# Physical Optics First Order Maximums

Multiple choice:

1. The middle of the first-order maximum, adjacent to the central bright fringe in the double-slit experiment, corresponds to at a point where the optical path length difference from the tow apertures is equal to: (a) h (=lander) (b) 0 (c) 1/2h (d) 1/4h (e) none of the above. Explain your choice.

2. A narrow slit of width D is illuminated by light comming from a monochromator so that the wavelength can be varied all across the visible spectrum. As h =Lander) decreases, the Faunhofer diffraction pattern, viewed in the focal plane of a lens: (a) shrinks with all fringes getting narrower (b) spreads out with all the fringes getting wider (c) remains unchanged (d) alters such that only the central maximum broadens (e) none of these. Explain your choice.

3. The effect of increasing the number of lines per centimeter of grating is to: (a) increase the number of order that can be seen (b) allow for the use of longer wavelengths (c) increase the spread of each spectral order (d) produce no change in the diffracted light (e) none of these. Explain your choice.

Problem:

Two vertical narrow slits seperated by 0.20 mm are illuminated perpendicularly by 500-nm. A fringe pattern appears on a screen 2.00 m away. How far (in mm) above and below the central axis are the first zeros of irradiance? [Hint: Horizontal slits create a vertical finge pattern composed of horizontal bands.]

#### Solution Preview

... for the diffraction minima is D * sin = n * h, where is the diffraction angle, n = 1,2,3... .

So we can write down for : sin = n * h / D. As we can see, if Lander h decreases, so does the angle for corresponding order minimum. And since the width of each diffraction maximum is related to the distance between adjacent diffraction minima, it also decreases with decreasing h. So the answer to this question is A) - the diffration pattern shrinks with all fringes getting narrower.

3. There is 2 kinds of diffraction maxima in the case of diffraction grating. I am not sure about the correct terms for them ...