Determine how many wavelengths will fit into the glass container when it is a vacuum. Since the light passes through the container twice, you need to determine how many wavelengths will fit into a glass container that has a length of 2L.

   # = 2L /λ

Explanation:

For this exercise we can use a direct proportion rule. If there is 1 wave in 1 wavelength in 2L wave how many lengths are there

                    # = 2L /λ 1 wave

let's calculate

                  # = 2L /λ

we see that the longer the wavelength the fewer waves fit in the container

the answer is f1 and d2 because if you put equal force on those two points the beam would balance trust me on this

the answer is 12 as i may remember

: )