Experiment 9 CD diffraction

Purpose:

The purpose of this experiment is to find the groove on the CD use the diffraction method

Equipment:

CD, laser pointer, CD holder structure, board, laser pointer holder, ruler.

Experiment and Data Collection:

We set up the structure like the picture does. Let the laser beam point to the surface of the CD, and use the white board receive the diffraction pattern from the CD.

From this experiment we get data:

The left first maxima distance = 12.9 cm

The right first maxima distance = 13.1 cm

And we use get the distance from the CD to white board L = 17.0 cm

We have the wavelength of laser wavelength from professor is about 670 nm

The equation we use is d = m*wavelength/sinθ, first maximum m = 1, for the angle is very small, so θ = tan^-1(x/L), use this equation we can find the angle θ, furthemore we can get sinθ. Then us equation d = m*wavelength/sinθ, we already know sinθ, m and the wavelength of the laser beam, we can find d.

We get two d one is for the left first maximum: d = 1.1 * 10^-6 m.

We get another one is for the right first maximum: d = 1.08 *10^-6 m.

we get the fact value of the distance between the CD grooves is: 1600nm.

But our experiment value range is smaller compare to the real value.

Conclusion:

in this experiment we learn how to use the diffraction method to find the distance between the CD grooves, and we finally find the distance, but our value is smaller compare to the expect value. We think there are some reason make these error percent, so our experiment is off a little bit. First reason is our measurement with the ruler have some error percent, we may record the value are not accurate, second is the laser pointer position is not vertical to the surface of the CD surface.. Third reason is our set up structure is not stable, may have some change during the measurement cause the error percent.