In concave lens, upper half prism has base in upward direction and that of lower half prism has base in downward direction central part is made from thin glass slab so its refraction angles decrease while moving from side to its center . Upper and lower prism refract light toward their base. This gives the light to move away from central straight light which is shown in the figure.
[caption id="attachment_14243" align="aligncenter" width="300"] concave lens refract light[/caption]
Because of the negative focal length for double concave lenses, the light rays will head towards the focal point on the opposite side of the lens. ... As the light rays enter into the more dense lens material, they refract towards the normal; and as they exit into the less dense air, they refract away from the normal.The concave lens is a diverging lens, because it causes the light rays to bend away (diverge) from its axis. In this case, the lens has been shaped so that all light rays entering it parallel to its axis appear to originate from the same point, F, defined to be the focal point of a diverging lens.
When the parallel rays of light pass through a concave lens to the principal axis, it deviates from its focal point and refracts the light rays. Due to this reason, it is called the Concave or Diverging lens. An incident light ray parallel to the principal axis of a concave lens, passes through it and diverges.The amount of bending depends on two things: Change in speed – if a substance causes the light to speed up or slow down more, it will refract (bend) more. Angle of the incident ray – if the light is entering the substance at a greater angle, the amount of refraction will also be more noticeable.It is a diverging lens, in case of an ideal concave lens, both the marginal and the paraxial rays gets diverged after getting refracted by the lens. Thus, the parallel rays will get diverged when they are permitted to go through the concave lens.