Thick lenses exhibit chromatic aberration due to: 

• Each colour has a distinct wavelength, as it passes through the thick lens, it refracts at different angles, making it impossible for them to meet at a single place.
• Due to this, the white light is dispersed into different colours. 
• This colour distortion that creates an outline of unwanted colour along the edges of the image of an object is also called chromatic aberration.

Therefore, we can say that chromatic aberration is caused due to dispersion of light.