Atmospheric Refraction

What is Atmospheric Refraction?

To explain atmospheric refraction in simple words, let us say that it is the change in the direction of propagation of electromagnetic radiation or sound waves traversing in the atmosphere. This refraction is caused by the light passing through the air. Air is made up of gas and dust particles with different optical densities. The velocity of light passing through air decreases with an increase in its density. Refraction can raise or lower and shorten or broaden the images of distant objects. Refraction near the ground results in mirages. A mirage is a naturally occurring phenomenon in which the light rays are refracted to produce a displaced image of a distant object or the sky.

One of the most common applications of atmospheric refraction is to determine the position of terrestrial and celestial objects. Due to atmospheric refraction, celestial objects appear higher than they are. Refraction is also applicable to sound and electromagnetic radiation. The atmospheric refraction diagram is given below:


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Atmospheric Refraction of Light

To define atmospheric refraction, we need to understand the concept of refraction. Refraction is the change in direction of a wave when passing from one medium to another caused by its change in speed. The atmosphere is made up of gas and dust particles with different optical densities. When a light ray passes through these particles of various densities, it causes a change in its speed which then changes the direction of the traveling light. This phenomenon is referred to as atmospheric refraction of light.

The most common effect of atmospheric refraction is observed at sunset and sunrise. At sunrise, the sun appears early due to the refraction of light and during sunset, the sun appears even after it disappears behind the horizon. This is an interesting application of atmospheric refraction.

The Various Causes of Atmospheric Refraction of Light are as Follows:

  • The presence of gaseous particles of different optical densities. 

  • Air pressure and temperature also affect atmospheric refraction. Higher air pressure and lower temperatures cause larger refraction.


Refractive Index

The Refractive index tells us how fast light will travel in a particular medium. The formula for refractive index is :

n = c/v

Where,

n = refractive index

c = speed of light in vacuum 

v = velocity of light in that material

The refractive index is a dimensionless quantity. The Refractive index for water is 1.33 which means that the speed of light in water is 1.33 times slower than that in a vacuum. The amount of refraction also depends upon the refractive index.

According to Snell’s Law,

n1Sinθ1 = n2Sinθ2

Where, 

n1 and n2 are the refractive indexes of the two mediums and θ1 and θ2 are the angle of incidence and angle of refraction respectively.

To further explain atmospheric refraction and the change in speed of a wave, let’s look at the refractive index of the atmosphere.

The refractive index of air or the refractive index of the atmosphere is approximately 1.002 which implies that the speed of light in air is 1.002 times slower than that in a vacuum. The different gaseous particles in the air have different densities which then change the speed of light travelling through the air. This change in speed causes changes in direction of light. 

The Atmospheric Refraction Formula is Given by-

R =(n0-1)cot ha

Where,

R = astronomical refraction

n0 = refractive index

ha = apparent altitude of the astronomical body


Conclusion 

The different gaseous particles present in the air with different optical densities help us to define atmospheric refraction. When light travels through these different particles it changes its velocity which results in a change in direction of light. Atmospheric refraction is a naturally occurring optical phenomenon that affects not only visible light rays but electromagnetic radiation as well. 

FAQs (Frequently Asked Questions)

1. What are Some Common Examples of Atmospheric Refraction?

Ans- Atmospheric refraction is a naturally occurring phenomenon that takes place when the direction of propagation of wave changes due to a change in the speed of light. This change in speed is brought about by the different particles in the air of varying optical densities. Some examples of atmospheric refraction are-

  • Far off objects such as stars twinkle or shimmer due to refraction of light. The starlight reaching our eyes undergoes constant refraction and therefore light reaches our eyes at different angles causing the twinkling of stars.

  • Mirage is an optical illusion caused when the ground is hot and the air is cool. The warm air above the ground causes large refractions of the light coming from the sky. It then appears that the light came off from an object on the ground.

  • Refraction causes the sun to appear when it’s crossed the horizon during sunset.

2. What is Astronomical Refraction? How Can it Be Calculated?

Ans- Astronomical refraction is the differential refraction that forms the shape of extended bodies such as the Sun and Moon and their angular position and appearance as celestial bodies. Refraction is greater at the horizon than that at the diameter of the earth due to the variability of temperature gradient near the horizon and geographical sensitivity. The distortion of the shape of the sun is also due to atmospheric refraction. The turbulence in the air also causes atmospheric refraction making them appear brighter and fainter continuously in a very short time span.


Astronomical refraction can be calculated using numerous formulas that correctly give the value of refraction and are fairly consistent.  

R = (n0-1)cot ha

Where,

R = astronomical refraction

n0 = refractive index

ha = apparent altitude of the astronomical body