Chapter 16 Notes

Optical Phenomena

of the

Atmosphere

A Rainbow

The Nature of Light

The light that forms the array of colors that constitute the rainbow, or the deep blue color of the sky, originates as visible light from the Sun. The interaction of sunlight with the atmosphere creates the numerous optical phenomena that take place in the sky.

Light from the Sun interacts with the gases of the atmosphere, as well as with ice crystals and water droplets, to create the optical phenomena through the properties of reflection, refraction, diffraction and interference.

Reflection of Light by a Smooth Surface

Reflection of Lightby a Rough Surface

Reflection

Light traveling through space travels in a straight line and at a constant speed. When the light enters a transparent medium, like glass, most of the light will travel through the glass at slower speed. Some of the light, however, will bounce off of the surface of the medium; this bouncing back of the light is called reflection.

The law of reflection – the angle at which the light strikes a surface or boundary and the angle at which the light is reflected are equal. If the surface or boundary is not smooth, light will strike and reflect at different angles.

Internal reflection – occurs when light that is traveling through a transparent material reaches the opposite surface and is reflected back in the transparent material. It is an important factor in the formation of optical phenomena, such as rainbows.

Refraction of Light

Refraction

Refraction is the bending of light as it passes obliquely from one transparent medium to another.

Refraction is caused because the velocity of light varies depending on the material that transmits it. In a vacuum, radiation (including light) travel at 3.0x108 meters per second, however other transparent media the speed of light will vary depending on the density of the medium through which it is traveling. The speed of light in a medium is always less than the speed of light in a vacuum.

The bending of light by refraction is responsible for a number of common optical illusions. These illusions occur because our brain perceives bent light as if it has traveled to our eyes along a straight path.

Refraction Causes Sun to Appear Higher

Hot Air Near Surface Refracts Light to Invert Images

Refraction over Air Cooled by Water Makes Ship Appear to Float in Air

Mirages

Mirage – an optical effect of the atmosphere caused by refraction in which the image of an object appears displaced from its true position, Types of mirages include inferior images, superior images, and towering.

Mirages occur when there is a significant difference in the temperature air near the ground to temperature of the air aloft.

Rainbow

Rainbows

An observer on the ground sees the rainbow as an arch-shaped array of colors that trail across a large segment of the sky. Two conditions are required for a person to see a rainbow:1)The Sun must be opposite of a rain shower2)The observer must be between the sun and the rain shower

Rainbows are caused be the refraction of sunlight as it passes through water droplets suspended in the sky. Each color of light travels at a different velocity in water; consequently, each color will be bent at a slightly different angle. Red light travels fastest through the water and is bent the least, while violet light travels slowest and is bent the most. The separation of sunlight into the colors of the rainbow is called dispersion.

Halo around Sun Caused by Cirrostratus Clouds

Halos, Sun Dogs, Solar Pillars

Halos appear as a narrow whitish ring having a large diameter centered on the Sun. Halos occur on days when the sky is covered with a thin layer of cirrus clouds, and is more often viewed in the morning or late afternoon when the Sun is near the horizon.

Sun dogs (or parhelia) are associated with halos and are seen as two bright regions on either side of the Sun

Sun pillars are vertical shafts of light that are most often viewed near sunset or sunrise, and appear to extend upward from the Sun.

Sun Dogs (Parhelia)

Sun Pillar

Halos, sun dogs, and sun pillars are caused by the dispersion of sunlight by the shape and motion of ice crystals in the atmosphere.

Ice crystals occur in four basic shapes: (a)plates, (b) columns, (c) capped columns, and (d) bullets.

A Glory

The Corona

The Glory

Most commonly seen by pilots, a glory consists of one or more colored rings that surround the aircraft’s shadow projected on the clouds below. The glory forms much like the rainbow, but the water droplets are much smaller and more uniform in size.

Corona

More commonly associated with the Moon than the Sun, it is a bright whitish disk centered on the Moon or Sun. A corona is produces when water droplets in a thin layer of water-laden clouds, usually altostratus, scatter light from the illuminating body.

The colors of the corona are the result of diffraction, the slight bending of light as it passes near the edges of cloud droplets, and the interference of the various components of white light.