Diffraction grating

Table of Content
Section Page
Experiment 1………………………………...4
Experiment 2………………………………...6
Areej Al-Jarb
Light is a form of electromagnetic radiation, just like Radio waves, television waves, radar, microwaves,
infrared waves, X-rays, and Gamma rays. One of the characteristic of light is that light is broken up into
discreet units. They are actually bundles of energy which we call photons. Just like in a stream of water,
it is actually water molecules (H2O) which are moving down the river. In a beam of light, it is actually
photons of light which are moving along at the speed of light.
Light, like all electromagnetic radiation, exhibit the properties of wavelength and frequency. So we
know that light acts like a wave.
As the light goes from left to right it actually follows the wavy line that goes up and down as it goes
toward the right. So we can see that the photon of light can vibrate up and down as it goes toward the
Now each photon is independent from the other photons. So we could have some photons vibrate up
and down, others vibrate in other directions. That is exactly what happens. Each photon vibrates in it's
own plane, or it's own direction.
What a polaroid lens does is to let through the light that vibrates only in the proper direction. The
picture shows the first lens (in both Experiment 1 &2), as only letting through the light that vibrates up
and down. All the other light is stopped.
Now, it's what we do with the second lens that determine the outcome of the experiment. If we have
the second lens oriented in the same direction as the first lens, (as in Experiment 1) then only the light
that vibrates up and down will pass through both lenses.
If on the other hand, the second lens is oriented as in Experiment 2, (letting only the light that vibrates
right and left) then no light will reach your eyes.
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Experiment 1
Linear polarization with polarizing lenses
A polarizer only allows light which is vibrating in a particular plane to pass through
it. This plane forms the “axis” of polarization. Unpolarized light vibrates in all planes
perpendicular to the direction of propagation. If unpolarized light is incident upon an
“ideal” polarizer, only half will be transmitted through the polarizer. Since in reality
no polarizer is “ideal”, less than half the light will be transmitted. The transmitted
light is polarized in one plane. If this polarized light is incident upon a second
polarizer, the axis of which is oriented such that it is perpendicular to the plane of
polarization of the incident light, no light will be transmitted through the second
However, if the second polarizer is oriented at an angle so that it is not perpendicular
to the first polarizer, there will be some component of the electric field of the
polarized light that lies in the same direction as the axis of the second polarizer, thus
some light will be transmitted through the second polarizer (see the bottom figure).
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The component, E, of the polarized electric field, Eo, is found by:
E = E0 cos Ø
Since the intensity of the light varies as the square of the electric field, the light
intensity transmitted through the second filter is given by:
I = I0 cos2 Ø
where Io is the intensity of the light passing through the first filter and Ø is the angle
between the polarization axes of the two filters.
Consider the two extreme cases illustrated by this equation:
• If Ø is zero, the second polarizer is aligned with the first polarizer, and the value of
cos2Øis one. Thus the intensity transmitted by the second filter is equal to the light
intensity that passes through the first filter. This case will allow maximum intensity
to pass through.
• If Ø is 90º, the second polarizer is oriented perpendicular to the plane of
polarization of the first filter, and the cos2 (90) gives zero. Thus no light is
transmitted through the second filter. This case will allow minimum intensity to pass
• These results assume that the only absorption of light is due to polarizer effects. In
fact most polarizing films are not clear and thus there is also some absorption of light
due to the coloring of the polaroid filters.
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Polarizing filters (2)
To show that the intensity of the transmitted light through two polarizers
depends on the square of the cosine of the angle between the axes of the two
1. Project the laser beam into the display tank and insert a polarizer into the
path of the beam so that it can be rotated in the plane perpendicular to the
2. Next place a second polarizer (analyzer) between the first polarizer and
the display tank. Rotate the analyzer so it make 0o with polarizer's axis.
