# Homework Help: Youngs double slit experiment

1. Aug 10, 2013

### 5ymmetrica1

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

using the formula Δy = λL/ d calculate the fringe separation (d) for the red and blue light used in the experiment

2. Relevant equations

as above

3. The attempt at a solution

So I rearranged the equation to read

d = λL / Δy

and I'm fine with the math, but I don't understand what wavelength to use, as the following question is.
Using (d) found in the previous question, find the wavelength of the red and blue light used in the experiment.

So say I have
Δy = 5.00 x 10-4m
L = 0.25 m

The equations would be

d = 0.25λ / 5.00 x 10-4

am I supposed to use a separate equation for this formula or do I just use an approximate length of red light such as 450nm?

2. Aug 11, 2013

### technician

I think that you have the terms in the equation confused.
Δy is the fringe separation. d is the distance between the slits (the slit separation)

3. Aug 12, 2013

### rude man

Does it matter?

4. Aug 12, 2013

### rude man

5. Aug 13, 2013

### technician

Only that the equation Δy = λL/d is the more or less standard equation to calculate fringe separation Δy.
Of course you can use any 4 symbols you like and give your own definition of each symbol. That would probably cause some confusion but you would probably get the correct answer.
There must be a good reason why Δy is commonly taken to be a separation between the fringes rather than the distance between the slits.
d is usually taken to be the distance between the slits, as it also is in analysing diffraction gratings.
Keep life (in physics) as simple and conventional as possible. But you are perfectly correct.....does it matter.