Why is violet in lowest part of rainbow?

Rainbows are usually visible after rain - or after watering plants using a hose! Rainbows are a product of light hitting water, in simple terms.
Light, though we usually perceive it as yellowish (sun) or whitish - though we can't actually see light - , is actually a combination of all colors (while black is when there isn't one). Each color of light corresponds to a certain wavelength - and a certain energy. That is, light is a combination of waves of different energies and wavelengths, and as humans, our eyes perceive this as difference in color. We cannot see every wavelength of light. We can only perceive those of a certain wavelength, referred to as the visible wavelength - the range 390-700 nm. There's more light beyond this - infrared, ultraviolet are one common terms you might hear.
When passing through a prism, light refracts and is separated into it's different components in a continuous manner. The prism separates the light into individual components, and bends them differently according to the wavelength of that component. Longer wavelength components get refracted less, and the shorter ones get refracted the most - or in other words, shorter ones get bent more than longer ones. Rainbows are formed when light hits water droplets in the atmosphere, which act as tiny prisms. Light is refracted, and we can see it's individual components as the colors of the rainbow. It is in this particular order - ROYGBIV - because of how light is refracted. Red has a wavelength of 700nm, Violet has a wavelength of around 390nm. Hence, red is refracted less and violet the most as they are at the ends of the visible spectrum. The other colors are of intermediate lengths and go from longer to shorter from orange to indigo. Hence, violet, as it is diffracted the most, is perceived to be at the bottom of a rainbow.
 
---In brief:
Rainbow is created by the refraction of light as it hits water droplets that act as prism. Violet has a short wavelength and is refracted more - red has a long wavelength and is refracted less. The arrangement of the colors of the rainbow is due to the difference in the extend to which they are dispersed or 'bent'. Violet is at the short end of the visible spectrum and red at the longer end and hence, they are at the ends of the rainbow - top and bottom.