Human beings are able to see the world when light reflected from the surface of any object creates an image on the retina and it is converted to a sensory signal with help of rod and cone cells and processed by the brain. This image is created by the lens present in our eyes. When the distance of the object changes the shape of the lens also changes to create the image on the retina. But when due to some reason the lens is unable to change its shape we need glasses to help the lens create an image on the retina.