Naturally Occurring Optical Phenomena

Apparent Depth

Dispersion Dispersion is the refraction of white light into separate wavelengths or colours.

Primary and Secondary Rainbows Three things must happen for you to see a rainbow’s colors: The sun must be shining The sun must be behind you There must be water drops in the air in front of you When sunlight shines into a water drop, it refracts the light and separates it into colors (like a prism). Light is refracted, reflected, and refracted again for primary rainbows. Light is refracted, reflected, reflected again, and refracted for secondary rainbows. (See the following diagram)

The smaller the wavelength of the light, the greater the angle of refraction as it exits (Violet light is refracted the most) The colors of the rainbow are divided as they are because the drops at the top are reflecting red light in the direction of our eyes and the drops at the bottom are reflecting violet light in the direction of our eyes (primary). Only the color that we see in each band is being reflected directly to our eyes. The other colors reflected by each drop are traveling at angles that are not incident with our eyes.

Mirages Both refraction and TIR play a role in forming a mirage. What different mediums are involved?

A mirage is a distant object produced as light refracts through air of different densities. The air closer to the ground is warmer (less dense) Since the light rays pass through layers of air with progressively lower indices of refraction, eventually the light is totally internally reflected.

Halos and Sun dogs A halo is produced by the ice crystals in cirrus clouds in the upper troposphere. Six-sided ice crystals that make up those clouds act as prisms and mirrors, refracting and reflecting sunlight between their faces. A sun dog occurs when light refracts through flat, horizontal ice crystals.

Red Suns and Blue Moons RED SUN Near sunrise/sunset, the angle of the sun’s rays strike the atmosphere at a lower angle than during the day. Therefore light enters a deeper layer of the atmosphere and most of the shorter wavelengths of visible light scatter away. The light that reaches our eyes are the longer wavelengths, orange-yellow (clean atmosphere) and red (polluted atmosphere). Blue moons occur when large particles scatter longer wavelengths of visible light and shorter wavelengths reach our eye.