – in experiment 2] - exposing light into a plane.
2.
Measure reflection angle reflecting plane of acryl bar.[cf : Be careful to not seeing laser
directly, and put on goggle when observing rays of light.
3.
Repeat this measurement with more than different 5 angles.
4.
Get a relationship between incident and reflection angle from graph.
2) Observe characteristic of light refracting between interface of different medium.
1.
Fill the water to a semicircle plastic instrument and put the center of graph paper.
2.
With incident angle, expose the light of laser pointer to the center of plane.
3.
Measure refraction angle refracting by water. .[cf : Be careful to not seeing laser directly,
and put on goggle when observing rays of light.
4.
5.
Repeat this measurement with more than different 5 angles.
Get a index of refraction for water n.
3) Observe the total internal reflection of ligh
1.
Fill the water to a semicircle plastic instrument and put the center of graph paper.
2.
With incident angle, expose the light of laser pointer to the center of plane.
3.
Measure refraction angle refracting by water. .[cf : Be careful to not seeing laser directly,
and put on goggle when observing rays of light.
4.
Repeat this measurement with other incident angle and find a incident angle that make
=90.
5.
Using a index of refraction got from 2), compare with theoretical value (3)
4) Observe the Brewster’s angle and polarization of light reflected with Brewster’s angle.
1.
Fill the water to a semicircle plastic instrument and put the center of graph paper.
2.
With incident angle, expose the light of laser pointer to the center of plane. Using the
direction of polarization getting from experiment (4) rotate the light source to direction
of polarization incident light be parallel with incident plane.
3.
Measure reflection and refraction angle from plane. .[cf : Be careful to not seeing laser
directly, and put on goggle when observing rays of light.
4.
Repeat this measurement with other incident angle and find a incident angle that make
amplitude of reflected light = 0
5.
Using a index of refraction got from 2), compare with theoretical value (6).
[cf : Don’t forgot to turn down switch after experiment.]
5) Do Experiment with Spectrum analyzer like that.
. At first, observe the instrument and understand the characteristic. There are two
tube, left lense of tube focuses the light. Right tube roles to observe. Both are
focused on center of the plate(Use diffraction grating or prism). Most important
part in experiment is focusing.
. Above picture an outline of experiment. Observe the split of light passing prism
or diffraction grating vary with a wavelength. Using tube’s length, angle and
grating number of diffraction grating(index of refraction of prism) calculate exact
value of wavelength.
. To rotate a #2 fixed screw while locking #1 fixed screw you can move the right
tube delicately.
) Using laser pointer observe the interference of light passing 600line/mm
diffraction grating.
. Calculate the angle emerging maximum amplitude n=1 and calculate grating
interval d using wavelength of laser
[cf: Never look laser directly.
Observe laser reflected at paper and don’t look long time. Observer should take a
goggle] Compare calculated value with real value and if there is difference find the
reason.
. Using in 1 and given interval of diffraction grating d=(1/600)mm calculate the
wavelength of a laser pointer.
3. Observe a interference of light and calculate the interval of diffraction
grating.
Background Theory
here are three methods making electromagnetic wave;
1.
Applying AC currents in an antenna
2.
Accelerating(or decelerating) the charged particle
3.
Exciting atom or molecule and changing energy level.
Applying AC currents are used when making long wavelength electromagnetic
wave longer than visible ray.
Antenna in FM radio(Left Fig) or Microwave plate antenna(right Fig) apply AC
currents to make electromagnetic wave. Wave emitted these antenna is polarized
determined by shape of an antenna.
One of the most practical mechanisms for producing X-rays is the rapid
deceleration of high-speed charged particles. The resulting broad-frequency
bremsstrahlung(German for “braking radiation”) arises when a beam of energetic
electrons is fired at a material target, such as a copper plate. Collisions with the
Cu nuclei produce deflections of the beam electrons, which in turn radiate X-ray
photons. Orbiting X-ray telescopes have given us an exciting new eye on the
Universe. There are X-ray microscopes, picoseconds X-ray streak cameras, X-ray
diffraction gratings, and interferometers, and work continues on X-ray holography.
Electromagnetic spectrum from X-ray tube composed by continuous spectrum
and steep line spectrum. Line spectrum comes from excitation of target atom and
continuous spectrum comes from decelerated atom.
At low temperatures, atoms tend to be in their ground state; at progressively
higher temperatures, more and more of them will become excited through atomic
collisions. When enough energy is imparted to an atom, whatever the cause, the
atom can react by suddenly ascending from a lower to a higher energy level. The
electron will make a very rapid transition, a quantum jump, from its ground-state
orbital configuration to one of the well-delineated excited states. Usually the
excited atom spontaneously relaxes back to a lower state, most often the
grounded state, losing the excitation energy along the way. This energy
readjustment can occur by way of the emission of light or by conversion to thermal
energy through interatomic collisions within the medium. This light is usually visible
light.
The excitation of atom. (a) The atom absorbs the energy and attains a higher
energy level. (b) With the emission of a photon, it drops back (c) and returns to
