Curvette

Meet Curvette, a concave mirror who gathers light to a point.

A concave mirror curves inward like the inside of a spoon. Light arriving parallel to its axis reflects and crosses at the focal point. For a spherical mirror, and rays close to the axis, the focal length is half the radius of curvature.

A mirror cut from a sphere of radius 40 cm therefore has a focal length of 20 cm. Rays far from the axis cross a little closer to the mirror, an effect called spherical aberration, which is why big telescope mirrors are often made parabolic instead.

What you see depends on where you stand. Closer than the focal point, you see an upright, magnified image, which is how shaving and make-up mirrors work. Further away, the image is upside down.

Isaac Newton built one of the first reflecting telescopes in 1668 using a concave metal mirror. Today the largest telescopes, car headlights, torches and solar cookers all use concave mirrors to collect or aim light.