After reading the chapter on convex lens, I saw several places where "converge" is used.

In the very beginning of the chapter, my book says "converging lenses bring light together". So I thought converging means gathering, sorta.

Then I saw the section explaining what happen when light rays come off an object placed "infinitely far" from a convex lens (100m counts as " infinitely far). It basically says in that case, light rays will pretty much all reach the lens parallel to one another, and they converge at the focal point on the far side/ the other side of the lens, creating an image.

I know image is created when light rays intersect. So I think converge in some sense means intersect and produce image.

Scientifically, I am not sure if my book is even correct, all the light rays coming off an object parallel to each other, there can't be any intersection. And yet my book says that an image is produced at f.

update I forgot to mention the third place where the word is mentioned. There, my book simply says "light parallel to the optical axis converges on the far focal point", without saying that any image is produced.


1 Answer 1


When it is saying the light rays converge, it means that they intersect. THe light rays intersect because the lens bends them so they all point at the same spot. I will explain more.

When a point source emits light, it emits in all directions. This is why if you are in a dark room and you put a candle in front of a sheet of paper, you will see a diffuse bright spot (instead of an image of the candle, for example).

Now suppose you use a converging lens. Then there is an angular range where light leaving the candle will pass through the lens. (Since the light is going in all directions, a lot of it miss the lens.) After the light rays, which had originally been divering in straight lines away from the candle, pass through the lens, they are bent by the lens so that they all begin to aim at the same spot in space. Now suppose you put a sheet of paper where that spot is. Then you will see a single bright point instead of the diffuse bright area that you had before.

Now suppose you add a second candle right next to the first one and keep the sheet of paper where it was. Then the light from both candles converges on the paper, but the bright spots from the two candles form in different places. Thus you are able to tell by looking at the sheet of paper that there are two candles.

The reason I talk about two candles is because every point in a real object emits light, so every point on the real object will have a different image point on the paper, and these will be arranged such that they form an image that looks identical to the orginal object (except it might be inverted).

Again this is all possible only becaues the lens is making the light rays coverge. Without the lens, your object would just create a big diffuse blurry bright area on the sheet of paper, since light from every point on the original object is hitting every point on the sheet of paper. Hopefully this explains what it means for the light rays to converge and why it is important for creating an image.

  • $\begingroup$ You helped me confirm that converge mean concentrate. But my other question is if there will be any image formed from the PARALLEL light rays coming off of an object placed infinitely far from the lens. $\endgroup$ Commented Jul 31, 2015 at 22:09
  • $\begingroup$ Will just be a dot since it's at the focal point? $\endgroup$ Commented Jul 31, 2015 at 22:10
  • $\begingroup$ Yes, the lens will bend the light rays so they all converge to a single point. To add a wrinkle, I will say that the location of this point will depend on what angle the incoming rays from infinity make with the lens. $\endgroup$ Commented Aug 1, 2015 at 2:58

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