LIST OF CONTENTS What is refraction Refraction through glass slab Laws of refraction Advanced sunrise and delayed sunset Twinkling of stars Why don’t planets twinkle.. Apparent flattening of sun’s disc at sunrise and sunset Mirage References
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 velocity in going from one medium to the 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. therfore, 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.
ADVANCED SUNRISE & DELAYEDSUNSET 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. 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. 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’SDISC 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 suns 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. 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 roads surface. The mind interprets this as a pool of water on the road, since water also reflects the sky.