A concave lens is a piece of curved glass or plastic that is thinner in the middle than at its edges, creating a slight inward “cave” shape. When light passes through it, the lens spreads the rays apart rather than bringing them together. This spreading effect is why concave lenses are also called diverging lenses, and it’s the property that makes them useful in everything from eyeglasses to telescopes.
Shape and Structure
The easiest way to identify a concave lens is by feel or profile. The center of the lens is noticeably thinner than the outer edges. If you looked at it from the side, you’d see the surfaces curving inward, like two shallow bowls pressed face-to-face. This is the opposite of a convex lens, which bulges outward and is thickest in the middle.
Concave lenses can have one or both surfaces curved inward. A lens with one flat side and one concave side is called plano-concave. One with both sides curving inward is biconcave. The degree of curvature determines how strongly the lens bends light.
How a Concave Lens Bends Light
When parallel rays of light hit a concave lens, they refract (bend) as they enter and exit the glass. Because the lens is thinner in the center, rays passing through the middle bend less than rays near the edges. The result is that all the rays spread outward after passing through, diverging away from each other.
If you traced those diverging rays backward, they would appear to meet at a single point on the same side of the lens as the incoming light. This point is called the focal point, but it’s a virtual focal point because light doesn’t actually converge there. The rays only seem to come from that spot when you look through the lens. This is a key distinction: a convex lens has a real focal point where light physically gathers, while a concave lens has a virtual one where light only appears to originate.
In optics, the focal length of a concave lens is always expressed as a negative number. This sign convention is a quick way to distinguish diverging lenses from converging ones in calculations.
Images Through a Concave Lens
Hold a concave lens at arm’s length and look through it. Everything appears smaller than it actually is, and the image stays upright, never flipped. That’s because a concave lens can only produce one type of image: virtual, upright, and reduced in size.
The image is virtual because it forms on the same side of the lens as the object you’re looking at. You can’t project it onto a screen the way you can with a magnifying glass (which uses a convex lens). The magnification is always positive but less than 1, meaning the image is always smaller than the real object. No matter where you place the object relative to the lens, this stays true. A convex lens, by contrast, can produce real or virtual images depending on distance, and those images can be magnified or reduced. A concave lens doesn’t have that flexibility.
Correcting Nearsightedness
The most familiar use of concave lenses is in prescription glasses for nearsightedness, or myopia. In a nearsighted eye, the eyeball is slightly too long or the cornea curves too steeply, so light rays come to a focus in front of the retina instead of directly on it. Distant objects look blurry because the image has already started to spread apart by the time it reaches the light-sensing cells at the back of the eye.
A concave lens placed in front of the eye diverges the incoming light just enough to push the focal point back onto the retina. The stronger the prescription (indicated by a larger negative number, like -3.00), the more the lens diverges light, compensating for a more elongated eyeball. This is why glasses for nearsighted people make your eyes look slightly smaller when someone looks at you through the lenses.
Other Practical Uses
Beyond vision correction, concave lenses show up in several optical instruments. Peepholes in doors use a concave lens to give you a wide-angle view of whoever is standing outside. The lens takes a large field of view and compresses it into a small, upright image you can see through the tiny opening.
In telescopes and binoculars, concave lenses are paired with convex lenses to sharpen image quality. A single convex lens can focus different colors of light at slightly different points, creating fringes of color around objects. Adding a concave lens to the system counteracts this effect, keeping colors aligned so the image looks crisp. High-quality telescopes and binoculars rely on these concave-convex combinations to minimize color distortion.
Camera systems and laser equipment also use concave lenses to widen beams of light or adjust how light spreads across a sensor. Flashlights sometimes include a small concave element to control the spread of the beam.
Concave vs. Convex at a Glance
- Shape: A concave lens is thinner in the center and thicker at the edges. A convex lens is thicker in the center and thinner at the edges.
- Effect on light: Concave lenses diverge (spread) light rays. Convex lenses converge (gather) them.
- Focal point: Concave lenses have a virtual focal point with a negative focal length. Convex lenses have a real focal point with a positive focal length.
- Image type: Concave lenses always produce virtual, upright, smaller images. Convex lenses can produce real or virtual images that may be larger, smaller, or the same size as the object.
- Vision correction: Concave lenses correct nearsightedness. Convex lenses correct farsightedness.
An easy way to remember the difference: a concave lens “caves in” at the center and spreads light out, while a convex lens bulges out at the center and pulls light together.