The reason why it is difficult to see the end of a rainbow is because the light is refracted and then reflected within the water droplets before exiting. When light leaves the droplets, it is bent and separated into colors due to the refraction and scattering of light. This causes the rainbow to make a full circle in the sky, but we only see the part of the arc that is above the horizon, making a semicircular arc. Furthermore, to see a complete rainbow, the observer would have to be high up or in the air, and the lighting and raindrop conditions would have to be perfect. This is quite rare, so in most cases we only see the semicircular arc. The end of the rainbow, where colors fade, is usually diffuse and difficult to distinguish clearly due to light scattering and other atmospheric factors.