Atmospheric Refraction: Atmospheric Refraction makes our day longer by 4 minutes! Yes, you read it right; we see the sun two minutes before the actual sunrise and can see the sun for two minutes more even after the actual sunset. These and many such phenomena occur in nature due to Atmospheric Refraction.

This article will deal with the concept of Atmospheric Refraction, its causes and a few natural occurrences.

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What is Atmospheric Refraction?

Atmospheric Refraction can be well understood by first understanding refraction then put both the words together to get a clear understanding of the concept of Atmospheric Refraction.
Refraction is the phenomenon of bending of the ray of light when it travels from one transparent medium to another that has a different optical density or refractive index. Say air and water have different refractive indexes. So when sunlight travels from air to water or from water to air, it bends at the interface of the two media.

Due to this reason, it becomes difficult for us to gauge the depth of the swimming pool just by looking at it from the air or get the exact details of an object in the air just by looking at it while being under the water in the swimming pool. And the atmosphere, as we all are well aware, is the layer of gases that surround our planet. So when light travels through the earth’s atmosphere, it undergoes refraction many times.

Definition of Atmospheric Refraction: It is the phenomenon of bending of light on passing through earth’s atmosphere.
Let us now understand in detail how and why atmospheric refraction occurs.

Causes of Atmospheric Refraction

Atmospheric Refraction is the refraction of light while passing through the earth’s atmosphere. The earth’s atmosphere is made up of several layers consisting of various gases. These layers of the earth’s atmosphere have different density, and the density goes on increasing as the light reach the earth’s surface from space. Also, the local conditions like temperature, etc., that affect the optical density of the layer are not constant.

Due to this reason, light has to pass through several layers having different optical densities in increasing order while reaching the earth’s surface from space, so it undergoes refraction multiple times before reaching the observer’s eye standing on the earth’s surface. This gives rise to many natural phenomena about which we will study in detail in this article.

Natural Phenomena Occurring due to Atmospheric Refraction

Atmospheric Refraction occurs when sunlight undergoes refraction many times while passing through the earth’s atmosphere. Some of the natural phenomena occurring due to Atmospheric Refraction are as discussed below:

1. Advanced Sunrise: Sunrise occurs when the sun crosses the horizon and become visible to us. It is when the light from the sun starts reaching us. But due to Atmospheric Refraction, the sun appears to rise above the horizon, two minutes before it actually is above the horizon. This is termed advanced sunrise. So due to Atmospheric Refraction, our day starts two minutes earlier, and we start getting sunlight two minutes earlier. Just before the actual sunrise, the light from the sun enters the earth’s atmosphere and travels through rarer to denser layers of the atmosphere, bending each time towards the normal and reaches the observer’s eye, who is now able to see the sun even before the actual sunrise.
   
