Atmospheric Refraction and Scattering of Light

Local Effects and Principles of Atmospheric Refraction

Atmospheric refraction is defined as the refraction of light by the Earth's atmosphere. A common observation of this phenomenon on a local scale occurs when viewing objects through a turbulent stream of hot air rising above a fire or a radiator. In such instances, the objects appear to waver or flicker randomly. This effect occurs because the air directly above the fire becomes hotter than the air located further up. The hotter air is lighter and less dense than the cooler air above it. Consequently, the hotter air possesses a refractive index that is slightly less than that of the cooler air. Because the physical conditions of the refracting medium, which is the air, are not stationary, the apparent position of the object as seen through the hot air fluctuates. This local wavering serves as a small-scale example of the broader phenomenon of atmospheric refraction.

The Stellar Scintillation Phenomenon: Twinkling of Stars

The twinkling of a star, known scientifically as stellar scintillation, is caused by the atmospheric refraction of starlight. As starlight enters the Earth's atmosphere, it undergoes continuous refraction before it reaches the observer on the ground. This atmospheric refraction occurs in a medium where the refractive index is gradually changing. Because the atmosphere bends starlight towards the normal, the apparent position of a star is slightly different from its actual position. When a star is viewed near the horizon, it appears to be slightly higher than its actual physical location, as illustrated in Figure $10.9$.

This apparent position of the star is not stationary but undergoes constant, slight changes. These fluctuations occur because the physical conditions of the Earth’s atmosphere are not stationary. Since stars are located at immense distances from Earth, they approximate point-sized sources of light. As the path of the light rays coming from the star varies slightly over time, the apparent position of the star fluctuates and the total amount of starlight entering the eye flickers. This result is that the star appears brighter at some moments and fainter at others, creating the characteristic twinkling effect.

Comparison of Stars and Planets as Light Sources

Unlike stars, planets do not exhibit a twinkling effect. This difference is attributed to the fact that planets are much closer to the Earth than stars. Due to their proximity, planets are perceived as extended sources of light rather than point-sized sources. If a planet is considered as a collection of a large number of individual point-sized sources of light, the total variation in the amount of light entering the eye from all these individual sources will average out to $0$. This averaging process effectively nullifies the twinkling effect, resulting in a steady appearance of planetary light.

Advanced Sunrise, Delayed Sunset, and Solar Disc Distortion

Atmospheric refraction is responsible for the fact that the Sun is visible to observers approximately $2$ minutes before the actual sunrise and continues to be visible for approximately $2$ minutes after the actual sunset. The term "actual sunrise" is defined as the moment the Sun actually crosses the horizon. Figure $10.10$ depicts the relationship between the actual and apparent positions of the Sun with respect to the horizon. Because of the bending of light in the atmosphere, the apparent position of the Sun is higher than its actual position when it is near the horizon. The total time difference between the actual sunset and the apparent sunset is quantified as being about $2$ minutes. Furthermore, the apparent flattening of the Sun’s disc observed at both sunrise and sunset is also a direct consequence of this same atmospheric refraction phenomenon.

The Tyndall Effect and Scattering of Light in the Atmosphere

The Earth's atmosphere consists of a heterogeneous mixture that includes smoke, tiny water droplets, suspended dust particles, and air molecules. When a fine beam of light strikes these particles, the path of the beam becomes visible due to the scattering of light. This phenomenon is referred to as the Tyndall effect. Examples of this include sunlight entering a smoke-filled room through a small hole or sunlight passing through the canopy of a dense forest. The color of the scattered light is dependent upon the size of the scattering particles. For instance, very fine particles scatter mainly blue light, while larger particles scatter light of longer wavelengths. If the size of the scattering particles is sufficiently large, the scattered light may appear white.