Atmospheric Refraction: Explanation and Applications

Key Concepts

• Atmospheric refraction
• Twinkling of stars
• Apparent higher position of a star
• Early sunrise and delayed sunset
• Flattened appearance of sun during sunrise and sunset

Introduction: 

In this session, we will get to know the reason behind phenomena like the twinkling of stars, early sunrise and delayed sunset, and flattening of the shape of the sun during sunrise and sunset. 

Explanation: 

Atmospheric Refraction: 

When light gets into a denser medium from a rarer medium, it bends towards the normal line and when it gets into a rarer medium from a denser medium, it bends away from the normal line.  

Our atmosphere is composed of multiple layers, each at a different temperature. The greater the temperature of air, the less optically denser it becomes. Hence, the temperature of a layer of the atmosphere is inversely proportional to its refractive index. As the temperature of the layers keep on decreasing when we go from the outer space toward the earth’s surface, the refractive index keeps increasing. This results in a bending of light continuously at each layer of the atmosphere before reaching the earth’s surface. In other words, the incoming light suffers multiple refractions to reach the earth’s surface in the atmosphere. This phenomenon is called atmospheric refraction. 

The refraction of light caused due to different layers of the atmosphere at different temperatures (different refractive indices) is called the atmospheric refraction. 

Applications of Atmospheric Refraction: 

Higher position of the star and twinkling of stars: 

The light from the star passes through several layers of the atmosphere before reaching an observer’s eyes. The temperature of the air decreases as we move closer to the earth’s surface from the outer space. Therefore, the atmospheric refraction of star light takes place through a medium of gradually increasing refractive index. The light from the star bends continuously towards the normal while reaching the observer on the earth. Finally, when the star light enters the observer’s eyes, it appears as if it is coming from a higher position (as compared to the actual position of the star), where the star is seen to be present. Thus, the star’s position appears to be slightly higher than its actual position. 

The optical density of the atmosphere changes continuously due to the continuous change in the physical conditions of the earth’s atmosphere. Therefore, the apparent position of the star is also not stationary. It changes slightly all the time. As the path of the starlight varies slightly continuously, the apparent position of the star fluctuates and the amount of starlight entering the eye also flickers. As a result, the star sometimes appears brighter and sometimes dull, which is the twinkling effect.  

Early sunrise and delayed sunset: 

The Sun appears to us 2 minutes before the actual sunrise from the horizon and 2 minutes after the actual sunset at the horizon. This phenomenon is due to the atmospheric refraction of sunlight. 

When the sun is just near the horizon, its light rays bend while entering the earth’s atmosphere as shown in the picture to reach an observer’s eyes. When this light enters the observer’s eyes, it appears to them as if the sun is already risen due to the alignment of the final refracted ray entering their eyes. 

A similar phenomenon takes place during sunset as well. The observer keeps receiving the sun light even if it is below the horizon due to atmospheric refraction. This whole process adds up 4 extra minutes to the duration of a day. 

Flattened shape of the sun during sunrise and sunset: 

The sun appears to be slightly flattened during sunrise and sunset. This phenomenon is also due to the atmospheric refraction of sunlight. 

Questions and solutions: 

1. What is atmospheric refraction? List down some of the phenomena that occur due to the atmospheric refraction. 

Answer: 

The refraction of light caused due to different layers of the atmosphere at different temperatures (different refractive indices) is called the atmospheric refraction. 

The following are the phenomena that occur due to the atmospheric refraction: 

1. Twinkling of stars 

2. Apparent higher position of a star 

3. Early sunrise and delayed sunset 

4. Flattened appearance of sun during sunrise and sunset 

2. Why do stars twinkle but the planets do not? 

Answer:  

The stars can be considered to be point sources of light as they are located far away from the Earth and thus the atmospheric refraction of their light leads to the twinkling effect. 

However, planets are located close to the earth and hence are considered to be extended sources of light. They can be considered to be a collection of a large number of point sized sources of light. If it is so, the total variation of the amount of light entering an observer’s eyes from all these individual point-sized sources, average out to zero. This nullifies the twinkling effect. 

Summary

• The refraction of light caused due to different layers of the atmosphere at different temperatures (different refractive indices) is called the atmospheric refraction.
• The atmospheric refraction of star light takes place through a medium of gradually increasing refractive index before reaching an observer on the Earth. This makes the star appear at a higher position as compared to its actual position.
• The atmospheric refraction is also responsible for the twinkling of stars, advance sunrise, delayed sunset and also the flattened appearance of the sun during sunrise and sunset.