Determine the number of grooves per cm on a CD
The groove spacing is also called the "track pitch". It is the distance between each track. the shorter the wavelength of the laser light they use the closer they can make the grooves and the more information they can get on the disk. Also notice however that the track pitch decreases more rapidly than the wavelength of the lasers in the above table, this is because you can also reduce the track pitch if you increase what is called the Numerical aperture of the lens you use. In fact how much information you can store on a disk depends on how small you can get the "spot size" of the laser beam and the radius of the smallest spot that you can get equals the laser light wavelength divided by the numerical aperture. (R=λ/NA) Method:
1. A table.
2. A CD or DVD (preferably blank)
3. A laser pointer with known wavelength. The wavelength is generally written on the pointer. The inexpensive red lasers are around 670 nm. 4. A ruler or meter scale.
5. A screen. However, the classroom wall can also be used as a screen. slide 3 of 5
In the adjacent figure, the schematic setup of the experiment is shown. 1. Place the CD vertically facing the screen and 1 meter away from it. The readable part of the CD (non-labeled part) should be toward the screen. 2. Fix the laser between the screen and the CD such that light from it incidents directly onto the CD. 3. Switch on the laser. Adjust the screen such that you see the fringes on it. slide 4 of 5
Observations and Calculations
Once the fringes are produced, the distance between the central fringe and the first fringe on one side of it should be measured. Let us call this distance D (in meters). This distance must be measured in order to calculate the angle θ. From the figure,
tan (θ) = D
=> θ = tan-1(D)
d = L/sin(θ) for n=1
=(53×〖10〗^(-2) )(633×〖10〗^(-9) )/(25.5 ×〖10〗^(-2) ) d=1.315 μm...