What causes chromatic aberration?
(a) Light is an electromagnetic wave and has intrinsic diffraction properties.

(b) Different wavelengths of light give different angles of refraction at the lens-air interface.
(c) The coefficient of reflection is different for light of different wavelengths.
(d) The outer edges of the lens produce a focus at a different point from that formed by the central portion of the lens. 

(e) The absorption of light in the glass varies with wavelength.