# Double slit interference

## Homework Statement

question (iii)

2. Homework Equations
D=sd/λ (where D is the distance from slit to screen, s is the distance to the central maxima, and d is the slit separation)[/B]

## The Attempt at a Solution

I plugged the values s=0.3*10^(-3), d=4.5*10^(-3) and λ=633*10^(-9), and got the answer 2.13m. However, the answer stays that I need to multiply this value by 2, and I don't know why as the formula doesn't say so. Thanks in advance![/B]

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## Answers and Replies

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Hello youmei,

I don't know why as the formula doesn't say so
What exactly is the formula saying (what is it for ? ) And what does the exercise ask ?

PKM
The formula ##\beta=\frac{\lambda D}{d}## denotes the distance of seperation between two adjacent maxima (or minima). But you are given the distance between a maximum & the adjacent minimum.
Can you figure it now?

youmei0426
The formula ##\beta=\frac{\lambda D}{d}## denotes the distance of seperation between two adjacent maxima (or minima). But you are given the distance between a maximum & the adjacent minimum.
Can you figure it now?
thanks a lot!!!

The formula ##\beta=\frac{\lambda D}{d}## denotes the distance of seperation between two adjacent maxima (or minima). But you are given the distance between a maximum & the adjacent minimum.
Can you figure it now?
I came across another similar problem regarding diffraction grating, and there the diffraction angle is from the normal to the first maximum. So I am a bit confused as to what exactly is the angle in the formula? Thanks!

PKM
I came across another similar problem regarding diffraction grating, and there the diffraction angle is from the normal to the first maximum. So I am a bit confused as to what exactly is the angle in the formula? Thanks!
The central maximum resides at the normal (or at the centre). Where should the first maximum occur then? Can you somehow approximately relate this distance to the angle you need?

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