2. Delayed Sunset: Sunset occurs when the sun crosses the horizon, and we are no more able to see the sun in the sky. It is when the light from the sun stops coming to us, and it becomes dark. But due to Atmospheric Refraction, the sun is still there in the sky for two minutes though it actually has crossed the horizon, and we keep getting the light from the sun for two more minutes. So due to Atmospheric Refraction, we keep getting sunlight for two more minutes even after the sunset. Thus, our day becomes longer by four more minutes due to Atmospheric Refraction, that is, two minutes during sunrise and two minutes during sunset. Just after the actual sunset, the light from the sun still keeps coming to the observer’s eye. This is because the light from the sun entering the earth’s atmosphere travels through rarer to denser layers of the atmosphere, bending each time towards the normal and keeps reaching the observer’s eye, who is still able to see the sun even after the actual sunset.
3. The sun appears flattened at sunrise and sunset but appears circular at noon: At sunrise and sunset, the sun is near the horizon. The rays of light from the upper part and the lower part of the sun’s periphery bends unequally while travelling through the earth’s atmosphere. These rays enter the observer’s eye to give the perception of flattening of the sun’s disk during sunrise and sunset on account of this unequal bending due to Atmospheric Refraction.
   At noon, the sun is overhead. So the rays of light from the periphery of the sun pass through the earth’s atmosphere normally, and no refraction takes place. Thus, the sun appears circular at noon.
4. Stars seem higher than they actually are: Stars are considered as point sources of light that are situated very far from the earth. The light from the stars travel through space then enters the earth’s atmosphere to reach the eyes of the observer on the earth. The optical density of the various layers of the earth’s atmosphere goes on increasing towards the earth’s surface from space. The light from the stars while travelling through these layers of the earth’s atmosphere keeps on bending each time towards the normal and reaches the observer’s eye. When the path of this ray just before entering the observer’s eye is retraced, it appears to come from a position a little higher than its actual position. Hence, the stars appear higher than they actually are.
5. Stars twinkle, but planets do not twinkle: We see stars at night. Stars are the independent sources of light that are very far away from the earth. At night when we see the stars, the amount of light coming from them increases and decreases continuously. So when the amount of light from the star increases, it appears brighter and when the amount of light from the star decreases, it appears dimmer. This phenomenon of appearing brighter and dimmer, again and again, is called twinkling. This occurs due to Atmospheric Refraction. The light from the star first travels through space where there is a vacuum; then it enters the earth’s atmosphere. This light further travels through various layers of the earth’s atmosphere that has an optical density in the increasing order towards the earth’s surface.
   Thus, the light from the star travels from rarer to denser layers bending each time towards the normal. Moreover, different layers of the earth’s atmosphere are constantly moving. Also, their temperature and optical density keep on changing continuously. Therefore, the apparent position of the star and the amount of light from it entering the observer’s eye keeps on changing continuously. This leads to the twinkling of the stars. Planets, on the other hand, appear bigger than stars as they are closer to the earth. Due to this reason, a planet is considered to be made up of a number of point sources of light. Due to atmospheric refraction, when some so the point sources look brighter, others appear dimmer. Therefore, on the whole, the net amount of light entering the observer’s eye from the planet remains constant, and so the overall brightness of a planet remains constant. Thus, planets do not appear to twinkle.

Applications of Atmospheric Refraction

Atmospheric Refraction is the phenomenon of bending the light many times while passing through the various layers of the earth’s atmosphere. This concept helps us to understand why stars appear higher than they actually are, why do stars twinkle but planets do not, why do we have advanced sunrise but delayed sunset, why the sun appears flattened during sunrise and sunset but appears circular at noon, and many such occurrences.

Atmospheric Refraction also helps us to understand why do objects appear moving when they are looked through the holi fire or any such turbulent stream of hot air rising from a fire or a radiator.

Summary

Atmospheric Refraction occurs when the light from the sun reaching the earth’s surface undergoes refraction many times due to the changing optical density of the various layers of the earth’s atmosphere. Due to Atmospheric Refraction, our day becomes longer by four minutes; that is, we see the sun two minutes before the actual sunrise and for two more minutes after the sunset.

Similarly, the stars in the sky appear higher than their actual position along with a twinkling effect, but planets being closer to the earth does not twinkle. Hence, Atmospheric Refraction is the reason behind these wonderful phenomena occurring in nature which has been elaborated in the article.

Frequently Asked Question (FAQs) on Atmospheric Refraction

Q.1. What is atmospheric refraction?
Ans: Atmospheric Refraction is the phenomenon of bending of light on passing through the earth’s atmosphere.

Q.2. What is the basic cause of atmospheric refraction?
Ans: The basic cause of atmospheric refraction is the variation in the optical density of different layers of the earth’s atmosphere.

Q.3. Which phenomenon causes advanced sunrise and delayed sunset?
Ans: Advanced sunrise and delayed sunset are caused due to atmospheric refraction.

Q.4. How do stars twinkle?
Ans: Stars are considered to be point sources that are very far away from the earth. Due to atmospheric refraction, the light from the stars undergoes refraction many times while passing through the various layers of the earth’s atmosphere. Moreover, the physical conditions like temperature and optical density of these mobile layers of the earth’s atmosphere keeps on changing continuously, due to which the amount of light from the stars keeps on changing continuously. Thus, the stars appear sometimes brighter and sometimes dimmer that makes them appear to twinkle.

Q.5. Why don’t planets twinkle?
Ans: Planets are very close to the earth, so they appear bigger than stars and are considered to be made up of a number of point sources. When the light from the planets enters the observer’s eye after undergoing atmospheric refraction, some of the point sources appear brighter while some of the point sources appear dimmer, and so the total amount of the light from the planet remains constant, thereby nullifying the twinkling effect. Therefore, planets do not appear to twinkle.