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
........................................................... 4
Experiment 2:Reflection Diffraction grating……………………………………5
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Areej Al-Jarb
Diffraction grating
2010
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 the grating acts as a dispersive element.
Because of this, gratings are commonly used in monochromators and spectrometers
For practical applications, gratings generally have grooves or rulings on their surface
rather than dark lines. Such gratings can be either transmissive or reflective.
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.
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Areej Al-Jarb
Diffraction grating
2010
Experiment 1: Diffraction grating
THEORY:
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) . The diffraction
grating acts as a "super prism", separating the
different colors of light much more than the
dispersion effect in a prism. 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. long-wave 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
which satisfy the relationship d sinθ/λ=|m| where d is the
diffraction grating constant and m is an integer. Thus, the diffracted light will have
maxima at angles
given by
A spectrometer is used to determine the
diffraction angle for each color. The diffraction
grating diffracts the incident light into identical
spectra on either side of the line of the
undiffracted beam. Note that To accurately
calculate wavelengths on the basis of
diffraction angles, the grating must be
perpendicular to the beam of light from the
collimator.
EQUIPMENT
–Diffraction grating.
–Spectrum tube power supply.
–Spectrum tubes such as Mercury, Helium, Cadmium, etc.
–magnifier.
OBJECTIVE
Measure the wavelengths of the used light.
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Areej Al-Jarb
Results(Diffraction Grating)
1.
Determine the diffraction grating constant
d=
(distance between successive grooves) .
2.
For two orders of the diffraction spectrum (m=1, m=2), find the angle of diffraction for each
color . The angle is measured from the incident beam to the refracted beam.
3.
Determine the wavelength for each color in the spectrum according to the formula:
When (m=1)
Color
(
)
( )
(
)
(
)
(
)
(
)
(
)
(
)
When ( m=2)
Color
(
)
( )
2