write 3 difference between concave and convex lens?

indhu15 has given absolutely correct answer. So, you can refer to it.
Here are few more things I like to add,
Convex lenses are curved outward and they cause light that goes trough it to converge or concentrate on one point. Think of a magnifying glass that is used to burn something. The light that goes through it concentrates on a point so you can burn things. Concave lenses are curved inward and when light goes through it diverges or spreads out. Think of a flashlight. The light comes from a small bulb goes through it and spreads out to a bigger beam on the other side.
Hope this helps,
@indhu15. Good answer. You get a thumb up. Keep posting!!

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 A concave lens is thinnest in its middle (the point where the line between the two focal points intersects the lens (at right angles to the plane of the lens).

A convex lens is thickest in its middle.

A convexo-concave lens is concave on one side and convex on the other. Virtually all spectacle (eye glasses) lenses are like this. Whether the particular lens is convex or concave is answered by the thickness at it middle compared with elsewhere on the lens.

You can also consider all the combos: single (the other side is flat), double convex and concave.

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