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Basic Experiments
Experiment 1:Diffraction grating ...... 6
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 laserstrikes 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. 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 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 θmgiven by
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, thegrating 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.
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.
ANALYSIS
Determine the wavelengths according to the formula:
where is the wavelength; d is the distance betweenlines 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 m=1
Color / ( ) / ( ) / ( ) / ( ) / ( )When m=2
Color / ( ) / ( ) / ( ) / ( ) / ( )1 / Areej Al-Jarb