ATMOSPHERIC REFRACTIONMade By: Qhudwa Sarmad X.B

R.I.S.Y

ContentWhat is Refraction?Laws of RefractionAdvanced sunrise and delayed sunset Twinkling of starsWhy don’t planets twinkleApparent flattening of sun’s disc at

sunrise and sunsetMirageReferences

What is Refraction of Light?

Refraction of Light is the phenomenon of change in the path of light, when it travels from one medium to another.

The basic cause of Refraction is change in velocityfrom one medium to other.

Laws of Refraction

The incident ray, refracted ray and the normal to the interface at the point of incidence, all lie in the same plane.

The product of refractive index and sine of angle of incidence/refraction at a point in a medium is constant

i.e., u * sin i= constant. therefore, u1 sin i1 = u2 sin i2 i1 = i and i2 = r hence, u2 / u1 = sin i / sin r and it is also known as SNELL’s LAW.

Atmospheric Refraction

Advanced Sunrise and Delayed Sunset The sun is visible to us

before the actual sunrise and after the sunset.

The density of air near the surface of earth is slightly higher.

The rays from the sun coming through space enter at the top of earth’s atmosphere and travel from rarer to denser medium.

Advanced sunrise and delayed sunset

They bend slightly towards normal at each refraction and appear to come from apparent position of sun.

Hence, the sun appears to rise a few minutes before the actual rise and for the same reason, it continues to be seen a few minutes after it has actually set

Twinkling of Stars The starlight on entering

earth’s atmosphere, undergoes refraction continuously before it reaches observer.

Since the refractive index of earth’s atmosphere is gradually changing, the atmosphere bends the starlight towards the normal, and the apparent position of star is slightly higher than its actual position.

Twinkling of Stars

As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of light entering the eye flickers..!!

Why don’t planets twinkle?

The planets are much closer to the earth, and thus are seen as extended sources.

If we consider a planet as a collection of large number of point sized sources of light, the total variation in the amount of light entering our eye from all the individual point sized sources will average out to zero, thereby nullifying the twinkling …effect..!!

Apparent flattening of sun’s disc at sunrise and

sunset If the sun (or moon) is low above the horizon, the optical path of the light through the atmosphere is very long, and the atmosphere usually has a layered structure: different temperature gradients and pressure.

Refraction of the light by these layers can cause the sun's disk to be deformed, flattened or distorted. Inferior and superior mirages are responsible for the distortions.

MIRAGE:MOST NOTICEABLE

A highway mirage is an inferior mirage which can be seen very commonly on roadways by day or by night.

Warm air is less dense than cool air, and the variation between the hot air at the surface of the road and the denser cool air above it creates a gradient in the refractive index of the air.

MIRAGE:MOST NOTICEABLE

Light from the sky at a shallow angle to the road is refracted by the index gradient, making it appear as if the sky is reflected by the road's surface.

The mind interprets this as a pool of water on the road, since water also reflects the sky.