Science Buddies: "Ask an Expert"

Hello,

I'm the dad of two 10 yr olds trying to duplicate the CD track spacing expiriment to demonstrate diffraction. Their project is due Friday, and we'd love to have this worked out by tomorrow so they can complete the displays.

We are also having trouble making sense of the formulas. I've read through the post a few weeks ago but it doesn't seem to help. We thought by using the sample measurements in the instructions, we could work with the formulas, making adjustments till they make sense (without throwing in measurement errors too). But, we have tried everything and cannot get the numbers to seem reasonable. Here is the formulas we are using:

Thanks in advance for your help!

Note: MSExcel SIN formula requires radians, which I suspect is part of our problem.

laser wavelength 655

incident beam
actual degrees = 70
degrees from normal (70-90) = -20
degrees from normal in radians = -0.349

reflected beam
order = 1
actual degrees = 138
degrees from normal (138-90) = 48
degrees from normal in radians = 0.838
distance 1*655/(sin(0.838)-sin(-0.349)) = 603.6

reflected beam
order = -1
actual degrees = 84
degrees from normal (84-90) = -6
degrees from normal in radians = -0.105
distance -1*655/(sin(-0.105)-sin(-0.349)) = -2758.0

reflected beam
order = -2
actual degrees = 56
degrees from normal (56-90) = -34
degrees from normal in radians = -0.593
distance -2*655/(sin(-0.593)-sin(-0.349)) = 6032.1

Hi there,

I helped a student in a posting last week on this.
The biggest mistake he made was he counted m = 0 as part of his angle.

https://www.sciencebuddies.org/mentorin ... ?from=Home
In the website above go to the picture under "Experimental Procedure" part 2 (the one that shows the protractor with all the laser beams refracted). Note that there are on the picture are m = +1, m = 0, m = -1 and m = -2.
So in your equation you would jot down Thetai = 20deg, Theta+1 = 50 deg, Theta-1 = -10deg (NOTE: USE MINUS DEGREES for "m" order to the right of normal, Theta-2 = -35 deg.

Double check this on your results first. After I helped him correct this mistake his results look great.

Good luck

Forgot to mention:
So do NOT use the angle for m = 0.
One mistake the previous student made was since m = 0 appeared to the left of normal, he would mistakenly count m = 0 as Theta+1 and m = 1 as Theta+2.

ghariman wrote:Hi there,

I helped a student in a posting last week on this.
The biggest mistake he made was he counted m = 0 as part of his angle.

https://www.sciencebuddies.org/mentorin ... ?from=Home
In the website above go to the picture under "Experimental Procedure" part 2 (the one that shows the protractor with all the laser beams refracted). Note that there are on the picture are m = +1, m = 0, m = -1 and m = -2.
So in your equation you would jot down Thetai = 20deg, Theta+1 = 50 deg, Theta-1 = -10deg (NOTE: USE MINUS DEGREES for "m" order to the right of normal, Theta-2 = -35 deg.

Double check this on your results first. After I helped him correct this mistake his results look great.

Good luck

Thank you.

The thing that was not clear (to me) from the instructions was that theta(i) should be a positive value even though it is right of normal. But plugging that in we get reasonable answers, about 1500 nanometers.

You are so generous to assist on this forum.

Randy

Thank you. The formula changes you helped us with worked. Now we are having a different problem. We can't seem to get more than two reflected beams on the DVD (one the . We don't know why. Please reply as soon as possible. Thanks...

---Matthew (the student)

Hi again,

The previous student that I helped out sent me pictures of his protractor with the laser lights being refracted. I can tell you that for his DVD's (not CD) that you will only be able to see m = +1 and m = -1.
He did his incident at 0, 20 and 30 degrees.

rice78746 wrote:Thank you. The formula changes you helped us with worked. Now we are having a different problem. We can't seem to get more than two reflected beams on the DVD (one the . We don't know why. Please reply as soon as possible. Thanks...

---Matthew (the student)