How is a rainbow formed? Understand The Formation of Rainbow

  • jeff_jo

    1. “Dispersion” is not synonymous with “separation of colors”. It only means that the refraction depends on the color of the light. If the initial conditions are the same for each color, as they are in some experiments with a flat surface like the face of a prism, separation will occur. If they are not, as for a curved surface like a raindrop, colors that are incident at different places can, and do, recombine into white light.

    2. Rainbows do not involve Total Internal Reflection (TIR) in any way. In fact, sunlight that enters a spherical raindrop cannot undergo TIR. The geometry when the light would exit the drop is symmetric with the geometry it had when it entered. Since light must enter at less than the critical angle, it cannot be more than the critical angle at the exit. Some light reflects and some refracts each time the light encounters the surface.

    3. A raindrop acts more like a lens than a prism. After one (not total) internal reflection, the light that strikes the entire surface gets reflected back in a cone that is about 40 degrees wide, like the beam of a flashlight. But the correct definition of dispersion implies that the beam has different half-widths for each color. And each color’s beam is brightest at the very edge of the cone.

    4. The diagram in the “Putting the pieces together” section is still correct. But it applies to these cones, not to individual rays. When the angle from the sun, to the drop, to the observer is about 42 degrees, the observer is in only the red cone and sees red. When it is 41 degrees, the observer is in the green, yellow, orange, and red cones. But the green is much brighter than the other cones, so the observer perceives it to be a pale green. At 40 degrees, the observer “sees” all colors but violet is dominant.

    5. The colors are not the same that one would see in a spectrum. The red is, but each successive color is paler than the previous one. And the rainbow has no inside edge – the violet band simple fades to white and continues all the way to the horizon! If you look at the photo in the article, this effect is quite clear.