Basic purpose of this lecture is to present on Lenses: Application of Refraction. There are two main types of lenses: convex lenses—these curve outwards and are fatter in the middle, and concave lenses—these curve inwards (a little like a cave) and are thinner in the middle. Convex lenses produce two different types of images, depending on where the object is located. If the object is at a distance greater than the focal length of the lens, a real image is formed. Concave lenses produce only virtual images. Rays coming into a lens from a distant object are almost parallel and form an image very close to the focus.