The Human Eye

Atmospheric Refraction

The deviation of light wave or other wave because of difference in atmospheric density at different height is called Atmospheric Refraction.

Temperature of air varies with variation in height. This variation in temperature depends on many factors.

Because of variation in temperature in atmosphere light coming after an object is not stationary rather flickering resulting in object appears flickering or fluctuated.

Example:

An object placed behind hot air rising from anything such as a radiator or fire appears flickering. This happens because the air above the fire or any such hot object becomes hotter than the further up. The hotter air is lighter and has refractive index less than that of cold air above and below this makes the zone of hotter air non stationary and object observed behind it apparently appears fluctuating.

Same conditions are always present in our atmosphere because of uneven heating and cooling effect which gives often flickering effect to the object seen through it because of uneven refractive indexes of air in our local environment. Such condition is called atmospheric refraction.

Twinkling of stars, delayed sunset and advance sunrise, apparent position of celestial bodies, etc. are some of the effect of atmospheric refraction.

Apparent Position of Stars

Stars are not seen at their actual positions rather they appear slightly above of their normal positions.

The earth's atmosphere is not uniform. The density of atmosphere and consequently refracting index changes continuously. Generally refracting index increases when coming down in atmosphere. Since, light coming from stars bends towards the normal when enters in the atmosphere, thus the apparent position of stars appear slightly above to their actual position. This is the cause that we see starts slightly above to their actual position when see from horizon.

Twinkling of Stars

Stars appears twinkling is due to the atmospheric refraction. The position of stars appears slightly above to their actual position because of change in refracting index of the atmosphere.

The refractive index is not stationary throughout because of many physical conditions of the atmosphere of earth. Change in physical condition of earth's atmosphere changes the refracting index because of that light coming from stars appears flickering.

Since stars are at very distant and are appeared as point-sized sources of light. As the path of rays of light coming from stars varying slightly, the apparent position of the stars fluctuates and amount of starlight entering the eye flickers. This makes stars twinkling.

Why do not the Planets twinkle?

Planets do not have their own light rather they reflect light coming from other celestial bodies, such as sun. On the other hand planets are too closer to us compare to stars and consequently appear larger in size.

Thus amount in variation of light coming to our eyes after reflection from planets will average out of zero, this nullifying the twinkling effect and we do not see planets twinkling as stars.

Advance Sunrise and Delayed Sunset

Space has no atmosphere while earth has dense atmosphere. Light coming from Sun at horizon when enter in dense atmosphere, it bends towards normal because of refraction and we see apparent position of Sun higher from its actual position. Because of this Sun becomes visible about 2 minutes before the actual sunrise.

Similarly, while sun set sun appears for about two minute even after sunset because of same phenomenon, i.e. refraction of light.

Sun appears flattening at sunrise and sunset because of same refraction of light.

Scattering of Light

White light comprises of seven colors. These seven colors of light have different frequencies and different wavelength. In the course of reaching sunlight to the earth, light has to travel through various layer of atmospheric density. Because of this uneven density of atmosphere light of different frequencies get refracted and scattered. This phenomenon is called scattering of light.

Scattering of Light gives many unique spectacular phenomenon in nature, such as blue color of sky, reddish color of sky while sunrise and sunset, etc.

Tyndall Effect

There are many minute particles present in atmosphere, such as smoke, tiny water droplets, suspended particles of dust and molecules of air, etc.

While a beam of light strikes such fine particles, the path of the beam become visible. The light reaches us after being reflected diffusely by these particles.

This phenomenon of scattering of light by the colloidal particles is called Tyndall Effect.

Example:

(a) When a beam of sunlight coming through a slit of window or door; many of the dust particles appear dancing through the beam. This happens because of Tyndall Effect.

(b) Similar appearance is seen when sunlight passes through a canopy of a dense forest. In such condition tiny water droplets in the mist scatter light.

The Size of Particles and Scattering of Light

Scattering of light depends upon the size of particles suspended in the way of beam of light. Very fine particles scatter blue light which has shorter wavelength while larger particles scatter orange and red light having longer wavelength.

And if the size of particles is larger enough, the scattered light after those particles may appear white.

Why is the Color of the Clear Sky Blue?

In the atmosphere many of the particles including air molecules are very small in size. These particles scatter visible light having wavelength shorter than them. Light at blue end have shorter wavelength while light at red has longer wavelength. Red light has wavelength about 1.8 greater than that of the blue light.

In clear sky fine particles scatter light of blue color more strongly than red which enters in our eye and we see the sky in blue color.

If there were no atmosphere present no scattering would take place and sky would have look dark. This is the cause astronauts look only dark sky when are in space.

Passengers flying at high altitude see dark color of sky because of non prominent scattering of light at such height.

Color of the Sun at Sunrise and Sunset

Sun is at horizon while sunset and sunrise. At horizon Sun is at greater distance compare to when at overhead. Sunlight coming from horizon has to pass through a thick layer of atmosphere, which scatter red and orange light of longer wavelength more strongly than that of light of shorter wavelength. Thus, scattering of red and orange light makes the color of sky and Sun orange red.

This is the cause that we see Sun in orange red color at sunrise and sunset.

In the noon Sun is comparatively nearer to us and fine particles scatter blue light having shorter wavelength more stronger than those of longer wavelength and we see the color of Sun as bluish and sky appear blue.