PASCO lab on the interferometer

[big man]

Hey guys,

I did the PASCO lab on the interferometer the other day and just have a problem that I wouldn't mind knowing what caused it.

here is the link to the PASCO lab manual:
http://www.nhn.ou.edu/~johnson/Education/Juniorlab/Optics/Pasco_Interferometers.pdf

Just in case you want to see what we had to do.

Anyway the problem occurred in Experiment 3 in that lab manual.
We had to measure the index of refraction of a laser by placing a glass plate in the arrangement given in the above manual.
Now I did that and tried to make it so that the glass plate was as perpendicular to the laser as I could make it, so as to reduce the effect of the secondary interference pattern. But I still had a very distinct pattern that made it hard to count the number of fringe transitions with varying the angle of the glass plate.

Now the problem is that when I started changing the angle of the glass plate, the fringe transitions were appearing to go to the left, but then when I had moved it maybe 2 degrees the fringe transitions momentarily stopped and then continued, but this time they were 'moving' in the opposite direction. It continued moving in the opposite direction (right) from 2 degrees to the end limit (10 degrees). I asked another group if that happened to them and they said it hadn't so I was just wondering if any of you guys knew what might cause something like that to happen?

I'd appreciate any ideas that I could further look into.

Thanks

 

[CarlB]

I'd be willing to bet that when you started with your glass plate, you weren't precisely perpendicular but were a little too far on the far side. In other words, you were probably perpendicular when you got to 2 degrees.

Try using 2 degrees as your "zero" instead of 0 degrees. If you do this, do you get the correct index of refraction for the glass?

Carl

 

[big man]

ahhh ok...that does work out to a better value.

Thanks for that : )