The Color Team
Sunlight looks white, but it's actually a team of seven different colors all traveling together so fast they blur into one. These colors are Red, Orange, Yellow, Green, Blue, Indigo, and Violet. When they enter a glass prism, they all slow down because glass is thicker than air.
Spreading Out
While all colors slow down, they don't slow down by the same amount. Violet light is 'clumsier' in glass and bends very sharply, while red light is 'sleeker' and only bends a little bit. This causes the team of colors to spread out into a fan shape.
Visible Spectrum
By the time the light comes out the other side of the prism, the colors have split up enough that we can see them individually. This colorful stripe is called a 'spectrum.' Every rainbow you see in the sky is created by millions of tiny raindrops acting just like these glass prisms.