# Wave Optics: Light in Water & Glass Wedge - Find Thickness/Angle

• lha08
In summary, the conversation discusses the use of a glass wedge submerged in water to illustrate constructive interference with light of a wavelength of 640 nm. The distance between successive bright fringes is given as 6mm. The solution involves finding the change in thickness of the glass between fringes and the angle of the wedge by using the formula 2t = (m+1/2)λ/nfilm, adjusting for the given wavelength in water, and comparing two adjacent fringes.

## Homework Statement

Light of wavelength 640 nm in water illuminates a glass (n = 1.5) wedge submerged in water (n = 1.33). If the distance between successive bright fringes is 6mm.
Find a) the change in thickness of the glass between these fringes
and b) the angle of the wedge

## The Attempt at a Solution

Since it's a bright fringe and n=1 to n=1.5 to n=1.33, it's constructive so 2t= (m+1/2)lambda/nfilm. WHen i plug in the values i get t=(m+1/2)(6.40X10^-7 m)/2(2.5) but I'm not sure how i can find m with the distance between the bright fringes (y=6X10^-3 m)...
And for the angle of the wedge i thought that it would be pi since n=1 to n=1.5 to n=1.33.

lha08 said:
Since it's a bright fringe and n=1 to n=1.5 to n=1.33,
Note that the wedge is submerged in water. Note also that the wavelength is given in water.

Between adjacent bright fringes, what must be the additional phase difference due to the thickness of the glass? What extra thickness is required to produce that phase difference?

Doc Al said:
Note that the wedge is submerged in water. Note also that the wavelength is given in water.

Between adjacent bright fringes, what must be the additional phase difference due to the thickness of the glass? What extra thickness is required to produce that phase difference?
so n=1.33 to n=1.5 to n=1.33 but my formula doesn't change? But i don't know how to use 6mm to get m...like the phase change formula is =2pi(path difference)/lambda or phase change=2pi(m)...? I'm kind of confused at that point..

lha08 said:
it's constructive so 2t= (m+1/2)lambda/nfilm.
This formula is almost right. That formula applies when the wavelength is given in air. How would you modify it to use the given wavelength in water?

Then compare two adjacent fringes, call them:
t1 at m = n
t2 at m = n + 1

Find the change in thickness.