# Homework Help: The Interference of Light:

1. Jul 28, 2009

### frederickcan

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

A double-slit experiment is performed with light of wavelength 600 nm. The bright interference fringes are spaced 1.8 mm apart on the viewing screen. What will the fringe spacing be if the light is changed to a wavelength of 400 nm?

2. Relevant equations

Fringe spacing equation
delta y = wavelength x L / d

3. The attempt at a solution

I tried to solve the fringe spacing for the wavelength of 600 nm by:

600 x 10^-9 m / .0018 m = 3.33E-4

then

plugged in the 3.33E-4 to solve what the spacing would be for 400 nm by:

dividing 400 x 10^-9 m / 3.33E-4 = 1200000

I think I'm interpreting the fringe spacing equation wrong, and I'm not sure where.

2. Jul 28, 2009

### ideasrule

Check your math; your equations are right, but you made a trivial calculation error.

3. Jul 28, 2009

### frederickcan

The only thing is I'm confused about L. The question didn't provide a value for L, and I have two unknowns (L, and the fringe spacing) so I feel like I'm using the wrong equation. However, because I couldn't find a better equation I set L to 1.

(Also, thank you for your help.)

4. Jul 28, 2009

### rl.bhat

In the problem L and d remain the same. So you can write equation for two wavelengths.
λ1 = x1*L/d....(1)
λ2 = x2*L/d....(2) From the first equation find L/d.
Substitute this value in the second equation.