Formation of Rainbow
There are three key effects that produce a rainbow:
Surface tension is critical to cause all the drops of rain to be very close to spherical independent of their (relatively small) size.
Refraction is the bending of light caused by the change in the speed of light going from one medium (air) to another (water). The ratio of speeds is called the refractive index and it varies slightly according to wavelength. Blue light bends slightly more than red light as is familiar from images of prisms. Prisms are wedge shaped so that the bend at the entry face is not simply cancelled by the inverse bend at the exit face - this cancellation is why colours come through a flat plane of glass almost undisturbed.
An internal reflection happens inside the spherical raindrop. This helps increase the dispersion of light caused by the refraction, and is why the rainbow is in front of you when the Sun is behind you. Halos (which don't have the internal reflection) around the light source (eg Sun, Moon, street light) also exist but these do not have such well defined colour separation.
The refraction on entry, internal reflection, and refraction on exit result in a total angle of about 42° between the Sun, the rainbow, and you - hence the familiar arc of a circle seen in the sky. This angle is consistent for a sphere of water so the effects from all the raindrops add up.
The second arc of a double rainbow is the result of a second internal reflection and so the colours in the arc are inverted.
There are of course multiple other parts of physics required for a full explanation, but these three are the key to understanding the principle properties of rainbows.
- Surface tension
- Refraction
- Reflection
Surface tension is critical to cause all the drops of rain to be very close to spherical independent of their (relatively small) size.
Refraction is the bending of light caused by the change in the speed of light going from one medium (air) to another (water). The ratio of speeds is called the refractive index and it varies slightly according to wavelength. Blue light bends slightly more than red light as is familiar from images of prisms. Prisms are wedge shaped so that the bend at the entry face is not simply cancelled by the inverse bend at the exit face - this cancellation is why colours come through a flat plane of glass almost undisturbed.
An internal reflection happens inside the spherical raindrop. This helps increase the dispersion of light caused by the refraction, and is why the rainbow is in front of you when the Sun is behind you. Halos (which don't have the internal reflection) around the light source (eg Sun, Moon, street light) also exist but these do not have such well defined colour separation.
The refraction on entry, internal reflection, and refraction on exit result in a total angle of about 42° between the Sun, the rainbow, and you - hence the familiar arc of a circle seen in the sky. This angle is consistent for a sphere of water so the effects from all the raindrops add up.
The second arc of a double rainbow is the result of a second internal reflection and so the colours in the arc are inverted.
There are of course multiple other parts of physics required for a full explanation, but these three are the key to understanding the principle properties of rainbows.
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