# images of objects at infinity

Is the focal point of our eye lens is on the retina? if it is so why dont we see point image of infinite object because light rays coming from them are parallel to each other and converges at focal point.

• "why dont we see point image of infinite object..." you mean like stars? – Asher Jun 15 '15 at 7:57

The focal length of the eye's lens changes based on what is being looked at. Ciliary muscles add or remove tensions from the lens to changes its shape, and thus its focal length. The closer the object, the shorter the focal length.

• focal length changes that's OK but why don't we see point images of objects. For example when we take focal length of a convex lens in lab we obtain a sharp image and we take the focal length all the rays are parallel because they are coming from infinity so why we don't see a point image on focus – raja aekant Jun 15 '15 at 8:33
• I think you are confusing the definition of focal length (where parallel rays are focused) with forming an image. A lens cannot form an image of an object at infinity. To form an image, light from different points on the object are projected onto different points on the image plane. When an object is at infinity, different points on the object are indistinguishable (much like stars, which appear as points even for powerful telescopes). – Mark H Jun 15 '15 at 8:50
• Now, for my lab, what we did was that we pointed the lens toward the far away (about 100 m away) trees outside the windows. Then, I assumed the objects were at infinity. So, in the thin lens equation : 1/f=1/s=1/s', 1/s tends to 0. Then, calculate the focal length, I just moved the screen forward and backward. When the image was clearest, I assumed the distance from the screen to the lens was the focal length. However, I find this weird when I came back to study for my lab test. How come I did not see a single point on the screen? – raja aekant Jun 15 '15 at 10:00
• Look at the lens equation: $1/d + 1/i = 1/f$ where $d$ is the object distance, $i$ is the image distance, and $f$ is the focal length of the lens. When $d$ is very large compared to the focal length, $1/d$ is very small, so the image distance is very nearly equal to the focal length. If you had a precise enough ruler, you would be able to tell the difference. Also, the light coming from the trees is not all parallel. Light from every leaf of the tree is hitting all parts of the lens. This is the reason you don't just see a point on the screen. – Mark H Jun 15 '15 at 11:24
• The image is always formed on the retina, which is about an eyeball-diameter away from the lens. In the lens equation, $i$ is constant and equal to the diameter of the eyeball. The eye changes the focal length of the lens to make sure this is always true. The only time the image distance is about equal to the focal length of the lens is when the object is very far away and the focal length is about equal to the diameter of the eyeball. – Mark H Jun 15 '15 at 13:08

The light rays coming from one point on the object are indeed parallel. However, the sets of parallel rays from different points come from DIFFERENT DIRECTIONS.

By the way, the object "at infinity" is a generalization. Of course, all real objects we observe are positioned at a finite distance from our eye. It is just that the divergence of the rays is negligible in some sense. For example, given the size of the Earth (~13000 km) the divergence of light rays coming from the Sun are almost parallel because the Sun is being so far away (~150 000 000 km) compared to any, even impossibly large lens we could have on Earth.

The rays from an infinite object are parallel for every point on the object but are not parallel for two different points on the object. Hence different points on the object form an image at different points on retina

• Now, for my lab, what we did was that we pointed the lens toward the far away (about 100 m away) trees outside the windows. Then, I assumed the objects were at infinity. So, in the thin lens equation : 1/f=1/s=1/s', 1/s tends to 0. Then, calculate the focal length, I just moved the screen forward and backward. When the image was clearest, I assumed the distance from the screen to the lens was the focal length. However, I find this weird when I came back to study for my lab test. How come I did not see a single point on the screen? – raja aekant Jun 15 '15 at 10:00
• You don't see a point image cause your object is not a point source. – SAKhan Jun 15 '15 at 11:58
• object is not a point source but rays converge at a point you agree that or not? – raja aekant Jun 15 '15 at 12:38
• object is not a point source then the rays will not meet at a point. – SAKhan Jun 15 '15 at 14:16