# Maximum slit width of double slit

1. May 9, 2014

### unscientific

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

Estimate maximum slit width of a double slit.

2. Relevant equations

3. The attempt at a solution

Letting the slit width be a,

The intensity profile of such a system is:

$$I = I_0 sinc^2\left( \frac{Nkasin \theta}{2} \right) cos^2\left( \frac{kd sin\theta}{2} \right)$$

I suppose the maximum slit width is when a = d? Such that all the interference maxima coincides with the minima from diffraction envelope. But physically this would mean that both slits collapse into one..

2. May 9, 2014

### dauto

The question is not about the width of the double slits. - It is about the width of the source slit (that's where the light comes from to begin with even before reaching the double slits.

3. May 9, 2014

### unscientific

Hmm, that's interesting. But the first thought is that why would it matter? If the beam is really wide, it would get diffracted the normal way.

4. May 10, 2014

### dauto

It matters because it determines whether or not the incoming light is coherent.

5. May 11, 2014

### unscientific

Well, but there is no path difference before approaching the slit, so why would it matter?