# Diffraction Grating Assessed

1) If a diffraction grating has 6,000 grooves per centimeter, to what angle will the first order constructive interference "line" of the Hydrogen Alpha (red) emission be diffracted?

2) Atoms of the hydrogen gas enveloping a particular star absorb the Hydrogen Alpha wavelength from the white light emanating from the surface of the star and reemit that color in random directions, causing the spectrum received by observers on Earth to have a dark line due to the missing light. (Discovered by Joseph Fraunhoffer) Suppose the measured wavelength of the dark line is found to be 6555 Angstroms. Is the radial motion of the star toward or away from Earth? Compute how fast the star is moving relative to the earth.

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...ight, 656.3 nm here.

So, theta = arcsin(lamba/d) (remember m=1 for first order)
theta = .226°

2) We know that since this line should correspond to the Hydrogen alpha line of 6563 Angstroms, the star must be moving relative to the earth, and the observed line of 6555 Angstroms must be due to ...