Monthly Archives: August 2001

The Beauty of The Physics of Rainbows


      A few days back my friend Manpreet asked me a question :

“why is the rainbow round ? ” . I didnt know the answer so I searched and found some very interesting things about rainbows and their Physics .

I thought that a rainbow is beautiful . The Physics of the Rainbow and the process of its creation is just as fantastic !!!

      The first systematic study of the Physics of the Rainbow was done by Rene Descartes in 1637 . He took a large glass sphere and held it in sunlight at various angles and positions . So he tried to simulate the water droplet in sunshine phenomenon .


      First thing u will notice is that when you are looking at a rainbow the Sun is behind your back . The rays come from the sun and enter a droplet.

While going into the droplet the rays are refracted and when the rays  go to the end of the droplet the rays may be refracted out or may be reflected back into the drop . The droplet acts like a prism and breaks the light into its constituent colors .  So the internal reflection gets the rays back to you broken up into their colors .


Now the angle is important . Descartes noticed that if the angle of the eye with the droplet is about 42 degrees u see a concentration of red color .

If it is 40 degrees u see blue . If it is anything lying outside the 40-42 degrees range u see just white light . So the rainbow is a circle with a angular radius of about 42 degrees .

      SOmetimes not all the light that enters the droplet comes out at one time . Sometimes some of it remains inside and undergoes another reflection and comes out after some time . For red light this comes out at 50 degrees instead of 42 degrees and for bule it is at 52 degrees .

So we see a secondary rainbow with the colors reversed and at an angle of about 50 degrees . This secondary bow is less bright .

      The rainbow is a semicircle because the earth comes in the way of what could be a full circle .

      Sometimes ( as Thomas Young observed ) the light near the top and the bottom of the rainbow interfere to form alternate bright or dark patterns . These are called supernumerary arcs .


      Notice that the difference in the brightness of  the sky inside and just outside the rainbow arc . The sky is  brighter inside the arc .

The reason for this is that the maximum angle at which light can come out a droplet is 53 degrees . It cannot be deflected more by our tiny droplet .

So beyond that no light comes out of the droplet and the sky is dark there .

      The light that comes out of the rainbow is almost completely

polarised . So if u look at it thru the poloroid glasses u will not

be able to see it .


      And sometimes u have lunar rainbows too . The light from a

full moon is bright enough to produce a faint rainbow .


      So the next time u look at a rainbow dont just admire the

beauty of the spectrum . Also admire the phenomena and the Physics of

the creation of the rainbow .