3. Place a detector (photometer) in the path of the transmitted beam and
record the intensity (Io)
4. Rotate the analyzer so it make an angle Ø with polarizer, record this angle.
5. From the detector record the intensity.
6. Continue to rotate the analyzer and record the intensity for each
7. Plot a graph to determine the relationship between the light intensity and
the square of the cosine of the angle
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Experiment 2: POLARIMETER
The polarimeter is used for the determination of the concentration of optically active
For this the sodium light with a wavelength of 589 nm is linearly polarized by means
of a polarizer. This light then passes through the solution to be investigated and is
observed through an analyzer. Optically active substances rotate the polarization
plane. By measuring the rotational angle by means of the analyzer, the concentration
of the solution can now be calculated.
1 Polarimeter
a) eyepiece with magnifying lens for
b) scales and vernier
c) adjustment screw for turning the analyzer
d) sample chamber with cover
e) polarizer
f) sodium lamp with cover
g) mains switch
2 Sodium lamp
3 Round cuvette 100 mm
4 Round cuvette 200 mm
Technical data
Measuring range:
Scale division:
Reading precision:
0 ... 180 °
0.05 ° (vernier)
sodium lamps
Wavelength: 589 nm
Round cuvette 100
Length: 100 mm
Diameter: 10 mm
Round cuvette 200
Length: 200 mm
Diameter: 10 mm
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Once the light goes through the first polarizer lens (just like a polarid lens) only the
light that vibrates up and down get through. Now the light enters the tube that is filled
with the active substance. When it goes through the solution, the light begins to twist.
The plane of light changes so that after the light comes out of the tube, it is now
vibrating in another direction, not up and down, but a different direction.
It is the job of the second polarizer lens (analyzer) to determine how much the light
has twisted or rotated. This second polarizer is rotated by the scientist until the light
disappears. Then the angle is noted and recorded. So a Polarimeter actually measures
how much the light has been rotated by a specific substance.
To test another substance, the scientist can replace one tube with another tube that
contains a different substance.
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To measure the coefficient of specific rotation for the sugar solution.
–sugar solution of different concentration (5% , 10% , 15% , 20% , 25% , 30).
1.Optical zeroing (calibration)
Switch on the sodium lamp and wait a short time for it to warm up.
Set the analyzer using the adjustment screw to approx. 90°.
Set the eyepiece to such a value that a clearly delimitated bright circle can be
Turn the analyzer with the adjustment screw to 0° so that the circle is uniformly
lit (dark), i.e. so that no central line is visible. When the analyzer is turned about
the zero position the central line and the two edge areas become light or dark.
For this reason deviations from the zero position can very easily be observed.
Reading off the angle by means of the vernier scale through the corresponding
magnifying lens in the eyepiece edge should now show 0°.
2.Filling a round cuvette
Hold the round cuvette vertical so that the thicker part is at the top.
Take off the upper screw ring; remove the internal cap, sealing ring and glass
Fill the round cuvette with the liquid to be investigated with as few bubbles as
Replace the glass disc, sealing ring and internal cap and tighten with the screw
3.Determination of the angle of rotation
Place a round cuvette with the liquid to be investigated into the sample chamber
in such a way that the thick part is on top. If necessary collect any air bubbles in
the area of the thick part so that they do not interfere with the path of light. Turn
the analyzer with the adjustment screw in such a way that the observed circle is
again uniformly lit.
Read the angle of rotation α from the scale by means of the vernier scale or work
out the average of the two readings to increase the precision.
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4.Determination of the concentration of a solution
The following applies:
concentration : c
angle of rotation : θ
length of the light path : l dm
specific rotation: s
ml ° /g dm
1. Fill the tube with the first concentration (5%) and maintain the situation of
darkness as the zero order. Record the angle of rotation.
2. Repeat the previous step with the other concentration. Tabulate your results .
3. Plot a graph between the concentration c (x-axis) and the rotation angle θ (yaxis) which will give a straight line.
4. Find the slop of the line then substitute in the relation :
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NAME --------------------------------------
Experiment 1
I0 = ---------------------------
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Experiment 2
l= 2 dm
Slope = --------------------------------s = ---------------------------------------
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