# Physics 312 Lecture: "Diffraction." Apr 27, 1998

• When light passes through a single narrow slit or aperture, it may interfere with itself
• The type of interference depends on the angle away from the center of the slit; the general formula is
```               a sin(theta) =  m lambda             destructive interference
```
where "a" is the width of the slit, and "m" an integer 1, 2, 3 ...
• A single slit produces a central bright maximum, with much fainter maxima to each side
• Any lens or eyeball of finite size is limited in its ability to resolve fine detail by diffraction; the limiting angle of a slit of width "a" is
```                    theta = lambda / a
```
• For a circular aperture of diameter D, the limiting resolution is
```                    theta = 1.22 lambda / D
```
• A mask with many, many slits equally spaced is called a diffraction grating
• A diffraction grating produces many narrow bands of nearly equal brightness; bright spots occur at any angle for which
```                    d sin(theta) = m lambda        m = 1, 2, 3, ...
```
where "d" is the spacing of the slits
• Light of different wavelengths will produce bright spots at different angles after passing through a diffraction grating; the grating creates a rainbow spectrum
• The resolving power of a diffraction grating is a measure of its ability to separate light of nearly equal wavelengths; it is equal to the number of slits times the order of the spectrum

This lecture discusses material in Chapter 38 of Serway.