A lens only reverses the effect of refraction that took place while entering.

but a prism does not do that
Chromatic aberrations: the index of refraction of glass is a little different for differentwavelengths of light (dispersion). Rays of different colors are separated by the lens, andblue light rays are deflected more than red light rays. Consequently the focal length of asimple lens is different for blue and for red light. The difference is not very large, butenough to cause trouble: red and blue polka dots on a dress can not be captured in goodsharp focus at the same time with a camera using a simple lens. If you focus on the reddots, the blue dots will be a little fuzzy and vice versa. You can see this imaginingto beperfectly in focus for the red dots. The blue dots will then be focused maybe 0.2 mm infront of the film and by the time the rays get to the film they will have already re-divergeda little, and produce a blurry blue dot image. The blur is worse if the lens aperture is large,because the rays diverge more over the 0.2 mm distance. This is because rays of lightfarther from the axis of the lens must be deflected more to be brought into focus. The morerays of light are deflected by the lens, the greater the effect of chromatic aberration. If youdon't need a large lens aperture (i.e. you want to take pictures only outside on sunny days)the solution is simple: make the lens aperture small.Blue light is deflected by the lens more than redlight, the focal point for blue is closer to the lensthan the focal point for red. Blue and red objectscannot be in focus at the same time. Closing aniris aperture, i.e. selecting the central rays only,or paraxial rays, reduces chromatic aberration.\\
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