A student holds a diffraction grating close to her eye and looks straight ahead

Question

A student holds a diffraction grating close to her eye and looks straight ahead at a source of blue light With the light source straight ahead the light enters her eye at normal incidence The undiffracted light from the grating falls on the back of her retina. The diffracted light from the grating falls to either side of her retina. If the diffraction grating has 6000 lines per centimeter, what is the angle between the two diffracted beams? If the source is changed to red light, is the angle larger or smaller? For blue light, the angle 2 theta = _______ deg For red light, the angle 2 theta = _________ deg In this lab you will be using Light-Emitting Diodes (LEDs) as a source of photons. A diode is a semi-conductor device that usually only passes current in one direction, a very useful property in circuits. Semiconductors come in several varieties. In a diode there are two adjacent materials, one that has a slight excess of free electrons (known as "n-type", because it has more negative carriers), and one that has extra "holes" for electrons to go (known as "p-type", because it has a deficit of free electrons, and therefore has net positive carriers). [You can think of a "hole" like an empty spot on a crowded freeway - one car can move into it, but that leaves the gap somewhere else. A "hole" has a positive charge of 1.6 times 10^-19 Coulombs, just the opposite of an electron. Both electrons and holes can carry current.] In a diode with no voltage across it (as shown at right), there is a small region, known as the depletion layer, where the extra electrons of the n-type material leak over to "fill in" the holes of the p-type material. In this region there are essentially no free charge carriers, and therefore, no possibility to conduct a current through the diode.

Explanation / Answer

3)nL = d.sin(w)
n = d.sin(w)/L = (1/600000) x sin(90)/586 x 10^-9 = 2.8 = 2

For blue light

sin(w) = 2L/d = 2 x 470 x 10^-9 x 6.0 x 10^5
w] = 34.41 deg

For red light
sin(w)= 2 x 700 x 10^-9 x 6.0 x 10^5
w= 57.14 degree

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