Lenses

• A lens forms an image by refracting light
• In a convex lens, parallel rays of light are brought to a focus at the principal focus
• The distance from the lens to the principal focus is called the focal length
• Ray diagrams are used to show the formation of images by convex and concave lenses
• The image produced by a convex lens can be either real or virtual
• The image produced by a concave lens is always virtual
• magnification = $\frac{image\;height}{object\;height}$
• Magnification is a ratio and so has no units
• In ray diagrams a convex lens will be represented by

  
• In ray diagrams a concave lens will be represented by