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Luminous objects give out light, non–luminous objects do not. All surfaces reflect light. Mirror–like surfaces reflect light in a regular way, other surfaces scatter light. For anything to be seen, light from it (either emitted or reflected) must enter the eye. Light enters the eye through a hole (the pupil) and is detected by the light–sensitive back surface (the retina). Beams of light can only be seen if the light hits something and reflects (or scatters) towards an eye.