Rainbows are caused by the reflection and refraction of sunlight in millions of raindrops.
The Sun must be behind the observer and fairly low for a rainbow to occur. This is why they're seldom seen in the middle of the day. Sometimes, if the light has been reflected twice inside each raindrop, a second fainter rainbow can be seen about 9 degrees outside the first. The colors on the outside one are reversed in order, red being in the inside. A third rainbow is occasionally seen and fourth rainbow is almost on top of the first so it is often not recognized.
A rainbow is actually a spectacular natural optical phenomenon. Individual raindrops act like tiny prisms, splitting the white light from the Sun (or other white light source) into a continuum of different hues, from deep violet to deep red. From any given point BETWEEN the Sun and the rain droplets, a single droplet would appear to refract a tiny speck of light in one particular hue, depending on its position. However, billions of raindrops combine their refractions to form the whole image, a circular arc containing a complete range of colors.
The colors of the primary rainbow are always the same - starting from the outside: red, orange, yellow, green, blue, indigo, violet.
The reason that it is IMPOSSIBLE to "approach" one end of the rainbow is that the rainbow phenomenon simply recedes as the viewer travels toward it.
If you use a garden hose to spray fine droplets you can see the entire circle of "rainbow" just a few feet away. A it is a circle there is no "end" to the rainbow.
When the sun is out (and preferably low near the horizon) and it is raining a rainbow occurs as the rays of the sun refract in and reflect from raindrops. Look away from the Sun when it is raining in that direction - away from the Sun - and you can often see 1, 2 or rarely even more rainbows centered around the "anti-solar point" (the shadow of your head).
Rainbows are formed by sunlight being refracted and reflected in a rain drop.
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