The central diffraction peak of the double-slit interference pattern contains exactly nine fringes. What is the ratio of the slit separation to the slit width?

Short Answer

Expert verified
The ratio of the slit separation to the slit width is given by: \[\frac{d}{a} = \frac{9\lambda}{2\arcsin\left(\frac{\lambda}{a}\right)}\]

Step by step solution

01

Finding the width of the central peak

The width of the central peak is defined as the angular range between the first minimum on either side of the central maximum. This can be found using the equation: \[\theta_{min} = \arcsin\left(\frac{m\lambda}{a}\right)\] where \(m\) is the order of the minimum (in this case, \(m=1\)), \(\lambda\) is the wavelength of the light, and \(a\) is the width of a single slit. To find the angular width of the central peak, we can calculate the difference in angles of the first minimum on either side: \[\Delta\theta = 2\theta_{min} = 2\arcsin\left(\frac{\lambda}{a}\right)\]
02

Finding the angular separation of two fringes

The angular separation between any two adjacent fringes in the interference pattern can be found using the equation: \[\delta\theta = \frac{\lambda}{d}\] where \(d\) is the separation between the two slits.
03

Relating the width of the central peak to the number of fringes

Given that the central diffraction peak contains exactly nine fringes, we can set up a proportion to relate the width of the central peak and the angular separation of two fringes: \[\frac{\Delta\theta}{\delta\theta} = \frac{9}{1}\]
04

Solving the ratio of slit separation to slit width

Substituting the expressions for \(\Delta\theta\) and \(\delta\theta\) into the proportion, we get: \[\frac{2\arcsin\left(\frac{\lambda}{a}\right)}{\frac{\lambda}{d}} = \frac{9}{1}\] Now, we need to solve for the ratio \(\frac{d}{a}\): \[\frac{d}{a} = \frac{9\lambda}{2\arcsin\left(\frac{\lambda}{a}\right)}\] Since we only need the ratio of slit separation to slit width, we can leave the answer in this form. Therefore, the ratio \(\frac{d}{a}\) is given by: \[\frac{d}{a} = \frac{9\lambda}{2\arcsin\left(\frac{\lambda}{a}\right)}\]

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