# Homework Help: Young's double-slit interference?

1. Mar 11, 2014

### kimlu

1. The problem statement, all variables and given/known data

In a Young's double-slit experiment, a set of parallel slits with a separation of 0.102 mm is illuminated by light having a wavelength of 580 nm and the interference pattern observed on a screen 4.50 m from the slits.
(a) What is the difference in path lengths from the two slits to the location of a second order bright fringe on the screen?
(b) What is the difference in path lengths from the two slits to the location of the second dark fringe on the screen, away from the center of the pattern?

2. Relevant equations

x/L = λ/d
x= 2λL/d

3. The attempt at a solution

2(580E-9)(4.5)/ (.000102)= .051m

I'm supposed to have it in µm and I put 51,176 but I kept getting the question wrong, what am i doing wrong?
Also for the second part I am confused on fringes since its the 2nd dark fringe does that mean the 2nd fringe that is dark which is between the 1st and 2nd order bright or a dark fringe that is 2 which is between the 3rd and 4th order bright. Am i even thinking about this the right way, each dark fringe is 1/2λ correct?

Last edited by a moderator: Mar 12, 2014
2. Mar 11, 2014

### Staff: Mentor

You calculated the distance between things on the screen (and got the conversion m -> µm wrong), that's not what you are supposed to calculate.

The differences in path lengths are comparable to the wavelength.

1/2λ is just one option.

3. Mar 11, 2014

### kimlu

thanx

Last edited: Mar 11, 2014
4. May 7, 2016

### Phil Lawless

Do a search on "Quantum interference with slits" by Thomas Marcella. It is in arxive.org. It gives a full quantum analysis of the two-slit geometry.

5. May 24, 2016

### Staff: Mentor

Be sure to check the date of the post you are replying to...