Is rainbow a real image or virtual image ? why?
Answers
In optical terminology, a “real image” is formed when light has converged to a particular place and proceeded on; when it’s detected downstream, it traces back to that “real image” location. A “virtual image” is formed when some optical element (mirror, lens, etc) launches light on a trajectory “as if” it had come from a point; the downstream light traces back to a point behind that optic, a place where the light never actually was. Your image in a mirror is virtual; the light was never there.
How to apply that terminology to a rainbow?
It would be easier to see if the situation was simpler: The Sun was more like a point, just one color, and there was one raindrop in the sky. In that case, when the raindrop was just in the right place, you’d be able to see the Sun’s light through it: just a dot of light. The drop is a _terrible_ optical element; it doesn’t really make an image. It’s more like light reflecting from a white wall than a mirror; the light is more scattered than it would be if a lens is focusing it.
A real rainbow has lots of raindrops and a bigger-than-a-point Sun, so you can see those points at lots of angles, forming an arc in the sky. And the Sun has lots of colors, each of which will form an arc at a different angle.
Those arc aren’t images, so we can’t really talk of them as “real images” or “virtual images”. They’re just a source of “scattered light” that comes from lots and lots of dots (raindrops) in they sky.
To put it in more practical terms: You can’t really focus a camera at a particular distance to make a rainbow sharp. It’s not an image at a particular distance. It’s a bunch of light coming from a lot of raindrops at different distances.