Why is a concave lens called a diverging lens?

Hint: The word diverging means that the incident rays are diverging away from a point. Hence we will first see how the incident rays behave when they are incident on a concave lens. Further, we will conclude on the behavior of the refracted rays and accordingly determine whether the concave lens is a converging or diverging lens.

Complete step-by-step solution:
To begin with, let us first draw the rays diagram of a biconcave thin lens when the incident rays are taken from infinity and focused at a point F i.e. focus on the principal axis.

In the above diagram, we see that the incident rays after refraction from the concave lens diverge. It looks like these rays are generated from a point F i.e. focus of the concave lens. The rays basically diverge because of the geometry of the above lens. Since these rays diverge from a point on the principal axis, which is the focus of the concave lens, we can conclude that the concave lens is a diverging lens.

Note: The concave lens has a wide amount of application due to its property. In the case of people who suffer from myopia, the image is formed in front of the retina. Therefore the optician will recommend the person to use spectacle with a concave lens as it will diverge the rays from the point where the image is formed to the retina of the eye. The power of a concave lens is always considered to be negative as its focal length is negative.