Diffraction grating
2010
Diffraction Grating
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Areej Al-Jarb
Diffraction grating
2010
Table of Contents
Section Page
Basic Experiments
Experiment 1:Diffraction grating
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Areej Al-Jarb
Diffraction grating
2010
Experiment 1: Diffraction grating
Diffraction grating :
A diffraction grating is an optical instrument consists of a large number of parallel,
closely spaced slits, which splits and diffracts light into several beams travelling in
different directions. The directions of these beams depend on the spacing of the
grating and the wavelength of the light so that the grating acts as the dispersive
element.
When light of a single wavelength , like the 632.8nm red light from a helium-neon
laser strikes a diffraction grating it is diffracted to each side in
multiple orders ,The condition for maximum intensity is the same
as that for the double slit or multiple slits, but with a large number
of slits the intensity maximum is very sharp and narrow, providing
the high resolution for spectroscopic applications. The peak intensities
are also much higher for the grating than for the double slit.
On the other hand if light of different wavelengths is incident on a diffraction grating,
it is diffracted at different angles (each wavelength of input beam spectrum is sent
into a different direction, producing a spectrum), each slit in the grating acts as a point
source propagating in all directions. The light in a particular direction θ is made up of
the interfering components from each slit, producing sharp intensity maxima. longwave light is deviated more than short-wave light. Generally, the phases of the waves
from different slits will vary from one another, and will cancel one another out
partially or wholly. However, when the path difference between the light from
adjacent slits is equal to the wavelength, λ, the waves will all be in phase. This occurs
at angles θm which satisfy the relationship dsinθm/λ=|m| where d is the separation of
the slits and m is an integer. Thus, the diffracted light will have maxima at angles θm
given by
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Areej Al-Jarb
Diffraction grating
2010
The diffraction grating acts as a "super prism", separating the different colors of light
much more than the dispersion effect in a prism.
EQUIPMENT
–Diffraction grating.
–Spectrum tube power supply.
–Spectrum tubes such as Mercury, Helium, Cadmium, etc.
–magnifier.
OBJECTIVE
Measure the wavelengths of the used light.
PROCEDURE
To accurately calculate wavelengths on the
basis of diffraction angles, the grating must be
perpendicular to the beam of light from the
collimator.
1. Align and focus the spectrometer as
described earlier (in the prism spectrometer experiment) . The telescope must be
directly opposite the collimator with the slit in sharp focus and aligned with the
vertical cross-hair.
2. Using the thumbscrews, attach the grating mount so it is perpendicular to the
engraved lines.
4. Insert the diffraction grating into the clips of the mount. To check the orientation of
the grating, look through the grating at a light source and notice how the grating
disperses the light into its various color components. When placed in the grating
mount, the grating should spread the colors of the incident light horizontally, so
rotation of the telescope will allow you to see the different colored images of the slit.
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Areej Al-Jarb
Diffraction grating
2010
6. Rotate the telescope to find a bright slit image. Align the vertical cross-hair with
the fixed edge of the image and carefully measure the angle of diffraction. as shown
in fig.
7. The diffraction grating diffracts the incident light into identical spectra on either
side of the line of the undiffracted beam. Rotate the telescope back, past the zero
diffraction angle, to find the corresponding slit image. Measure the angle of
diffraction for this image.
8. If the grating is perfectly aligned, the diffraction angles for corresponding slit
images will be identical.
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Areej Al-Jarb
Diffraction grating
2010
ANALYSIS
Determine the wavelengths according to the formula:
where
is the wavelength; d is the distance between lines on the diffraction grating
(d =-----------------
line/mm grating);
is the angle of diffraction; and m is the order of the diffraction spectrum under
observation.
When
Color
When
Color
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m=1
(
)
(
)
(
)
(
)
(
)
(
)
(
)
(
)
(
)
m=2
(
)
Areej Al-Jarb