Viva Questions On Diffraction Grating Pdf Download
Niki Wienberg
A diffraction grating is an optical element, which separates (disperses) polychromatic light into its constituent wavelengths (colors). The polychromatic light incident on the grating is dispersed so that each wavelength is reflected from the grating at a slightly different angle. The dispersion arises from the wavefront division and interference of the incident radiation from the periodic structure of the grating.
The dispersed light is then re-imaged by the spectrograph and the required wavelength range is directed to a detection system. Gratings consist of equally spaced parallel grooves, formed on a reflective coating and deposited on a substrate. The shape of the grooves (blaze angle) influences what wavelength range the grating is best optimised for.
The dispersion and efficiency of a grating are dependant on the distance between adjacent grooves and the groove angle. Gratings are generally better than prisms - they are more efficient, they provide a linear dispersion of wavelengths and do not suffer from the absorption effects that prisms have which limits their useful wavelength range.
where: n is the order of diffraction, λ is the diffracted wavelength d is the grating constant (the distance between successive grooves) θi is the angle of incidence measured from the normal and θd is the angle of diffraction measured from the normal. The diagram above shows the orders of the diffracted wavelength. As well as positive orders, light can also be diffracted in the opposite direction (i.e. n = -1, -2 etc.) Higher orders may also appear, but these decrease in intensity. Usually the first order lines (n=1 or n=-1) are the most intense.
These influence the choice of grating line density, blaze angle/wavelength, master (different masters for a given line density and blaze angle yield different efficiency and polarisation characteristics) and grating size.
A diffraction grating is constructed by scratching a flat piece of transparent material with multiple parallel lines. The material can be scratched with a great number of scratches per cm. The grating to be utilized, for example, contains 6,000 lines per cm. The scratches are opaque, but the spaces between them allow light to pass through. When light falls on a diffraction grating, it forms a multiplicity for the source with a parallel slit.
If a peak continually falls on a valley, the waves cancel and there is no light at that location. Also, if peaks constantly fall on peaks and valleys regularly fall on valleys, the light is brighter at that place. Diffraction is an alternative to using a prism to detect spectra.
Consider two rays that originate from the line at an angle θ to the straight line. If the difference in their two path lengths is an integral multiple of their wavelength λ, constructive interference will occur, which is given as:
Problem 3: A grating containing 5000 slits per centimetre is illuminated with monochromatic light and produces the second-order bright line at a 30 angle. Determine the wavelength of the light used? (1 = 10-10 m)
Consider two rays which emerge making an angle with the straight through line.Constructive interference (brightness) will occur if the difference intheir two path lengths is an integral multiple of their wavelength() i.e., difference =n where n = 1, 2, 3, ... Now, a triangle is formed, asindicated in the diagram, for which
n = d sin( )
and this is known as the DIFFRACTION GRATING EQUATION. In thisformula is the angleof emergence (called deviation, D, for the prism) at which awavelength will be bright, d is the distance between slits (note thatd = 1 / N if N, called the grating constant, is the number of linesper unit length) and n is the "order number", a positive integer (n = 1, 2, 3, ...) representing the repetition of the spectrum.Thus, the colors present in the light from the source incident on thegrating would emerge each at a different angle since each has a differentwavelength .Furthermore, a complete spectrum would be observed for n = 1 andanother complete spectrum for n = 2, etc., but at larger angles.Also, the triangle formed by rays to the left of 0o is identicalto the triangle formed by rays tothe right of 0o but the anglesR and L (Right and Left)would be the same only if the grating is perpendicular to the incident beam.This perpendicularity is inconvenient to achieve so, in practice, R and Lare both measured and their average is used as in the grating equation.PROCEDURECalibrating the Spectrometer Read and follow the proceduresfor calibrating the spectroscope found in the previous experiment.The calibration can be performed with the grating in place on the table.Measuring
- CAUTION: The diffraction grating is a photographic reproduction andshould NOT be touched. The deeper recess in the holder is intendedto protect it from damage. Therefore, the glass is on the shallow side ofthe holder and the grating is on the deep side.
