Question

what are concave and convex lenses

Answer 1

convex lenses can be used to get magnified or larger images of objects.as it shows small images,it can be used as spectacles for short sightedness (myophia).it can also be used in flashlights, peepholes, binoculars, telescopes, and in photography.it is also kept in curves of road to get a better view of the coming vehicles which cannot be seen by the other driver.it can reduce accidents.
concave lens can be used as reflecting surfaces in headlights of cars, in telescopes, in satellite dishes, as shaving mirrors, in torch and etc
concave mirror 
has a curved in shape or structure.it goes in like a cave.
they are thinner in middle
they passing in rays spread out.i.e they diverge.
image formed is real and also virtual.
convex mirror
it has a bulging out structure
they are thicker in the middle
it will not let the rays diverge instead they converge.
the image formed is always virtual

Answer 2

Answer :-

Spherical Lenses Has been broadly classified into two types :-

• Concave lens.
• Convex Lens.

Explanation :-

Concave Lens !!

A Concave lens is made up of a transparent medium bounded by two spherical surfaces such that it is thin at the middle and thick at the edges.

Convex Lens !!

A Convex Lens is made up of a transparent medium bounded by two spherical surfaces such that it is thick at the middle and thin at the edges.

It is also called Bi-Convex Lens or a Double Convex Lens.

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Extra Information !!

Some Important Terms !!

APERTURE

The area of the lens available for Refraction is called the aperture of the lens.

PRINCIPAL AXIS

A straight line Passing through the centres of curvature of the two surfaces of a lens is called the Principal axis.

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These terms are so important you have to read it, it will come in use in the chapter of Reflection and Refraction (Chapter of Light of any Class).