4.Light and Sight
Without light we wouldn't be able to see. Reflected light has to enter our eye. Try standing (safely) in a dark cupboard or try to read a book under your bed covers - you shouldn't be able to see anything without any light getting in. Light travels in a straight line from the source, reflects off the object and enters our eyes for us to see.
The Pupil of the eye is a muscle that can expand and contract to let more or less light into our eyes. When there is not much light (darker) our pupils open up wider (expand) to try and let more light in to see.
Did you know that because light travels in straight lines we actually see everything upside down? It is believed babies actually see the World upside down until we train our brain to realise to flip the image. Look at the image below - the leaf is projected upside down on to the retia at the back of the eye.