Optical Magic of Nature

There is an "Eye of Desert" in the most desolate dead silence place with the diameter of 48km， emerald green， like a giant deep eye staring at the sky... Like a magician， mystical nature uses colorful light to perform optical magic one by one and show us the colorful world.

Blue Sky · White Clouds · Sunset Scattering of Light

Blue sky with white clouds always brings us good mood. Why the sky appears blue， while the clouds appear white？ In fact， the nature utilizes the principle of light scattering to realize the changes in color.

Sunlight is broad-spectrum， and includes infrared light， visible light， and ultraviolet light. Visible lights include lights of different colors， such as red， orange， yellow， green， cyan， blue， and purple lights whose wavelengths decrease in turn with 580nm yellow light as the center wavelength. These lights of different colors get together， and produce white vision in our eyes， so the essential color of the sunlight is white.

Dust， atmospheric molecules and other particles in the atmosphere with the dimension less than the wavelength of light will scatter the light passing through them， such scattering is characterized by the shorter the wavelength of light being， the greater the scattering effect being， so bluish light has strong scattering effect， while reddish light has weak scattering effect. This is why the sky is blue. Clouds in the sky are composed of water droplets in the atmosphere； the dimension of these water droplets is larger than the wavelength of visible light. When light waves of various wavelengths go through water droplets， there is scattering of almost the same intensity. Therefore， the clouds are white for human eyes. Early in the morning or in the evening， the sun looks like a red ball of fire because at the moment the sunlight is almost parallel to the ground plane. The scattering effect is most significant if the atmosphere going through is the thickest. Almost all shortwaves in sunlight side-scatter， only red light of longer wavelength comes to the ground， so we can see fiery-red sunrise and sunset. Should there is no atmosphere scattering， we will see the same sky in the day as that at night， and stars twinkling on a black background， while the only difference is that the very bright sun sparkling on the black background. Such is the scenery we can see on the moon.

You must have heard the story that an expedition is lost in a desert. Suddenly， a water source appears ahead， and reflection of sand dune is rippling in water. The expedition are ecstatic， and run to see as if they are clutching at straws， but what makes them disappointed is that there is nothing. In fact， this is a magic of "mirage" performed by the nature. Temperature in desert is high， and the air temperature is getting higher， the density is getting smaller， and the refractive index is getting lower if it is closer to the desert surface so that the air layer with gradient refractive index is formed. If light from distant sand dune irradiating to ground is refracted when it enters hot air layer of smaller refractive index， the incident angle increases gradually， the route curves gently， and finally total reflection appears when the incident angle increases to the critical angle， the phenomenon is equivalent to specular reflection effect. People will see the reflection of distant sand dune as if distant sand dune is reflected from the surface of the water when looking against the reflected light. While the incident angle for the light shining from nearby sand dune to the ground is small， therefore， total reflection conditions cannot be met， the light will be absorbed by the desert， thus， the reflection of sand dune cannot be seen， and the illusion that there is water will disappear once approached. The explorers are fooled by the nature in this way.

Rainbow · Secondary Rainbow ·

Brocken Bow

Refraction and

Reflection of Light

"There are red， orange， yellow， green， cyan， blue， purple in the sky， like someone dancing with a ribbon in hands，" This is Chairman Mao's praise on rainbow. When showers clear up， and water vapor in the sky does not disappear， you look the direction opposite to the sun， it is like the sky wearing a colored hair band given by the nature， and this is rainbow. Rainbow is generated by refraction and reflection of sunlight on the surface of water droplets in the atmosphere. The light will be refracted on the surface of the droplets after it irradiates on the water droplets， then light of various colors will disperse and deviate， purple light has the maximum deviation， red light has the minimum deviation， and light of other colors deviates in between. Refractive rays come into our eyes after a reflection and refraction. Thus we can see a colorful rainbow. If light comes into our eyes by means of refraction when it produces secondary internal reflection， the sequence of the colors is just the opposite of that of a rainbow， called secondary rainbow. Brocken bow appearing in the mountain clouds is also formed by refraction and reflection inside the water droplets in the clouds. People believe that they see a Brocken bow because there is decree by destiny between them and Buddhahood， but are not aware that Brocken bow is just a little trick played by the nature.

Lunar Corona · Solar Halo

Diffraction and

Refraction of Light

Sometimes when enjoying the moon at night， the bright moon in the dark night sky appears to wear colorful clothes， very beautiful， this is lunar corona. Because there are a lot of tiny water droplets or ice crystals in clouds， the incident sunlight and moonlight will be diffracted in clouds when diameters of these tiny water droplets or ice crystals are several times to dozen times larger than the wavelength. Diffraction is a physical phenomenon propagating deviating from the straight line when light waves meet obstacles of the same size with the wavelength. Light of different wavelengths separates due to diffraction. Therefore we can see small colorful halos around the sun and the moon. The inner ring of aperture is cyanine （purple is less obvious）， the outer is yellow， and the outermost is red， in addition， the smaller the water droplets are， the stronger the diffraction is， and the larger the corona ring is. Because the sunlight is too bright， generally， solar corona is difficult to be observed， however， lunar corona is more common.