December 16, 2013

Senior Inter New Syllabus - Ray Optics Important SA Questions

Q: Explain the formation of mirage.
A: Mirage is an optical-illusion. It is formed due to total internal reflection of light. On a hot summer day, the ground is heated due to radiation of sun. Then the air in lower levels gets heated and its density decreases. As a result its refractive index and optical density decreases. Now rarer medium is formed at lower layers and denser medium exists at upper levels when light ray coming from sun (or) from a tall tree will pass through denser to rarer medium and satisfy i > ic and total internal reflection takes place. As a result an observer gets an impression that reflection of distant object is occurring due to presence of water, but water will not be there. Such
inverted images cause an optical illusion to the observer.

Q: Explain the formation of rainbow.
A: Rainbow is due to dispersion of white light through water drops. Rainbow is the combined effect of dispersion, refraction and total internal reflection of Sunlight by spherical water droplets of rain. Conditions for observing a rainbow are that the Sun should be shining in one part of the sky, while it is raining in the opposite part of the sky. Formation is atleast a three step process. Sunlight is first refracted as it enters a rain drop which causes different colours (different wavelengths).

Red will bend less, blue and violet will bend more. These components will suffer
total internal reflection inside water drop at water-air boundary, and again get refracted at the opposite surface and come out. In this way rainbow is formed on a rainday.

Few important points:

Sunset and Sunrise appear red because least scattering of red colour reaches the
earth and other colours are scattered more and are removed from it.

Sky appears blue due to scttering of sunlight in atomsphere.

Red is used as danger signal because red light is scattered the least and violet is scattered the most.

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