A body at a very distant point, such as the ship on the horizon line, is said to be at infinity.
We know that the image focus is located on the retina and the light rays, coming from the object we are observing, fall on our eyes practically parallel. The illustration below gives us an idea of how light rays strike the human eye.
In this type of situation, where the observed object is in “infinity”, our eyes do not strain their muscles, so we have the sensation of rest in the eyes. Therefore, the farthest point at which an object can be so that the human eye can see it clearly is called the remote point. Therefore, we can say that the remote point of the human eye is located at infinity.
Now let’s imagine that the boat is approaching the beach, that is, it is coming very close to you. The minimum distance at which an object can be seen clearly, without being out of focus , is 25 cm for a normal adult human eye. This minimum distance at which an object is clearly seen is called the near point . See the illustration below.
In order to focus on this near point, you can perform a simple experiment. Take a pencil and hold it at a distance from one of your eyes, fix your gaze on the tip of the pencil and slowly bring it closer to your face, trying to determine the smallest distance that allows a clear vision. Distances smaller than this should give a blurred image.
We can feel the great muscular effort that our eyes make to keep images of very close objects sharp. For this reason, after carrying out a prolonged reading, it is recommended to fix a remote point, that is, a point farthest from the eyes.
