Experiment 12: CD Diffraction

Objective: The purpose of this experiment is to study the diffraction interference. A laser, a DVD, a meter sticks, and a stands and clamps are used. Students have to use the concept of diffaction interference to calculate the distance d between the grooves on the DVD. The calculated value will then be compared to the actual value--760nm.

First, put the laser between the DVD and the white board. Point the laser beam to the groove of the DVD. Adjust the position of the DVD and the laser so that the central reflected point falls back on the laser.  Measure the distance between the first order maxima and the central point (X) . Measure the length between the laser pointer and the DVD (L). It is given that the wavelength of the laser is 632*10^-9nm. Use equation a= wavelength/sinθ to calculate the experimental groove distance on DVD.


Figure 1: the groove distance on the DVD has to be calculated.


Figure 2: the laser is put between the DVD and the whiteboard.


Figure 3: First order maxima can be seen as red dots.

Calculation:

λ=632.8x10^-9m

L=11.7 +/- 1 cm

x=19.65 +/- 1cm

 

 θ=tan^-1 (X/L)
  =59.23
a=λ/sinθ
  = 632.8x10^-9/sin(59.23)
  =736nm

percent error= |(736-760)/760l x100%
                     = 3.158%

Conclusion: The distance of grooves on the DVD is calculated to be 736nm. Compared to the actual value of 760nm, the percent error is 3.158%. This small percent error means the result is reliable and accurate. The zero order maximum of the diffraction pattern striked back to the hole of the laser pointer. The first order maxima is used in this experiment. The higher order maxima are also existed. However, because of the low intensity, they are not seen.