Optics II Mirages and Rainbows

White Clouds and Scattered Light Reflection – occurs when sunlight bounces off a surface at same angle Scattering – occurs when sunlight is deflected in all directions Why is the bottom of thunderstorms dark? White Clouds and Scattered Light

Blue Skies and Hazy Days Selective scattering – ability of molecules to scatter one wavelength better than others The larger the particle, the more they scatter all wavelengths. Haze is white because of this Crepuscular rays – scattering of sunlight by dust and haze to produce white bands of light

The Sun can appear red because it travels through more atmosphere near sunset than at midday Volcanoes can cause red suns for many days Red Suns

Figure 15. 9: Red sunset near the coast of Iceland Figure 15.9: Red sunset near the coast of Iceland. The reflection of sunlight off the slightly rough water is producing a glitter path. Fig. 15-9, p. 420

The Mirage: Seeing Is Not Believing Mirage – an image that appears displaced from its true position Caused by light moving through different air densities Inferior mirage – images that appear lower and inverted from original Superior mirage – images that appear higher than original

Inferior Mirage Warm surface, decreasing temperatures aloft The proper conditions for an inferior mirage are commonly found over a hot road surface during summer, or over an unfrozen lake on a very cold day.

Figure 15.16: Inferior mirage over hot desert sand. Stepped Art Fig. 15-16, p. 424

Superior Mirage Cold surface, warmer temperature aloft The proper conditions for a superior mirage are commonly found over water and over snow-covered surfaces.

Rainbows Water droplet phenomenon When sunlight enters rain drop, it refracts and disperses Red refracts less than violet This light is reflected off backside of raindrop at critical angle Each wavelength is reflected at different angle to our eyes (dispersed) It takes many different raindrops to produced a rainbow (each raindrop gives a different wavelength) Must face the falling rain with sun at your back The primary rainbow, the brightest of the rainbows Rainbows

Figure 15.26: When you observe a rainbow, the sun is always to your back. In middle latitudes, a rainbow in the evening indicates that clearing weather is ahead. Fig. 15-26, p. 429

Figure 15. 28: The formation of a primary rainbow Figure 15.28: The formation of a primary rainbow. The observer sees red light from the upper drop and violet light from the lower drop. Fig. 15-28, p. 430