Phys 4910 Spectroscopy
Spring 2018 Assignment 3

  1. A square diffraction grating is 10 cm on a side, and is ruled with 600 lines per millimeter.
    1. Assuming that light falls on the whole grating, how many lines are illuminated?
    2. What is the maximum resolving power of the grating in 2nd order?
    3. What is the minimum wavelength difference of two green lines (wavelength ~ 500 nm) which can be seen as two separate lines?
  2. Light falls on the grating from question 1 at normal incidence. Diffracted light is to be detected with photomultiplier with a sensitivity which runs from 300 nm to 800 nm.
    1. How many full orders are seen? (A full order is one for which the whole range of wavelengths can be seen for a given value of the order number). For each of them, what is the range of angles covered by that order?
    2. How many partial orders are seen? (A partial order is one for which only a portion of the range of wavelengths can be seen. Other wavelengths are forbidden if the sine of the angle becomes greater than one). For each of them, what is the range of wavelengths that can be seen?
  3. The grating is now placed inside a monochromator, with exit slits which are 75 cm away. What is the spectral width of the exit slits if they are physically
    1. 5 µm wide?
    2. 100 µm wide? (As in question 1, take the wavelength to be 500 nm in 2nd order)
  4. Comment on the relative size of the answer from question 1.3, and the answer you got for each part of question 3.