We know that a part of light is reflected when a light ray undergoes refraction. If it does undergo, then it will never come out of the droplet. Now if these separated light rays, strike the water-air interface of the water droplet, they may or may not undergo total internal reflection according to their incident angle. But what happens is, since there are many wavelengths of light in white light, each of them bends according to their own wavelength and hence we see them separately. So, when a polychromatic light like our sunlight(i.e., white light) travels from air to water, it bends towards the normal. And this dispersion happens when the sunlight strikes at the surface of the water droplet(i.e., at the air-water interface) As you know, light rays bend towards the normal when it travels from a rarer to denser medium. It's formed due to the refraction(or better to say dispersion) of white light from the sun. This is something confused by many students.Īctually, a rainbow isn't formed due to the Total Internal Reflection of water droplets. See my answer here Difference between primary and secondary rainbow, and here Dispersion in Rainbows for more information, as well as Jearl Walker's amazing article on this.ĮDIT: Here's a fun little fact, if you're interested in rainbows: since everybody always sees a rainbow at exactly the same angle (from $\angle40^\circ$ to $\angle42^\circ$) centered around their head, any rainbow you see is your own personal rainbow! Your friend standing next to you is observing a rainbow caused by an entirely different set of drops. You can actually do the calculations and show that all rainbows occur at exactly the same elevation from the line drawn between your head and the head of your shadow on the ground (called the anti-solar point). The effect is also smaller for higher-order rainbows, which makes them much weaker. The effect is tiny, but the sheer number of drops leads to it being visible. This angle is also dependent on the refractive index, and thus is different for different colours, leading to the "bands" of different colours appearing at different angles and forming the "rainbow". When the incident rays strike the inside of the drops at certain special angles of "minimum deviation", all the the reflected rays end up getting "bunched" together rather than scattered apart. A tiny amount of light is reflected from the inner wall of the droplet (while the majority of the light is transmitted). You're right, it isn't Total Internal Reflection that is responsible for a rainbow, just ordinary reflection at the interface.
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