Focusa

Meet Focusa, a converging lens who brings parallel rays together at one point.

A lens bends light because light travels more slowly in glass than in air. A converging lens, thicker in the middle, bends rays toward its axis, so rays arriving parallel to the axis all cross at the focal point, a distance f from the lens. The thin lens equation links the object distance d_o and the image distance d_i: 1/f = 1/d_o + 1/d_i.

Example: an object 30 cm from a lens with f = 10 cm forms a real, upside-down image 15 cm on the other side, since 1/10 − 1/30 = 1/15. Its magnification, d_i/d_o, is one half. Bring the object inside the focal length and the lens becomes a magnifying glass, making an upright virtual image.

Cameras, eyeglasses, projectors, microscopes and the lens of your own eye all follow this equation as a first model.

It is the thin lens approximation for rays near the axis. Real lenses suffer from aberrations, and because the refractive index varies with colour, different colours focus at slightly different points.