Atmospheric Refraction

We’ve always heard the nursery rhyme Twinkle-Twinkle little star, but have you ever asked why stars twinkle at night? It is certain that everyone has seen the twinkling star at once in your life, but do you know when astronaut travel in space they are no longer to see stars twinkling in the sky, the only thing they can see is the beautiful lighted dots. So this makes one thing clear that the stars only twinkle or seem to twinkle from our earth but why? Well, this is due to our earth’s atmosphere which causes refraction and due to which star twinkles in the night.

To understand the term atmospheric refraction, it is very important to understand what is refraction? And then we can understand that why on earth we can see the twinkling effect.

Twinkling of a star due to atmospheric refraction.

What is Refraction of Light?

Well, we all familiar with the reflection of light that when the light hits a polished surface then it bounces back and this phenomenon is known as reflection. But sometimes, the light bends when it passes from one medium to another, and this bending of light is known as refraction. Basically, the cause of refraction is due to the difference in optical density of the two mediums.

Refraction of light in a different medium

In the figure, one can easily understand refraction, firstly the light travels in the air (optical rarer medium) and the moment it strikes the glass slab, it bends as it enters into an optically denser medium.

Causes of Refraction

This theory is quite different from what we have studied about light, in that it always travels in a straight direction. So why does the light refract Well, the main cause of refraction is the speed of the light. Basically, light travels at different speeds in different media. Thus, when the light travels from one medium to another medium then at that point it faces a change in speed. Now it is known whenever light moves from one medium to another then its speed gets alter which causes refraction. Let’s discuss what happens when light enters from air to the glass slab, and then it comes out from the glass slab.

• When the light ray travels from air to glass slab, then it bends toward normal as the speed gets reduced in the optically denser medium.
• When the light ray travels from the glass slab to air, then it bends away from the normal as the speed gets increased in the optically rarer medium.

This can be understood that why the speed of light is different in a different medium, by understanding a simple analogy. Suppose you have two pools, one, which is full of water and the other one, which is full of honey. So, tell me in which pool you can swim easily? Of course water, it is less dense than the honey pool.  Now apply the same analogy to light, so when it travels in a rarer medium then it travels with more speed than in a denser medium.

Laws of Refraction

When a beam of light travels from two medium then it follows two laws of refraction:

1. The first law of refraction states that the incident ray refracted ray and the normal to the interference of the medium lies on the same plane.
2. The ratio between the sine of the angle of incidence and sine of the angle of refraction is constant to the interference of the medium, also called Snell’s law of refraction.

K = sin i / sin r

where k is constant, i is the angle of incident and r is the angle of refraction. 

Atmospheric Refraction

In this section, we try to understand why does the stars twinkle in the night? Are we seeing stars at correct positions?  And let me tell you one more interesting fact, we see the sun 2 min before sunrise. This is quite interesting but why? So we are going to answer these questions in this section.  As we all now aware of the term refraction and the cause of refraction. So let’s understand atmosphere refraction.

Since we know the change in medium causes refraction likewise our earth is covered with the atmosphere having different layers, and these layers possess different temperatures at different heights, some atmospheric layers are warm and some are cold.  Now the warmer layer of the atmosphere behaves like an optically rarer medium whereas the cooler layer behaves like an optically denser medium. So now we again have different media thus when light passes through different layers of the atmosphere it gets refracted.

In short, the refraction cause due to the earth’s atmosphere is called atmospheric refraction.

Examples of Atmospheric Refraction

Now let’s discuss some examples of atmospheric refraction:

1. Twinkling of stars: As we discussed earlier the temperature of the atmosphere’s layer is different at a different height which creates different media for refraction. Also, the air is not constant which led to a change in temperature and causes refraction. So, when the atmosphere refracts more light to us the star seems bright. However, when the atmosphere refracts less light than the star seems dim, and this process happens so fast that it seems to us twinkling.
2. Advanced sunrise and Delay in the sunset: Have you ever wondered that the actual sunset happens much earlier than what you usually see? Yes, you heard it right. When we see sunsets, the light ray coming from the sun goes under refraction and bends towards us. At that time, the sun already crossed the horizon. The sun, we see, is an image that is formed due to refraction that is actually higher than its actual position. This results in the delay of sunset and the same thing happen during sunrise which makes it earlier than the actual one.
3. The apparent position of the stars: As we discussed in the twinkling of a star that our atmosphere has different layers which cause light to refract, so our atmosphere bends the starlight towards normal,  which causes the apparent position of the star to appear to be slightly different from its actual position.
4. The Rainbow: In our childhood, we all love to see a rainbow after the rain. The formation of a rainbow involves Reflection, Refraction, and Dispersion. Firstly, the sunlight enters into the water droplet during which it undergoes refraction as well as dispersion. But here we are only concerned about refraction. So, the light bends inside the droplet (due to change in medium) and performs total internal reflection (will discuss in another article) and then comes out where the light again refracts which results in the formation of a rainbow.

Sample Problems

Problem 1: Explain why stars seem higher than their actual position?

Solution:

Due to Atmospheric refraction, our atmosphere has different air layers having different temperatures at different heights. Thus, this difference of temperature in a different layer of the atmosphere becomes the rarer and denser medium and causes Atmospheric refraction. Hence, the atmosphere bends the starlight towards normal, which causes the apparent position of the star to appear higher than its actual position.

Problem 2: Why only stars twinkle in the night but not the planet?

Solution:

Since the star appears very small as it is very far from the earth and seems a point source so a continuous change in atmosphere refracts the light causes the twinkling effect. However, the planet which is not so far from our earth appears as collections of a point source, so when light causes dimming the effect produced by the point source on one side of the planet would get canceled out by the brighter effect produced by the point source of light in other parts.

Problem 3: By how much atmosphere refraction causes a delay in the sunset and early sunrise?

Solution:

The sun appears 2 minutes before the actual sunrise and sunset 2 minutes later than the actual sunset.

Problem 4: Explain why the object seems moving if we look at the object through the hot air over the fire?

Solution

This is because of the refraction of light, as the hot air above the fire behave as rarer medium and the colder air further above the hot air behaves as an optically denser medium,  hence refraction takes place and the object seems shaky or moving. 

Problem 5: Write at least three applications of atmospheric refraction?

Solution:

Following are the application of atmospheric refraction:

1. Advance sunrise and delay sunset
2. The apparent position of stars
3. Formation of rainbow

Problem 6: We know that when light moves from one medium to another causes refraction. Now our atmosphere covers with air only so how does light refract in one medium? Explain in brief.

 Solution:

Well the atmosphere’s layer is not the same, as at different height it has different temperature due to which some layer become optically rarer medium and some become optically denser and this causes atmospheric refraction.