Concepts

Class 10
Chapter 11 Class 10 - Human Eye and Colourful World

Have you seen objects being blurry above a fire?

Like in the photo below

This random flickering (objects being blurry) is due to atmospheric refraction.

We know that when light travels from medium to another, refraction takes place

In the atmosphere, the layers of air have different optical densities. That means that different layers of air have different refractive index.

Air in upper atmosphere is optically rarer but air in lower atmosphere is optically denser

So, when light rays pass through air, refraction takes place

Now, we can define atmospheric refraction

## What is Atmospheric Refraction?

Refraction of Light due to different layers of earth's atmosphere is called Atmospheric Refraction

Let's look at different examples of Atmospheric Refraction

• Twinkling of Stars
• Stars appear higher than they actually are
• Advance Sunrise and Delayed Sunset
• Flattening of Sun's Disk

Let's look at them one by one

## Twinkling of Stars

We know that Stars emit their own light.

Due to this light, they shine at night.

We observe that, the intensity of light of the star changes - it becomes very bright, then becomes dull.

This change in intensity of light coming from the stars is called Twinkling of Stars

Reason for Twinkling

The twinkling of stars is due to atmospheric refraction of star's light.

The Optical Density of Air continuously changes from one moment to another

When atmosphere refracts more starlight towards us, stars appear to be bright

When they reflect less light, star appear to be dim

Hence, the star light reaching our eyes change continuously and stars appear to twinkle.

## Stars appear higher than they actually are

Star light comes from vaccum in Space to Air in Earth's atmosphere

As we move towards the earth, this air becomes more optically denser

That is, the refractive index increases as we move closer to the earth

When star light enters the earth atmosphere, they bend

It appears that they are coming from higher point than they actually are

## Why does an object waver or move over a fire?

When we put an object over fire, it flickers (appear to move slightly).

It is because of refraction of light in air

Air above fire is very hot (optically rarer)

While air away from fire is cooler (optically denser)

When we place object over fire, light rays appear to move from optically denser air to optically rarer air

This causes refraction of light and object appears to waver or move

Get live Maths 1-on-1 Classs - Class 6 to 12