Thin Lenses and Ray Diagrams Model

Object distance 30.0 cm

objectimagelight rayextended backwards (not real rays)

The lens diagrams are drawn to scale with the readings: one grid square is 5 cm, and the focal length is 10 cm. Each lens is treated as a thin lens, so every ray bends once, at the centre line of the lens, as in exam diagrams. The eye diagrams are not to scale: a real eye is about 2.5 cm across, the near object would be about 25 cm away (about ten eye-widths, here less than two), and the focusing errors are exaggerated so that you can see them.

How to write it in the exam

A converging lens refracts rays parallel to the principal axis so that they meet at the principal focus (focal point), F.
A diverging lens refracts a parallel beam so that the rays spread out. They seem to come from the principal focus on the side the light enters.
The focal length is the distance from the centre of the lens to the principal focus.
Ray diagram: a ray parallel to the principal axis passes through F after the lens; a ray through the centre of the lens goes straight on. The top of the image is where these rays cross.
Object beyond 2F: the image is real, inverted and diminished. Object at 2F: real, inverted, same size. Object between F and 2F: real, inverted, enlarged.
A real image is formed where rays of light really meet, so it can be shown on a screen. A virtual image is formed where diverging rays seem to come from when they are extended backwards; it cannot be shown on a screen.
A magnifying glass is a converging lens with the object closer to the lens than the principal focus. The image is virtual, upright and enlarged.
Short sight is corrected with a diverging lens; long sight is corrected with a converging lens. Each lens makes the eye bring the light to a focus on the retina.