{"id":439,"date":"2026-07-07T23:47:55","date_gmt":"2026-07-07T23:47:55","guid":{"rendered":"https:\/\/physicsfundamentalsinfo.com\/blog\/?p=439"},"modified":"2026-07-07T23:47:57","modified_gmt":"2026-07-07T23:47:57","slug":"concave-convex-lens","status":"publish","type":"post","link":"https:\/\/physicsfundamentalsinfo.com\/blog\/waves\/concave-convex-lens\/","title":{"rendered":"Concave and Convex Lens"},"content":{"rendered":"\n
Definition<\/div>

\n\nA concave and convex lens are the two basic types of thin lens that bend light in opposite ways: a convex (converging) lens is thicker in the middle and focuses parallel rays to a point, while a concave (diverging) lens is thinner in the middle and spreads rays apart. Both obey the lens formula 1\/f = 1\/v \u2212 1\/u.\n\n<\/p><\/div>\n\n

Pick up a magnifying glass and hold it over a page. The letters swell. Tilt it toward the sun and it will burn a tiny dot into the paper. Now think of the peephole in a front door \u2014 the whole hallway shrinks into one little fisheye view. Same idea, opposite effect.<\/p>\n\n

Both tricks come down to a single curved piece of glass or plastic. One kind of lens pulls light together; the other pushes it apart. Learn which is which and the rest of optics \u2014 cameras, spectacles, telescopes, even your own eyes \u2014 starts to make sense.<\/p>\n\n

What Are Concave and Convex Lenses?<\/h2>\n\n

A lens is simply a shaped piece of transparent material that bends light in a controlled way. Change the curve of its surfaces and you change where the light ends up.<\/p>\n\n

Convex lens (converging)<\/h3>\n\n

A convex lens bulges outward \u2014 it is thicker in the middle than at its edges. When parallel light passes through, the lens bends every ray toward the centre line, so the rays meet at a single point called the focal point. Because it gathers light to a point, a convex lens is also called a converging lens<\/strong>.<\/p>\n\n

Hold one up to a distant window and you can catch a small, upside-down picture of the view on a card behind it. That is light converging.<\/p>\n\n

Concave lens (diverging)<\/h3>\n\n

A concave lens caves inward \u2014 it is thinner in the middle than at its edges. Parallel light passing through spreads apart, as though it were fanning out from a point behind the lens. That spreading is why a concave lens is called a diverging lens<\/strong>.<\/p>\n\n

Look through one and everything appears smaller and further away. It can never focus the sun to a burning point, no matter how you angle it.<\/p>\n\nF<\/text>Convex (converging)<\/text>light in →<\/text>F<\/text>Concave (diverging)<\/text>light in →<\/text><\/svg>\n\n

A convex lens converges parallel light to a real focal point; a concave lens diverges it, so the rays only appear to come from a virtual focal point.<\/p>\n\n

Here is a memory hook that sticks: a con-cave<\/strong> lens caves in, so it spreads light. A convex<\/strong> lens is the opposite \u2014 it bulges out and gathers light. Caved-in glass diverges; bulging glass converges.<\/p>\n\n

Concave vs Convex Lens: 7 Key Differences<\/h2>\n\n

The two lenses are near-mirror opposites. This table lines up the seven differences that matter most in class and in exams.<\/p>\n\n

\n\n\n\n\n\n\n\n\n\n\n\n
Feature<\/th>\nConvex lens<\/th>\nConcave lens<\/th>\n<\/tr>\n<\/thead>\n
Shape<\/strong><\/td>\nThicker in the middle, bulges outward<\/td>\nThinner in the middle, caves inward<\/td>\n<\/tr>\n
Effect on parallel light<\/strong><\/td>\nConverges rays to a real focal point<\/td>\nDiverges rays; they appear to come from a virtual focal point<\/td>\n<\/tr>\n
Other name<\/strong><\/td>\nConverging lens<\/td>\nDiverging lens<\/td>\n<\/tr>\n
Focal length sign<\/strong><\/td>\nPositive (+f)<\/td>\nNegative (\u2212f)<\/td>\n<\/tr>\n
Power sign<\/strong><\/td>\nPositive (measured in +dioptres)<\/td>\nNegative (\u2212dioptres)<\/td>\n<\/tr>\n
Image of a real object<\/strong><\/td>\nReal or virtual, depending on distance; can be inverted and magnified, or upright and magnified<\/td>\nAlways virtual, upright and diminished<\/td>\n<\/tr>\n
Typical uses<\/strong><\/td>\nMagnifying glass, cameras, projectors, the eye, long-sight correction<\/td>\nPeepholes, short-sight correction, laser beam expanders, telescope elements<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n\n

The Lens Formula<\/h2>\n\n

One equation ties together where the object sits, where the image lands, and the strength of the lens. It works for both lens types \u2014 as long as you keep the signs straight.<\/p>\n\n

1\/f = 1\/v \u2212 1\/u<\/div>\n\n

Each symbol has a precise meaning and, in SI units, is measured in metres:<\/p>\n\n