How much we see of the arc depends on where we are and how high the sun is in the sky.
Every observer will see a slightly different angle of refraction, so each rainbow is unique to that person. If the angle is right, some of that light will be reflected off the internal surface of the raindrop, exiting the drop and refracting again, so that ultimately we see the rainbow with the red band on the outside. This is known as refraction - each colour with its different wavelength will bend at slightly different angles so that the light splits into its different component colours. Sunlight is made up of a spectrum of different colours that will slow down and change direction when entering water.
© Cammie Czuchnicki via Royal Meteorological Society / Natural History Museum