What a converging lens does
A converging (convex) lens is thicker in the middle than at the edges. When a beam of light rays travelling parallel to the principal axis passes through it, the rays are refracted so that they meet at a single point called the principal focus (F). The distance from the centre of the lens to F is the focal length (f). A fatter lens bends light more strongly and has a shorter focal length.
Key idea
Linear magnification m = image height ÷ object height = v ÷ u, where u is the object distance and v the image distance from the lens. m > 1 means the image is larger than the object.
The images formed
The type of image depends on where the object is placed:
- Beyond 2F: real, inverted, diminished (used in a camera).
- At 2F: real, inverted, same size.
- Between F and 2F: real, inverted, magnified (used in a projector).
- At F: no image forms (rays emerge parallel).
- Inside F: virtual, upright, magnified — this is the magnifying glass.
A real image can be caught on a screen because the rays actually meet; a virtual image cannot, because the rays only appear to come from it.
Worked example
An object 3 cm tall forms an image 12 cm tall on a screen. Magnification m = 12 ÷ 3 = 4. The image is real, inverted and four times larger than the object.
Remember
- Only real images can be projected onto a screen.
- A magnifying glass works with the object inside the focal length.
- Draw ray diagrams using the two standard rays: one parallel to the axis then through F, and one straight through the lens centre.