How, without touching the lenses of the glasses with your hand, to determine whether

How, without touching the lenses of the glasses with your hand, to determine whether they are intended for myopia or hyperopia?

As you know, a lens is a transparent body bounded by two spherical surfaces.

If the thickness of the lens is less than the radius of curvature of the spherical surfaces, then the lens is thin. Otherwise, a flat concave lens. For myopia, concave lenses are used. For hyperopia – thin lenses.

In order to determine without touching the lens, for myopia or farsightedness of the eyes, they are intended to be directed to the lens a beam of light. If, after that, a shadow from the lens appears on the screen, then the lens is for myopic glasses.