# Homework Help: Physics-Young's Double Slit Problem

1. Sep 29, 2006

### frankene

Hello! I am stuck on a problem that seems so easy. The question is:

Two slits are 0.143 mm apart. A mixture of red light (wavelength = 665 nm) and yellow-green light (wavelength = 565 nm) falls on the slits. A flat observation screen is located 2.29 m away. What is the distance on the screen between the third-order red fringe and the third-order yellow-green fringe?

I solved for Theta for both red light and the yellow-green light. (m = 3)

Theta = sin-1((m*lamda) / d)

Red: Theta = sin-1((3 * 665e-9) / .143e-3) = .799 degrees

Yellow-green: Theta = sin-1((3 * 565e-9) / .143e-3) = .679 degrees

From there I solved for the distance and subtracted the two:

y = L * tan theta

Red: y = 2.29 * tan(.799) = 0.032m

Yellow-green: y = 2.29 * tan(.679) = 0.027m

I get an answer of 0.005m but the site says my answer is wrong. Any hints as to what I am doing wrong?

Thank you for any help!!

2. Sep 29, 2006

### jpr0

try inputting your answer to the same degree of accuracy as the slit seperation, try 4.804 mm

3. Oct 1, 2006

### frankene

Thank you! It had to be in meters so it took 4.804e-3 meters! I am glad it was an easy fix. I thought I had a grasp of this concept and I was frustrated when I couldn't get the correct answer.

Thank you again!!!