Optics is an area of physics which looks at how light behaves and how we can observe it. This usually involves using mirrors and lenses to magnify, focus, and direct the path of light. An everyday example of this is the use of corrective lenses in glasses which improve people's sight. The lenses change the focal length of the light. The focal length is the distance from the centre of the lens to the focus point of the light. This allows the wearer to see a focused image instead of a blurry one.
The main use of optics in astronomy is in telescopes. Telescopes use lenses or mirrors to magnify the light coming from distant objects in the Universe. This lets us see fainter objects. And lets us see more detail in the image. We also use lenses and mirrors to focus the light. This lets us look through the telescope with a camera and collect get data from the observations. Instruments called interferometers combine the light collected from 2 or more telescopes.
A ray diagram shows the path light takes as it travels. It includes what happens when it reaches a surface, such as a lens or a mirror. The direction and angle light travels through a telescope will vary depending on design of the telescope.