How do I determine the number of fringes in Thermal Expansion Lab?

[ricola_pak]

Thermal Expansion Lab

I'm trying to do the Prelab question 2 in the above lab.

These are the steps I'm taking, please correct me if I am wrong, and please assist me where I need help, thanks!

1) The expansion coefficient of copper is 16.7*10^-6 K^-1, and the length of the bar of copper is 0.15m. To find out how much the bar contracts as it cools from 50 to 22 degrees, do I simply multiply the coefficient by (50-22) and multiply this to the length of the bar?

2) Now, to find out how many fringes I will see as this process occurs, what should I do?

In the first step I have obtained delta L, the change in length of the bar. I also know the relationship delta L = wavelength/2. So I can find out the wavelength, but how does that tell me how many fringes I will be seeing?

3) On the bottom of page 1, it says that the relative changes in length will result in changing the intensity from zero to twice of the original beam... I am not too sure where I can see this relationship?


Thanks a lot!

 

[dextercioby]

For the first part,i'm sure the formula that u have to use is

$$l_{T}=l_{T_{0}}\left[1+\alpha \left(T-T_{0}\right)\right]$$

Daniel.

 

[ricola_pak]

Thanks Daniel. Can anyone else help me to relate this to how many fringes I should observe?