I am nearsighted. So i am using glasses. In glass i see a convex shaped lens. But to correct my vision i need divergengt lens that is concave. When i am using the glass in correct position the upper face is convex and the light should focus at some point. When i use it front side back then first will become concave then light should diverge. During both position i see clear image. Why i see image clearly despite the lens is concave or convex? Is there is some trick?


The refraction of light occurs at both surfaces with a convex surface converging the light and a concave surface diverging the light.

The direction in which the light is travelling, from convex to concave surface or from concave to convex surface does not change the total amount of deviation of the light.

You can check this by putting your lens a small distance above some print.
If the print looks smaller when looking through the lens then overall the lens is diverging and correcting the eye for nearsightedness (this is your condition) whereas if the print looks larger when looking through the lens then overall the lens is converging and correcting the eye for farsightedness.
Turning the lens over makes no difference to the size of the print. You will note that if you look up the lens makers formula there is no order given for the radii of the two surfaces this is because to the first approximation that order does not matter.

The focal length $f$ of a lens does not depend which of the two surfaces of radius of curvature $R_1$ and $R_2$ the light hits first

$$\dfrac 1 f \approx ({\mu_{\rm lens}-1})\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$$

where $\mu _{\rm lens}$ is the refractive index of the materila of which the lens is made.

The type of lens used for your spectacles is called a meniscus lens and in section looks something like this.

enter image description here

For the positive meniscus lens you will note that the convex side (left) is curved more than the concave side and so overall the lens is converging whereas the opposite if true for the negative meniscus lens which is diverging.

On this HyperPhysics web page about the lens maker's formula you can design your own meniscus lens.

Bifocal and varifocal lenses work because the curvature of the lens surface is not the same over all the area of the lens.

Here is a picture of one of my spectacle lenses.

enter image description here

You will note that I am nearsighted because the squares on the graph paper appear smaller when observed through the lens.
You will also note that the squares are much smaller at the top of the lens which I use for distance vision than those at the bottom of the lens which I use for near vision.

My lenses are varifocal.

  • $\begingroup$ @RedGrittyBrick Point taken and I have added a little bit to my answer in order to address your comment. $\endgroup$ – Farcher Feb 15 '18 at 19:30

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