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More Phenomena

The Nature of Light

Page 5:  More Phenomena

Different wavelengths (colors) of light have different refraction angles. For white light, which is a combination of colors, each of the component colors of the white light will have a different refractive index (a different speed of light in the dense optical medium) so that the component colors each have a different refraction angle.

Figure 5.1:  Refraction angle depends on color (drawing not to scale). [NASA]

That is why raindrops cause rainbows. Sunlight enters and exits each raindrop through interfaces that are not parallel.

In lens design, the dispersal of light directions, resulting from different refraction angles for different colors, is called chromatic aberration. Lens designers try to account for this by using special shapes and materials.


Polarization

Polarization is a phenomenon of light that is not usually directly observed by humans. Some animals, however, readily observe and use the polarization of light.

Electromagnetic waves, including light, have electric and magnetic waves. Each electromagnetic wave has an electric wave and a magnetic wave.

Figure 5.2:  Electric and magnetic wave. [NASA]

The electric wave is perpendicular to the magnetic wave, and both are perpendicular to the direction of travel of the wave.

Polarization refers to the orientation of the electric field, whether it is vertical, horizontal, or any other angle. Sunlight is randomly polarized, with each photon (electromagnetic wave) having a different polarization. Reflection changes light polarizations depending on the approach angle of reflection.

“When reflected off a smooth surface, light becomes partially or completely polarized. Reflected off water, the electric field is mainly polarized horizontally.”
— 
Basic Sciences in Ophthalmology, p. 14

Polarization causes glare, which some insects use to locate water (by detecting high intensity light with horizontal polarization).

Reflection of sunlight in the atmosphere polarizes the sunlight. Sunlight in the atmosphere becomes most polarized 90 degrees away from the sun. Some insects are able to detect that polarization in the sky, and use that for navigation.

“polarized signals might be more effective in aquatic than in terrestrial environments. On the one hand, the low refractive index difference between water and natural objects markedly reduces the ‘polarization noise’ …On the other hand, the angle and degree of polarization do not substantially change with increasing distance from the water surface, but the spectral content of light does. Hence, in close-range social interactions, cephalopods and crustaceans seem to have swapped colour for polarization signals.”
— 
Rudiger Wehner

Polarimetry

Satellite remote sensing can emit radar signals of known polarizations to see what polarizations are reflected back, to help determine what type of objects are reflecting.

“A radar antenna can be designed to transmit and receive electromagnetic waves with a well-defined polarization, which is defined as the orientation of the electric field vector in the plane orthogonal to the wave propagation direction. By varying the polarization of the transmitted signal, synthetic aperture radar systems can provide information on the polarimetric properties of the observed surface. These polarimetric properties are indicative of the structure of the surface elements within a resolution element. Oriented structures such as buildings or naturally aligned features (e.g. sand ripples) respond preferentially to oriented polarizations and tend to preserve polarimetric coherence, whereas randomly oriented structures lead to depolarization of the scattered signals.”
— 
NASA Jet Propulsion Lab

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