# Thin lens is actually prism?

I just found in one book that thin lens is nothing but combination of small angled prisms. How is it even possible ? Can someone provide me satisfactory explanation for it?

Here is a diagram to show how you might think of a lens as a collection of prisms.

There is also a Fresnel lens is made from a series of prisms as shown below.

Which looks like this in real life.

This supplements the images given in the answer by @Farcher.

Underlying the images is the Fermat Principle of Least Time.

This passage from Feynman's QED: The Strange Story of Light and Matter (pp. 57-58) describes the process using his approach to quantum mechanics. (bolding mine)

Now, let's have some fun. Let's "fool the light," so that all the paths take exactly the same amount of time. How can we do this? How can we make the shortest path, through M, take exactly the same time as the longest path, through A?

Well, light goes slower in water than it does in air; it also goes slower in glass (which is much easier to handle!). So, if we put in just the right thickness of glass on the shortest path, through M, we can make the time for that path exactly the same as for the path through A. The paths next to M, which are just a little longer, won't need quite as much glass (see Fig. 36). The nearer we get to A, the less glass we have to put in to slow up the light. By carefully calculating and putting in just the right thickness of glass to compensate for the time along each path, we can make all the times the same. When we draw the arrows for each way the light could go, we find we have succeeded in straightening them all out--and there are, in reality, millions of tiny arrows--so the net result is a sensationally large, unexpectedly enormous final arrow! Of course you know what I'm describing; it's a focusing lens. By arranging things so that all the times are equal, we can focus light--we can make the probability very high that light will arrive at a particular point, and very low that it will arrive anywhere else.

FIGURE 36. A "trick" can be played on Nature by slowing down the light that takes shorter paths: glass of just the right thickness is inserted so that all the paths will take exactly the same time. This causes all of the arrows to point in the same direction, and to produce a whopping final arrow--lots of light! Such a piece of glass made to greatly increase the probability of light getting from a source to a single point is called a focusing lens.

• This reasoning applies individually to the segments of a Fresnel lens, but not to the whole thing. Commented Jul 8, 2022 at 14:03
• I concur with John Doty. In the case of a focusing Fresnel lens the light has multiple paths available from a single point of emission to a single point of reception, and those paths have different duration. This means that the notion of "least time" is not applicable in the case of a Fresnel lens. The criterion of "least time" is applicable if and only if: (1) There is only a single available path or: (2) the setup is arranged such that while allowing multiple paths all paths have the same duration Commented Jul 9, 2022 at 3:15