RAINBOW [WHAT IS RAINBOW]

                              RAINBOW                           

WHAT IS A RAINBOW?

A RAINBOW IS AN TYPE OF DISPERSION OF SUNLIGHT BY THE WATER DROPS IN THE ATMOSPHERE. THIS IS A PHENOMENON DUE TO THE COMBINED EFFECT OF DISPERSION, REFRACTION AND REFLECTION OF SUNLIGHT BY SPHERICAL WATER DROPLETS OF RAIN. THE CONDITIONS FOR OBSERVING A RAINBOW ARE THAT THE SUN SHOULD BE SHINING IN ONE PART OF THE SKY [SAY NEAR WESTERN HORIZON] WHILE IT IS RAINING IN THE OPPOSITE PART OF THE SKY [SAY EASTERN HORIZON]. AN OBSERVER CAN THEREFORE SEE A RAINBOW ONLY WHEN HIS BACK IS TOWARDS THE SUN.

FORMATION OF RAINBOW?

WHEN THE SUNLIGHT IS FIRST REFRACTED AS IT ENTERS A RAINDROP, WHICH CAUSES THE DIFFERENT WAVELENGTH COLOURS OF WHITE LIGHT TO SEPARATE. LONGER WAVELENGTH OF LIGHT [RED] AND BENT THE LEAST WHILE THE SHORTER WAVELENGTH VIOLET ARE BENT THE MOST. THESE COMPONENT RAYS STRIKE THE INNER SURFACE OF THE WATER DROP AND GET INTERNALLY REFLECTED IF THE ANGLE BETWEEN THE REFRACTED RAY AND NORMAL TO THE DROP SURFACE IS GREATER THAN THE CRITICAL ANGLE OF 48 DEGREES. THE REFLECTED LIGHT IS REFRACTED AGAIN AS IT COMES OUT OF THE DROP. IT IS FOUND THAT THE VIOLET LIGHT EMERGES AT AN ANGLE OF 40 DEGREES RELATED TO THE INCOMING SUNLIGHT AND THE RED LIGHT EMERGES AT AN ANGLE OF 42 DEGREES. FOR OTHER COLOURS, ANGLES LIE IN BETWEEN THESE TWO VALUES.

WE SEE THAT RED LIGHT FROM DROP 1 AND VIOLET LIGHT FROM DROP 2 REACH THE OBSERVERS EYE. THE VIOLET FROM DROP 1 AND RED LIGHT FROM DROP 2 AND DIRECTED AT LEVELS ABOVE OR BELOW THE OBSERVER. THUS THE OBSERVER SEES A RAINBOW WITH RED COLOUR ON THE TOP AND VIOLET ON THE BOTTOM. THUS, THE PRIMARY RAINBOW IS A RESULT OF A THREE-STEP PROCESS, THAT IS, REFRACTION, REFLECTION AND REFRACTION.

                                   

WHEN LIGHT RAYS UNDERGOES TWO INTERNAL REFLECTIONS INSIDE A RAINDROP, INSTEAD OF ONE AS IN THE PRIMARY RAINBOW, A SECONDARY RAINBOW IS FORMED. IT IS DUE TO THE FOUR-STEP PROCESS. THE INTENSITY OF LIGHT IS REDUCED AT THE SECOND REFLECTION AND HENCE THE SECONDARY RAINBOW IS FAINTER THAN THE PRIMARY RAINBOW. THE ORDER OF THE COLOURS IS REVERSED IN ITS CLEAR FORM.