# Can we see a point with only one ray or do we need two rays?

I am just a high school student trying to self study, so please excuse me if you find the question silly.

My teacher says that we can see a point when a ray of light from the point is reflected into our eye.

But almost everyone on the internet say that when two diverging rays from a point enter our eye we can see the point.

Which one is correct A or B. (Don't laugh I only had paint)

• Mar 10 '19 at 8:43
• @Farcher That is also my question. Mar 10 '19 at 9:32

Both are correct in ways.

The eye changes its shape so that when you focus on a point, all the diverging light from that point is collected and redirected onto (roughly) the same detector on the back of your eye. (as in image B) However, in theory, you only need to collect a single photon to "see" a point (this corresponds more closely to A). Very sensitive light detectors can see a single particle of light, and IIRC, so can the eye if it is very dark.

In reality however (literally in ray optics, which is a very good approximation for visible light), most objects emit light in all directions, so in realistic situations, B is the better picture. It's important to note that there are "an infinity" (in reality a large but finite number) of rays emitted in a sphere from every point. There is nothing special about the ray distinguished in A. If you blocked the top ray in B, you'd still see the point, it would be the same situation as A, but the light would just be bent by the eye. Each additional ray simply adds to the amount of light collected by the eye, making the image more clear.

• Just to make sure I understood it correctly. You are saying that one ray of light would not give our brain sufficient information to form a clear image, but a ray of light is the minimum amount of light needed to sense something although not very clearly. If two or more rays enter our eyes then we can recover more information about the object and we will be able to see a clearer image.Right? Mar 10 '19 at 9:44
• What do you mean by "can see a single particle of light" and "but the light would just be bent by the eye" Mar 10 '19 at 9:48

Both your teacher and strangers on the Internet are correct because you can see diverging rays of light coming from an object when you're focused on that object.

Let's take a look inside your eye. Light interacts with three parts of your eye: the cornea, the lens and the retina. The cornea and the lens work together to focus light onto the retina.

When you look an an object your unconscious brain will tell your eye to focus on the object. "Focusing" on an object means stretching out your lens to just the right focal length. "Just the right focal length" means that rays of light originating from the thing you're looking at converge at the same point on the retina.

You can see a point from one (or more) diverging rays of light because they all hit the same point on the retina. More is better because more light is easier to see.

• Can you tell me why it is that more light rays from a single point means a much clearer picture. Mar 10 '19 at 10:21
• I think this question deserves to be asked on the top level. Mar 10 '19 at 10:31