- Place the grating on the center of the table with its scratches running vertically, and with the base material (glass) facing the light source. In this way, one can study diffraction without the complication ofrefraction (recall from the previous lab how light behaves when travelingthrough glass at other than normal incidence). Fix the grating in place using masking tape.
- Rotate the table to make the grating perpendicular to the incident beamby eye. This is not critical since the average of R and L accommodates a minor misalignment.
- Affirm maximum brightness for the straight through beam by adjusting thesource-slit alignment. At this step, the slit should be narrow, perhaps afew times wider than the hairline. Search for the spectrum by moving thetelescope to one side or the other. This spectrum should look much like thatobserved with the prism except that the order of the colors as you moveaway from zero degrees is reversed.
- Search for the second- and third-order spectra. Do not measure thehigher-order angles, but record the order of colors away from zero degrees.
- For each of the seven colors in the mercury spectrum, measure the anglesR and L to the nearest tenth of a degree by placing the hairline on the stationary side of the slit.
- Average the right and left angles for each color.
- Use the grating equation with d=(1/6000) cm to find the wavelength for each color.Remember that 108 angstrom = 1 cm.
- Calculate the percent deviation for each wavelength using% deviation = (data-theory)/theory x 100%where "theory" is the tabulated wavelength from the last experiment.Do not ignore the sign; it contains information. A positive % deviation means that the value is above the theory; a negative % deviation means that the value is below the theory.
- Do you notice any systematic problems in your seven % deviations?
- Use the grating equation with the tabulated values of from last time and your measured values of to calculate seven different values of N, the grating constant (N=1/d).
- Average the seven values of N. For the error on N, use thestandard deviation on the mean (SDOM). Compare your answer to the accepted value of 6000 lines/cm. Does your value of N agree with themanufacturer's value within the error range? See Taylor page 5 if you are confused.
- What could be causing any discrepancy?
- Why is it necessary that the base side of the grating face toward thelight source? Draw a ray diagram for the two cases: a) base toward thesource (correct) and b) grating toward the source (incorrect).
- A certain color emerges at 15o in the first-order spectrum.At what angle would this same color emerge in the second order if the samesource and grating are used?
In this experiment, you determine the laser wavelength using a diffraction grating. To cover all the physical concepts associated with this experiment, I have created this quiz. You can attempt it and learn more about the experiment.
You may know that one inch is equal to 2.54 cm. So, if you want to determine the grating element, you can calculate it mathematically. This can be done during a diffraction grating practical lab using any laser.
Interference is the phenomenon based on the principle of superposition. Where intensity distribute of two or more than two waves in a defined manner. For the interference the two sources should emit waves in phase and light of same frequency.
In this experiment grating is a glass plate ruled with very fine, close and parallel lines. Each line is said to be slit. The number of lines may vary in one inch dimension of the grating. For an example 300 LPI (line per inches), 15000LPI, etc.
LASER characteristics are those which remain same for different lasers, whether that is Ruby Laser, Semiconductor laser or He-Ne Laser, etc. In all cases emitted light radiation will be highly direction. That you call uni directional, highly collimated beam, monochromatic, etc. as mention in option one.
For a given diffraction grating, first you see what text is mentioned on that. This text tells about the number of slits in this glass plate. For an example in this case 15000 lines per inch given. So from the result we will get standard value of grating element that is single slit width d. Use it for percentage error.
For the LASER action population inversion is must. In an atom, the electrons absorbs the energy and goes in higher states and after spending the life time come back in its original state. This is transition of the electron from one state to the other. Because this reason electronic configuration changes of the atoms. The change in electronic configuration is termed as excited state of the atom correspond to the absorbed energy.
When atoms stay for longer duration in higher energy state, the amount of atoms in ground state decreases, known as population inversion. This result is opposite to the general case for atoms, where more number of atoms in ground state and few atoms in the excited state.
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