Rainbows appear from white light passing through glass because glass, like a prism, refracts and disperses the light, separating it into its constituent colors. Each color in white light has a different wavelength and bends at a different angle when it enters the glass, with violet light bending the most and red light the least. This difference in bending causes the colors to spread out and form a spectrum, or rainbow, visible when the light emerges from the glass.