Practical: Light passing through a glass prism
White light is a mixture of several colours. When it passes through a glass prism rays of different colours bend by different amounts: red bends least and violet most, so the white beam is split into a spectrum. By tradition seven colours are named: red, orange, yellow, green, light blue, blue, violet; in reality the colours blend continuously. A rainbow forms when raindrops do the same job: sunlight enters a drop, bends, reflects off its inner back wall and bends again on leaving. We see a rainbow with our back to the Sun and rain in front; the bow shape arises because light leaves the drops at about 42° from the point opposite the Sun (the textbook’s “spherical atmosphere” explanation is not quite right). An object’s colour depends on the light it reflects: a green leaf mostly reflects green light and absorbs the rest; a white object reflects all colours and a black one absorbs almost all. A single-colour beam sent into a prism is only bent and not split into other colours.
Making a spectrum: on a sunny day set a glass of water on a windowsill and hold a white sheet behind it to look for a coloured band. Or send a flashlight beam through a narrow slit onto a glass (or plastic) prism and catch the spectrum on a screen. Safety: never look directly at the Sun or the flashlight; prism edges can be sharp so handle with care; do not shine the beam into anyone’s eyes.