The Physics
Opus in profectus

Diffraction and Interference (Light)

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  1. Measuring the thickness of human hair.
  2. Write something else.
  3. Write something different.
  4. Write something completely different.


  1. Suppose you were the owner of a radio station and that you wanted to extend the range of your broadcast. Would it be wise to erect a second transmitting antenna to broadcast simultaneously with the first? How will a second transmitter affect reception? Explain your reasoning.
  2. For what portion of the electromagnetic spectrum could a picket fence be used as a diffraction grating? Explain your reasoning.
  3. For each of the following examples, state whether the source of light produces a spectrum that is mostly continuous or mostly discrete. Compile your results in a table like the one below.
    Identify the spectral type with a check mark (√)
    source continuous discrete
    blow torch    
    candle flame    
    cathode ray tube    
    emission nebula    
    fluorescent tube    
    glow stick    
    incandescent bulb    
    neon sign    
    plasma display    
    the Sun    
    red hot metal    


  1. The prominent yellow color of excited sodium vapor comes from two lines with nearly the same wavelength — the D lines at 589.0 nm and 589.6 nm. A diffraction grating with 531.5 lines per mm is to be used to observe the discrete spectrum of sodium.
    1. Determine the angular separation between the D lines.
    2. How far away would an observing screen have to be placed for the D lines to be separated by 1.0 mm?
  2. A diffraction grating has 5360 lines per cm.
    1. What range of angles do the visible wavelengths (400–700 nm) span in first order maxima?
    2. How wide would a screen have to be to capture this range of wavelengths if it was placed 1.00 m away?
  3. An unmarked diffraction grating was used to observe the spectrum of hydrogen. A screen was placed 1.0 m away and the distance from the central maximum to the first order bright fringe was measured for each of the four visible lines in the spectrum. The wavelengths of the lines were taken from the National Institute of Standards and Technology Atomic Spectra Database. The data were compiled in a table like the one below.
    The spectrum of hydrogen
    name color λ (nm) x (mm) λ (mm) sin θ
    α violet 410 452    
    β violet 434 483    
    γ blue 486 558    
    δ red 656 874    
    1. Complete the table by converting the wavelength to millimeters and computing the sine of the angle of the fringe.
    2. Construct a graph using the data in the last two columns.
    3. Determine the line of best fit.
    4. Determine the number of lines per millimeter on the unmarked diffraction grating.