the ground state in about 10-8s. Right Fig. about ultraviolet photo shows the
matter with body temperature emits long wavelength invisible light(ultraviolet).
General visible light emitted from matter is incoherent light. But laser can make
coherent amplified light using the stimulated emission method.
These Fig shows a principle of laser(coherent light). In laser (a) excited atom emit
laser and stimulates other atoms (b) these lights reflect between mirrors and
amplifie (c) and emit some fraction.
Speed of a light in vacuum is c, this is constant from nature. In the matter speed
of light becomes slower, the ratio of speed of light c and velocity in matter v is
called index of refraction.
1. Law of reflection
1) The incidented light and reflected light are always in same plane.
2) The angle-of-incidence equals the angle-of-reflection.
2. Law of refraction
1) The incidented light and reflected light are always in same plane
2) There is relation between angle-of-incidence (세타) and angle-of-refract (세타)
(1)
N12 is defined the ratio of speed of light in both medium.
As the angle of incidence exceeds critical angle (세타), there is no refracted ray
and all the light is reflected; this effect is called total internal reflection. Total
internal reflection has found many applications in medical technology, for example,
like optical fiber.
To find (세타), we use (1); we arbitrarily associate subscript 1 with the medium
and subscript 2 with vacuum, and then we finding
(2)
Which gives us
(3)
Optical fiber can transfer information without loss. It is essential in optical
communication. Below fig shows well total internal reflection of light inside optical
fiber.
When light passes medium, speed is slower than that of vacuum c. These
phenomena are occur.
-In sea level, speed of light passing air is 70km/s slower than passing vacuum.
-In water, speed of light is 25% compare with vacuum.
-In glass, speed of light is 25% compare with vacuum.
To get a speed of light in these medium we should consider index of refraction. i.
e.
(4)
There are index of refraction in air, water, glass
A Frenchman, Fizeau, shone a light between the teeth of a rapidly rotating toothed wheel. A mirror
more than 5 miles away reflected the beam back through the same gap between the teeth of the
wheel. There were over a hundred teeth in the wheel. The wheel rotated at hundreds of times a
second - therefore a fraction of a second was easy to measure.
By varying the speed of the wheel, it was possible to determine at what speed the wheel was
spinning too fast for the light to pass through the gap between the teeth, to the mirror, and then
back through the same gap. He knew how far the light traveled and the time it took. By dividing
that distance by the time, he got the speed of light. Fizeau measured the speed of light to be
313,300 Km/s
But using optical fiber you can measure speed of light more accurately. If light passes the length d
optical fiber while time t speed of light is given by;
(5)
We can represent the wave’s polarization by showing the directions of the electric field oscillations
in a head-on view of the plane of oscillation. The electromagnetic waves emitted by a television
station all have the same polarization, but the electromagnetic waves emitted by any common source
of light are polarized randomly, or unpolarized. We can distribute the polarized light into a linearly
polarized or circular polarized whether it rotates or not. General polarization combine linearly and
circular is called elliptically polarized.
Certain materials are inherently dichroic because of an anisotropy in their respective crystalline
structures. The electric-field component of an incident light wave that is perpendicular to the
principal axis is strongly absorbed by the sample. So we can use these materials as polarizers.
When electric field linearly polarized passes polarizer, we can resolve E into two components relative
to the polarizing direction of the sheet. Because transmitted parallel component shrinks as cos(세타),
the emerging wave is given by;
(6)
To linearly polarize the light in He-Ne gas laser, using the phenomenon that when light is incidented
with Brewster’s angle only polarized element vertical with incident plane is reflected.
This is occurred when reflected and refracted angle have 90. When assume angle-of-refract is
angle-of-reflect is
, index of refraction of medium is n, Brewster’s angle is given by;
(7a,b,c))
(8)
Diffraction grating can be considered as small windows with interval d. When two lights from nearby
window focus at on one point P on the screen, interference amplitude will be maximum if path
difference is a integer times of wavelength.
I.e. condition to be maximum amplitude is given by;
(9)
So longer wavelength, maximum occurs in larger angle. If light composed by many coherent
monochromatic wave, there are color spectrums by wavelength. When wavelength is not long, there
is more higher order maximum(n>1).
-Characteristic of diffraction grating
When barrier’s size is like wavelength, diffraction which is characteristic of wave occurs. If diffracted
waves from several barriers meet, interference occur. The place of maximum is different with
wavelength. I.e We can distribute the light vary with wavelength. Diffraction grating uses this
characteristic. With diffraction grating we can choose special wave length from continuous spectrum
or know wavelength of light. If you have interest, do experiment refer past experiment “The thing
occur in diffraction grating”.
References

Constant current(voltage) power supply

Measurement of specific heat of an object - calorimeter

Measurement of temperature by the thermocouple and the digital thermometer

Treatment of measurement data

Analysis method based on the graph

James Joule - The importance of precise measurement
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