Diffraction Grating Experiment 

This video explains Diffraction Grating Experiment in detail.
Aim: To determine the wavelength of light using diffraction grating.
Objectives: To understand the phenomenon of diffraction and use it for determining wavelength of light.
Apparatus: Spectrometer, Hg vapour lamp, diffraction grating, magnifying lens, etc.
Theory: A grating is an arrangement of a large number of parallel slits of same width and separated by equal opaque spaces. This is achieved by scratching a large number of parallel lines on a piece of a glass plate.
When a plane wave front of monochromatic light is incident normally on a grating every point in each slit acts as a source of secondary wavelets spreading in all directions. The central maxima is formed where the secondary wavelets traveling in the same direction as the incident light, is bought to focus. At any other point where the rays travel with an inclination with respect to an incident light, the waves reach in different phases. As a result dark and bright bands are formed on either side of central maximum.
We use a spectrometer, which consists of a collimator to collect light, a prism table to mount the diffraction grating and a telescope to focus the diffraction pattern. The position of the telescope and the prism table can be noted on a circular scale graduated in degrees.
Formula:
(a+b) sin θ = n λ
Where (a+b) is the grating element = 2.54 /15000 (cm/lines),
n is the order of spectrum
λ is the wavelength
Procedure:
1. Level the spectrometer table with a spirit level and focus the telescope and collimator for parallel light.
2. Mount the diffraction grating on the prism table such that it lies normal to the collimator. This is done as follows:-The telescope is brought in line with the collimator so that the image of the slit is seen through the telescope. Now keeping an eye on the scale the telescope is rotated through an angle of 90 degrees. Diffraction grating is mounted on the prism table. The spectrometer table is now rotated towards the telescope till the image of the slit is observed in the telescope. At this point the diffraction grating makes an angle of 45 degrees with the collimator. So looking at the scale the spectrometer table is rotated through an angle of 45 degrees. Now the diffraction grating is normal to the collimator.
3. Move the telescope to observe the diffraction pattern.
4. Begin with second order yellow band. Note down its position in both the windows of spectrometer.
5. Keeping the sense of rotation same, move over to another band of a different colour in second order and note down its position in both the windows of the spectrometer.
6. Now note down the position of the yellow band in the first order and so on till you come to yellow band in second order on other side of central maxima.
7. Repeat the procedure again in the other direction.
8. Note down the diffraction angles for various colours & using the formula calculate the wavelengths of those colours.
Observations:
Least count of Spectrometer table = 30”
(a+b), the grating element = 2.54/15000 (cm/lines)
Conclusion:
The phenomenon of diffraction was understood by calculating λ of light diffracted by diffraction grating.
Questions and Discussion:
1. Explain what is diffraction?
Diffraction is the phenomenon of bending of light about the corners of obstacles.
2. Short note on diffraction grating.
Diffraction grating is a piece of glass on which a large number of lines are scratched. The scratched portions behave like an obstacle and the space in between as a slit. The distance between two scratches/opaque portion is called as the grating element. Grating element controls the amount by which a particular wavelength will diffract.
3. What are the different orders of diffraction and their colours.
Orders of diffraction are taken with respect to the central maxima, which is white in colour, as all the rays pass through un-deviated at the center, giving a central white band (for a rectangular aperture). The first maxima, away from central maxima, on either side of it, are called as Ist order and so on. These maxima show colours as different wavelengths are diffracted through different extend and hence, separate.
4. What is the least count of the spectrometer?
½ min
5. Can diffraction pattern be seen with naked eyes?
Yes. We use telescope in this experiment, to pin point the diffraction pattern, so as to determine the diffraction angle.
6. What are the applications of diffraction?
i. To determine flaws in material
ii. To channelize photons in photonic devices.
iii. To determine lattice constants and hence the type of material and its purity.