This has been confusing me. Let's take convex lens. Below in the picture, is it a double convex lens or just the symbol of convex lens?

enter image description here

For example: In convex glasses and contact lenses, the lens used is only half of what is shown above.

Upon some further searching I found this picture:

enter image description here

So, does it mean the lens we use glasses is Plano-convex lens? and the lens we use for cameras and magnifying glasses are double convex lenses?

  • 1
    $\begingroup$ The "lens" of a fine camera is an assembly of multiple "elements" made of different kinds of glass and/or plastic, some of which may be cemented together, and some not. They can be any of the above shapes, and more. $\endgroup$ Commented Aug 19, 2016 at 21:33

3 Answers 3


If one wants to be rigorous then the lens in the first picture should be called double convex or bi-convex since it is convex on both sides.

For camera lenses and magnifying glasses you mostly want to enlarge the picture and use convex lenses.

For the glasses you have to distinguish if the person is far- or nearsighted. For farsighted people you would take convex lenses, for nearsighted people you would take concave lenses. This is related to the length of the eyeballs and less to magnifying the picture.

Edit: In the first picture of your question the rays on the left side of the biconvex lens seem to run parallel to each other. This representation is often misleading since in reality the rays would converge and focus at one point (even if the point is far away.) For example look at this plan-convex lens:

rays through a plan-convex lens

In a plan-convex lens the rays on the left are coming parallel and may focus only at infinity. You can always construct such pictures yourself just remembering that the angles of rays with respect to the normal are larger in the medium with lower density (air). See for example the red ray in the picture above. By drawing the same picture for a bi-convex lens you would see that the rays would not remain parallel on the left side after leaving the lens, they would converge and focus in a single point similar to the right side of the lens.

In reality this is what you want to achieve for your farsighted glasses, magnifying glasses and so on. So you would use bi-convex lenses although the bending on both sides might be different. For nearsighted glasses you would rather use bi-concave lenses. Of course there are applications where plan-convex lenses would be preferred, e. g. to reduce unwanted optical effects (in telescopes and so on... see this question here).

  • $\begingroup$ That's my question. You said convex lens but what kind of convex lens for magnifying glass, eye glasses and contact lens? Is it single, double, or both as shown in picture? $\endgroup$
    – user963241
    Commented Aug 20, 2016 at 14:48
  • $\begingroup$ Edited my answer $\endgroup$
    – Statics
    Commented Aug 20, 2016 at 15:47
  • $\begingroup$ Is the lens of concave glasses actually concave from both sides? For example: The glasses I have seen are always convex in shape from the front even in concave lens. $\endgroup$
    – user963241
    Commented Aug 21, 2016 at 4:07
  • $\begingroup$ Most glasses are indeed convex-concave. However the curvature of the convex side and of the concave side differ depending on whether you need a farsighted- or a nearsighted correction. So farsighted glasses would still be convex in total even though the back side may appear concave. I believe this total curvature is due to the roundish shape of the eye and thus better focusing but you better ask an optician. $\endgroup$
    – Statics
    Commented Aug 21, 2016 at 10:44
  • $\begingroup$ I'm not sure I understand why the incident rays for a bi-convex lens couldn't be parallel? (The first few sentencese after your "Edit:") Surely the shape of the lens has no effect on the angles of incoming light? $\endgroup$
    – craq
    Commented Mar 20, 2018 at 21:31

There seems to be a confusion between two different aspects of a lens.

A converging lens takes a parallel beam of light and brings it to a focus at a point. It is generally thicker at the centre than at the edge. Examples of this are: a magnifying glass, and eyeglasses for an individual who is farsighted and cannot focus on nearby objects.

A diverging lens takes a parallel beam of light and spreads it out, as if it were coming from a point. Such a lens could be used for glasses for a near-sighted person who cannot focus on distant objects

From http://www.sciencefacts.net/diverging-lens.html

enter image description here

None of the above says anything definitive about the shape of the each of the two surfaces of the lens; only about the relative thickness at the centre and edge.


  1. Convex-convex - always converging; always thicker at the centre
  2. Plano-convex - always converging; always thicker at the centre
  3. Concavo-concave - always diverging; always thinner at the centre
  4. Plano-concave - always diverging; always thinner at the centre
  5. Convex-concave - can be converging, diverging or neutral

All these lenses will behave the same way, no matter which way the light goes through them.

Eyeglass lenses will almost always be convex on the outer surface, the one farthest from the eye, simply to fit it to the curvature of the face.

If the inner surface is concave, and more sharply curved than the outer, then the lens is diverging.

If the inner surface is more gently concave, flat, or convex, then the lens is a converging one.

Stage glasses, as part of the costume for an actor with normal vision, would have a convex outer surface and a concave inner surface, with the same radius of curvature for both surfaces, and would be neither converging or diverging...

  • $\begingroup$ Are there really no diagrams for half convex and double convex lens for example? $\endgroup$
    – user963241
    Commented Aug 20, 2016 at 15:20

Spectacle lenses for short sight are essentially biconcave, although the whole lens is curved making it concave (inner surface) / convex (outer surface). For long sight they will be biconvex but again curved making them concave / convex (but less concave than lenses for short sight).

Camera lenses have multiple elements and are complicated to describe: modern camera lenses are the highly-optimised result of two hundred years of evolution and (more significantly) extensive computer-optimised design: zooms in particular hardly existed before numerical simulation techniques.

Yes, I know there were zoom lenses before computer design: the reason people still worship primes is largely because of how rubbish such lenses were, not to mention how big. The tiny zoom on your P&S camera with its huge range is entirely a result of computer design.

  • $\begingroup$ Also, some of the lens elements on your P&S camera are molded plastic with computer-designed, aspheric surfaces. Aspherics are another game changer. Back in the day, there was no technology to make high quality lenses from plastic, and the cost of griding and polishing glass aspherics has always made them way too expensive for any kind of mass-market application. $\endgroup$ Commented Aug 19, 2016 at 21:51
  • $\begingroup$ Another reason for worshipping primes: No moving elements. Fixed elements can be aligned with better precision than moving elements. And yet another reason: Fewer surfaces (But, of course, we also get fewer surfaces by using aspherics.) $\endgroup$ Commented Aug 19, 2016 at 21:55
  • $\begingroup$ @jameslarge It's OK, I worship primes too (I have, perhaps, 30?). But there is a lot of myth and lore about them: my single favourite lens -- a Zeiss C-Sonnar -- is also the most rattly lens I own: however its elements are aligned, it is not with great precision... $\endgroup$
    – user107153
    Commented Aug 19, 2016 at 22:22

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