What Causes Atmospheric Refraction? Key Principles and Real-Life Applications
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:
[Image will be uploaded soon]
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 on Atmospheric Refraction: Learn the Science Behind What You See
1. What is atmospheric refraction as explained in the CBSE Class 10 syllabus for 2025-26?
Atmospheric refraction is the phenomenon of the bending of light as it passes through the Earth's different atmospheric layers. According to the CBSE Class 10 syllabus, this occurs because our atmosphere has layers of varying optical densities and temperatures. As light from a distant object like a star enters the atmosphere, it continuously travels from a rarer to a denser medium, causing it to bend. This principle is used to explain several natural phenomena.
2. What are some common real-world examples of atmospheric refraction?
Some of the most common and important examples of atmospheric refraction include:
The twinkling of stars: Light from stars is constantly bent by turbulent atmospheric layers, causing their apparent position and brightness to change.
Advanced sunrise and delayed sunset: The Sun appears visible about two minutes before it actually crosses the horizon and remains visible for about two minutes after it has set.
Mirage formation: An optical illusion, especially on hot days, where light bends to create a displaced image, often resembling a pool of water.
Flattened appearance of the Sun: During sunrise and sunset, the Sun appears oval because light from its lower part is refracted more than light from its upper part.
3. Why do stars twinkle but planets do not?
This difference is due to the vast distances and apparent sizes of these celestial bodies. Stars twinkle because they are so far away that they act as point-sized sources of light. As their single ray of light passes through Earth's unstable atmosphere, it is refracted multiple times, causing the light path to waver, which we perceive as twinkling. In contrast, planets do not twinkle because they are much closer and appear as extended sources (a collection of many light points). The refraction effects from different points on the planet's surface average out, cancelling the twinkling effect and resulting in a steady glow.
4. How does atmospheric refraction cause an advanced sunrise and a delayed sunset?
Atmospheric refraction makes the Sun visible even when it is physically below the horizon. When the Sun is just below the horizon, its light rays travel from the vacuum of space (a rarer medium) into the Earth's atmosphere (a denser medium). The atmosphere bends these light rays downwards. For an observer on the ground, these bent rays appear to be coming from an apparent position that is above the actual horizon. This effect makes us see the Sun about two minutes before the actual sunrise and for about two minutes after the actual sunset.
5. What is the primary cause of atmospheric refraction?
The primary cause of atmospheric refraction is the variation in the optical density of Earth's atmosphere. The atmosphere is not uniform; its density, temperature, and pressure change with altitude. Air is generally densest near the surface and becomes progressively rarer (less dense) at higher altitudes. When light enters our atmosphere at an angle, it continuously passes through layers of changing density, which causes its path to bend. This continuous bending of light is known as atmospheric refraction.
6. How is atmospheric refraction different from the scattering of light?
While both are atmospheric optical phenomena, they are distinct processes:
Atmospheric Refraction is the bending of a light ray as it passes through mediums of different densities. It explains the change in the apparent position of objects, like the twinkling of stars or advanced sunrise.
Scattering of Light is the process where light is absorbed and re-emitted in different directions by particles (like dust or gas molecules) in the atmosphere. It explains why the sky appears blue and why the Sun looks red during sunrise and sunset.
7. How does atmospheric refraction make the Sun appear oval or flattened at sunrise and sunset?
The Sun appears oval at sunrise and sunset because of differential refraction near the horizon. Light rays from the Sun's lower edge travel through a denser part of the atmosphere compared to the rays from its upper edge. As a result, the light from the lower edge is bent more than the light from the upper edge. This greater bending makes the vertical diameter of the Sun appear shorter than its horizontal diameter, giving it a flattened, oval shape.
8. What is a mirage and how does it relate to atmospheric refraction?
A mirage is an optical illusion caused by an extreme form of atmospheric refraction and total internal reflection. On very hot days, the layer of air near the ground is hotter and less dense than the cooler air above it. Light from a distant object (like the sky or a tree) travelling towards the ground bends away from the normal as it enters this hotter, rarer air. Eventually, the angle of incidence becomes so large that the light ray undergoes total internal reflection and travels upwards towards an observer's eye, creating an inverted, shimmering image that often looks like a reflection in water.
