PhysicsLAB Lab
Reflection Gratings: Wavelength of a Helium-Neon Laser

The purpose of this lab is to experimentally determine the wavelength of a helium-neon laser by using a reflection grating.  
 
 
Setup your equipment so that the laser beam can reflect off of the surface of an LP record in such as way that its initially strikes the record at as nearly a perpendicular angle as possible and its reflection is displayed on an adjacent screen.
  
Refer to the following information for the next four questions.

Record which "track" of the record the laser strikes. Then record the length of that track's song in minutes as well as the physical width of the song's "band" in centimeters. Knowing these values will allow you to calculate the distance between the grooves in that section of the record.
time for song = _____ minutes 

width of song's band on the record's surface = _____ meters 

If the record's angular velocity of record is 33 1/3 rpm (revolutions per minute), calculate the number of revolutions - a.k.a., number of grooves - made during the song's duration. 

Calculate the distance between two adjacent grooves. 

Refer to the following information for the next two questions.

Next measure how far the record is from a screen. This will be the value for L. Place a piece of paper on the screen and mark three consecutive bright fringes. By measuring how far apart adjacent fringes are on the screen, you can calculate an average "y" or linear displacement. Remember to include your "target paper" in your final lab report -- don't lose it!
L = L' = _____ meters 

y = _____ meters 

Refer to the following information for the next four questions.

Young's equation for interference states:
 
where
 
Remember that for bright fringes the EPD = mλ where m ∈ {0,1,2,3 ....}.
Use Young's equation to calculate the wavelength of the laser's light. You may let m = 1 since all fringes are symmetrical and are equally bright and equally spaced.
 
λexperimental = _____ meters 

Ask your teacher for the actual wavelength of the laser.
 
λactual = _____ meters 

Calculate a percentage error for your experimental wavelength's value. 

If a CD were to be substituted for the LP record, what effect would it have on the linear deviation, y, measured on the screen (provided L remained relatively constant)?

 



After submitting your results turn in your target paper and your handwritten calculations for each blank in the webform.


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