Huygens Principle

Chapter 29 Light Waves

Huygens’ Principle  Every point on a wave front can be regarded as a new source of wavelets, which combine to produce the next wave front, and so forth. This principle can be used to predict the laws of reflection and refraction and the phenomena of diffraction.

Diffraction Light bends as it passes around an obstacle or through narrow slits We find that diffraction (amount that the wave is spread out):  Increases with wavelength o May explain why AM radio is easier to pick up among large buildings  Increases as the entrance slit decreases

Diffraction patterns are series of bright and dark areas (lines) on the observing screen 2-D diffraction patterns can also be observed. The pattern from a screen might look like this Diffraction Screen Pattern

Generally, the diffraction is greatest for slits that are about the same size as the wavelength of light passing through. This sets a limit to microscopy. When the size of the object is about the size of a wavelength of visible light, the diffraction is great enough to make image resolution difficult.

Interference Light can be made to interfere with itself. The interference depends on the wavelength and the phase relationship between the waves. If the two waves are 180° out of phase, total destructive interference results. If the wavelengths are different, a shifting complex wave results. Thin Film Interference results when the incident light reflects from both surfaces of the film. The two reflected waves can interfere and the resultant depends on the path length difference between the reflected waves. Some path length differences will be appropriate to make blue light show constructive interference, while another site line might make a path length difference for constructive for green.

Thin Film The effects can be seen for visible light and path length differences are measured in fractions of a wavelength. Interference effects can therefore be used to measure very small distances.

Polarization Long, stringy molecules can be made in colloidal suspensions that allow light vibrating in the plane parallel to the molecular strings to pass. This demonstrates that light is a transverse wave. If light passing through one polaroid is then passed incident upon a second polaroid with transmission plane perpendicular to the first, no light passes through the second. Think of the polaroid material as a picket fence. Only light vibrating in the same direction as the pickets makes it through. But this light can be blocked by a fence oriented perpendicular to the first. Common light sources are not polarized, but sunlight reflecting off of the sky 90° from the sun is partially polarized. Polaroid sunglasses are effective at eliminating glare in the sky 90° from the Sun. Light reflecting off of non-metallic surfaces is also partially polarized and the sunglasses work here too. Combining polarization effects and thin film interference, materials engineers can study the imperfections in materials or the effect of stress on materials.