Interpreting Interference Pattern Shift in Double Slit Experiment

[darkar]

Here's the question,

A double slit experiment uses a helium-neon laser with a wvaelength of 633nm and a slit separation of 0.120 mm. When a thin sheet of plastic is placed in front of one of the slits, the interference pattern shifts by 5.50 fringes. When the experiment is repeated under water ( assuming the laser still works!) the shift is 3.50 fringes. Calculate

(a) the thickness of the plastic sheet and
(b) the index of refraction of the plastic sheet.

THere's some extra question I would like to ask, what exactly does that interference patter shift means? Less 5.5 fringes? or all of the fringes move by 5.5 fringes?

Thanks ~

 

[Doc Al]

The entire pattern shifts (all the fringes move).

 

[darkar]

So, how to solve the questions?

 

[siddharth]

The wavelength of light changes in different medium. So you have to find the optical
path length (ie $\mu d$).If you find the net phase difference(using the optical path length), you can calculate where on the screen the the points of maximum and minimum intensity occurs in each case.