Wave theory predicts diffraction of light (the spreading of light into a region behind an obstruction), but this is not easily observed unless the obstruction has dimensions comparable to the wavelength.

It is possible to see dark fringes between the thumb and forefinger when held very close together. Fresnel demonstrated in 1816 that diffraction phenomena are explained by a wave nature for light.

Diffraction gratings have as many as 12,000 equally spaced parallel grooves. When illuminated with white light, each slit produces a new wave front. These wave fronts interfere and produce pairs of continuous spectra equally spaced on opposite sides of the principle image.

If illuminated with monochromatic light, successive pairs of slit images will appear on either side of the principle image. The first pair are called the first-order images, the second pair are the second-order images.

Through a series of derivations that I don’t particularly care to show you, the following equation emerges: l = d sinqm /m or sinq = m•l/d

l = d sinqm /m d is the distance between the slits, and is called the grating constant l is the wavelength qm is the diffraction angle

A diffraction grating is ruled with 6. 50 x 103 lines per centimeter A diffraction grating is ruled with 6.50 x 103 lines per centimeter. The grating produces a second-order image of a monochromatic light source at a diffraction angle of 55.0°. Calculate the wavelength of the light source in nanometers.

Ex. 10 - A mixture of violet light (l = 410 nm in vacuum) and red light (l = 660 nm in vacuum) falls on a grating that contains 1.0 x 104 lines/cm. For each wavelength, find the angle q that locates the first-order maxima.

For white light, the ordered maxima show all colors For white light, the ordered maxima show all colors. At the first order maxima, the spectrum would range from 24° to 41°. For higher orders the spectra from adjacent orders may overlap. The central maximum is still white.

Common light sources are incoherent Common light sources are incoherent. (radiate in all directions, wide range of frequencies, random phase) If waves have identical wavelengths and are in phase they are said to be coherent (ripple tank).

Light Amplification by Stimulated Emission of Radiation Single frequency-monochromatic Constant phase relationship

Light flash into a ruby crystal sends photons into the crystal Light flash into a ruby crystal sends photons into the crystal. This sends some atoms into a higher energy state, electrons drop into lower energy levels releasing more photons. These photons excite more atoms, etc. This results in an amplification of the photon beam.

Since the crystal is of a specific length, a standing wave of single-frequency light is produced. As intensity increases, photons pass through a partially reflective end in intense pulses of coherent red light.