Light that helps us see. Light travels at the universal speed, whether from a light bulb to the eye or from a torch to the sock in the cupboard and then to the eye.
Light always travels at about:
300 million metres in each second or: 3 × 108 metre / second
That's the universal speed, often called the
speed of light. In deep space that's just what it does. When light travels through air, water or glass (three examples of different mediums) it appears to travel more slowly.
Some speeds of travel: vacuum, 299 792 000 metre/second ; air, 299 703 000 metre/second ; water, 225 408 000 metre/second ; glass, 199 862 000 metre/second.
Because light travels at such a high speed, seeing books, cars and dogs seems to happen without delay. Only if you look at things very far away do you notice the delay.