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 h_a

Where,

R = astronomical refraction

n₀ = refractive index

h_a = 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.