What's at the end of a rainbow? I recently saw a video where someone saw the end of a rainbow as it went into a lake. How is this even possible considering the fact that rainbows have no ends and are circular in nature?
Edit: I could not find the original video. However, I found one that is slightly similar to the original. https://www.youtube.com/watch?v=tPKQwbF3h24
 A: It was probably an optical trick requiring the lake to be in a certain position relative to the rainbow. Here is why.
We take the case of an ordinary rainbow (where the sun is behind you and you are looking away from it). In front of you stretching off into the distance is a cloud of fine mist (tiny rain droplets) extending up into the sky. Each droplet acts like a combination of a prism and a mirror. The prism part splits up the sun rays striking it into their constituent colors while the mirror part bounces those colors back towards the point where you are standing.
When you are looking "at" a rainbow, you are looking through a very, very large number of those droplets, each one of which is reflecting a little spread of colors back at you. These all add up in a line to form a cone-shaped source of colored light with the tip of the cone where you are and the big end of the cone pointing away from you.
This means the rainbow cone is suspended in space in front of you and only really distant objects are actually behind the rainbow.
If the lake is right in front of you and the droplet cloud is behind the lake, then it is possible for you to see a reflection of the rainbow coming from the surface of the lake which makes it look as if the rainbow was somehow "emerging" from the lake surface- even though it is not.
A: Although not that common there is nothing special about a rainbow "hitting the ground" as the image below shows.

You can think of the ground as getting in the way of the rainbow being a complete circle as shown in this next image taken from an airplane
during a thunderstorm with the ground not causing an obstruction.

Without getting too far into the details of the formation of a rainbow the diagram below illustrates the location, in terms of angle not distance, of a rainbow.

Notice the line from the Sun passing through the observer to the anti-solar point which is the centre of the rainbow.
During the morning the line rotates towards the vertical so the anti-solar point goes further down below the ground level and less of the rainbow is seen.
If the Sun is high enough in the sky then no rainbow is seen.
On the other hand when the observer is in an airplane the anti-solar point can be well above ground level and so a complete rainbow can be seen.
There are many videos of "the end of the rainbow" seen whilst travelling in a car.

This video illustrates that “the end of a rainbow” depends on the position of the observer (in the car) because as the observer moves so does “the end of the rainbow”.
Atmospheric optics is a website which is worth a visit as it illustrates the formation of various types of rainbows including those formed by reflection.
A: This looks more like a rainbow produced with a garden hose. This means it not due to dropplets high in the sky but by a mist very close to the ground. The driver is heading towards the rain and the rainbow disapears when he enters the rain with big droplets.The mist looks like it is at the edge of the rain area, on the left.  There are many images showing this type of rainbow, if you never "did" one yourself. Here is an example:
Garden hose rainbow
You can see a hint of a small rainbow produced by the mist spread by the back wheels of the car he's passing.
