# Light interference

1. Feb 3, 2005

### matpo39

im a little stuck on part b of this question.

Light of wavelength 633 nm from a helium-neon laser is shone normally on a plane containing two slits. The first interference maximum is 85 cm from the central maximum on a projection screen 14 m away.

(a) Find the separation of the slits.

(b) How many interference maxima (including the central maximum) can be observed, assuming your projection screen can be as large as you want?

i was able to get part a by using the small angle formula y=(R*m*x)/d
where y= the maxima seperation, R=the distance to the screen, x=the wave length, d=the slit seperation and in the case for part a m=1
i found d to be 10.4 micro meters which turned out right. now for part b) my initial reaction was that since the screen is infinit and that m=+-1,+-2.... and since the wave length is fixed at 633 nm that there would be infinite maxima points.

can any one verify this for me since i am unsure if this is correct.
thanks

2. Feb 3, 2005

### Pseudopod

I'm not sure if that's correct - it could be. The way I would go about solving part b would be to find an equation relating the angle $$\theta$$ (as viewed from the slit's perspective, $$\theta$$ would be the angle between the middle maxima and any given "mth" maxima) to the integer number m of the maxima you are looking at. You could then plug in $$\theta=90$$ and solve for the mth maxima.