What is a lens? Distinguish between a convex lens and concave lens. Which of the two is a converging lens : convex lens of concave lens?

A lens is a piece of transparent glass bound by two spherical surfaces, which converge or diverge a beam of light.

The difference between a convex lens and a concave lens are as follows:

Convex Lens	Concave Lens
1. It converges the light rays to a point.	1. It diverges the light rays in different directions.
2. It is thicker at the middle as compared to the edges.	2. It is thicker at the edges as compared to the middle.
3. It has a positive focal length.	3. It has a negative focal length.
4. It has a real focus.	4. It has a virtual focus.

Among these two lenses, the convex lens is a converging lens. 

Explanation

Converging Lens or Convex Lens -  It is a lens that possesses at least one surface that curves outwards. In other words, it is thick across the middle and thin at the upper and lower edges., because of which the light that enters the lens, gets focused, or converged. Due to this effect, it is also known as a positive lens or a converging lens.

The image formed by a convex lens is both real and virtual depending on the position of the object, which means both diminished and magnified images can be produced.

Diverging Lens or Concave Lens - It is a lens that possesses at least one surface that curves inwards in the middle. In other words, it is thin across the middle and thick at the upper and lower edges, because of which the light that enters the lens, gets spread out, or diverges, which results in forming a smaller image. Due to this effect, it is also called a negative lens or a diverging lens.

The image formed by a concave lens is virtual & erect, which means it will appear to be farther away than it actually is, and therefore smaller than the object itself.