grating - Diffraction Grating and Interference APPARATUS 1...

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Diffraction Grating and Interference APPARATUS 1. Spectrometer 2. Diffraction grating 3. Mercury arc lamp 4. Board for mounting glass plates 5. Two plane parallel plates of glass 6. Aluminum stand equipped with a lens, a mirror inclined at 45 , and an index. 7. Sodium lamp 8. Metric ruler (30 cm) On the instructor’s desk the student will find: 9. A hydrogen Geissler tube 10. Tissue paper for cleaning the glass plates 11. A thin strip of paper 12. A small strip of steel INTRODUCTION Part I: The Grating Spectrometer A diffraction grating consists of a large number of fine, evenly spaced parallel slits. There are two types: transmission and reflection gratings. There are two kinds of transmission gratings; one kind has lines ruled on glass, the unruled portions acting as slits, the other kind is a replica of the reflection type. It consists of a piece of gelatin mounted between two pieces of glass, the thinner portions of the gelatin acting as the slits. The reflection grating is formed by ruling lines on a polished metal surface; the unruled portions produce by reflection the same result as is secured by transmission with the other type. The purpose of this exercise is to measure the wavelengths of several spectral lines. The transmission grating, to be used in conjunction with a spectrometer, is a replica. It has from 5,000 to 6,000 lines per cm; the exact number is usually found on the grating. Let the broken line, MN, in Fig. 1 represent a magnified portion of a diffraction grating. Waves start out from all of the slits in phase, so that the phase difference at F between waves from A and C corresponds to the path difference AB. This same difference at F will be present between waves from each two successive slits in the grating. Hence, if AB is equal to λ , or 2 λ , or 3 λ , etc. where λ is the wavelength of the light, waves from all the slits will constructively interfere at F and we shall get a bright image. The images at F when AB = λ , 2 λ , 3 λ , etc. are called the first order spectrum, second order spectrum, third order spectrum, etc., respectively. It is seen from Fig. 1 that, if θ is 1
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M N F G G A C B A B = A C s i n G e n t r a n c e s l i t o b j e c t i v e c r o s s - h a i r s l i g h t s o u r c e 4 5 Figure 1: Schematic layout of the spectrometer.
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