Michelson Interferometer question

[jader]

Homework Statement

A Michelson interferometer is operated in a vacuum, using monochromatic
light of wavelength 589 nm. The interferometer is set up so that the distances
between the moving mirror and the beam splitter and the fixed mirror and the
beam splitter are equal. A parallel-sided object 1.2 cm in length and refractive
index 1.4900 is then placed between the fixed mirror and the beam splitter.
i)Calculate the path length of the interposed material.
ii)What is the wavelength of the light in the interposed material
iii)What is the change in the optical path length of the fixed arm that results?
iv)What would be the required dimension of a parallel sided object of refractive index 1.6618 placed between the moving mirror and the beam splitter to ensure that both arms had the same optical path length?

Homework Equations

I don't understand what iii) and iv) are asking. What are the arms?

The Attempt at a Solution

r1=1.2cm
λ=589x10^-9m
n1=1.4900
I think I can do i) and ii)
I have reasonable answers for them now :)

I'm so confused

 

[dlgoff]

Take a look at this interferometer drawing. Lm and Lf are the two arms.

 

[jader]

Okay, thanks. :)
I'm still not sure how to go about this... I can't find any equations with Lf and Lm in, except: Optical Path Difference = 2Lm - 2Lf