The power of the human eye's lens

[Asmaa Mohammad]

Hello,
I read in my physiology book that when the ciliary muscle relaxes, the power of the eye lens decreases, hence we get the ability to see far objects.

I am new to physiology, and when I googled "the power of the eye lens" I didn't find articles explain the sentence above simply.

So I would appreciate if someone explains for me how would the decrease of the lens power makes us see far objects?

Thanks in advance!

 

[.Scott]

The power of the lens refers to how much it bends the light.
When the lens is relatively flat, it will not bend the light very far, so it is well suited for distant objects.
When it is more bulgy, its forward and rear surface are more curved, and the light is bent further - it is more powerful. This is what is needed to focus on item in the foreground.

 

[Drakkith]

To understand how the lens works, you need to understand a little about how the light coming from an object behaves as the distance between the object and the eye changes. Let's consider 3 examples:

A. An object placed far away from the eye.
B. An object very close to the eye.
C. An object in between.

First, note that light reflected (or emitted) by a single point on the object moves outwards away from the object in a large cone. Part of this expanding cone of light is then captured by the cornea and pupil. So we can say that the light that is captured by the eye forms its own cone with a smaller angle than the larger cone, whose light goes everywhere else. In the image below, object O₁ corresponds to example A, object O₂ corresponds to example B, and object O is example C.

The dashed/solid lines coming from O₁, O₂, and O₃ represent the outermost light rays of our expanding cone that the eye captures. The key thing to notice is that the angle between the two extreme light rays of this cone increases as the object gets closer to the eye. In other words, as the object gets closer, the light rays of the cone hit the cornea at an increasingly large angle as the object moves closer. If we imagine a single glass lens instead of the eye, then as the angle of these light rays increases, they get focused further and further back behind the lens. So an inflexible glass lens by itself only has a very small range of object distances where it can bring the light down to a tight focus. Everything outside of that range would appear blurry. If you had this single lens and a camera sensor behind it, you would need to move the sensor or the lens back and forth to get objects at other distances in focus (which would make the objects previously in focus become blurry).

The problem here is that the cornea (the most external part of the eye, and the part which has most of the refractive power of the eye. The lens just does "fine focusing") is not flexible and does not bend. Or, more accurately, it's flexible, it just doesn't have anything attached to it to bend it. This means that if we had no lens, or if our lens was inflexible, we would have only a small range of distances wherein objects would be in focus. Luckily for us, our lens is flexible and is attached to muscles which cause it to change shape when flexed. When you focus on an object at a closer distance, such as O₂ in the image below, the muscles in your eye flex and bend the lens so that it is more strongly curved and has more power. This increased power brings the light rays from O₂ (the short dashed lines) to focus closer to the lens. Ideally, it brings them to focus on the retina itself.

However, the light rays from objects further away, represented by O₁ in the picture, are then brought to focus before they hit the retina. This is why objects far away look blurry when you focus up close.

If you then shift to look at O₁, the muscles in your eye relax and your lens goes back to its original shape and becomes less strongly curved. The light rays aren't bent as much and now the rays from O₁ come to focus on your retina and the rays from O₂ are focused behind the retina (or would be if they weren't absorbed by the retina first). Hence, objects up close look blurry when you focus into the distance.

If you then focus on object O, then both far away objects nearby objects look blurry. This is what is being shown in the image below. The rays from O are focused onto the retina, while the rays from O₁ come to focus before the retina and the rays from O₂ are brought to focus behind the retina.

 

[Andy Resnick]

Hello,
I read in my physiology book that when the ciliary muscle relaxes, the power of the eye lens decreases, hence we get the ability to see far objects.

I am new to physiology, and when I googled "the power of the eye lens" I didn't find articles explain the sentence above simply.

So I would appreciate if someone explains for me how would the decrease of the lens power makes us see far objects?

Thanks in advance!

Try googling "accommodation of the eye".

 

[swampwiz]

The lens of the eye is functionally similar to the lens *system* in that it has a range of focal length that is achieved by the muscles there; in a camera, this range is achieved by varying the distance between a pair of fixed focal-length lens (i.e., the pair of lens at particular distance from one another has an equivalent focal length from some reference principle plane that would apply to a single lens). For both spherical & cylindrical focus separately, an there is a minimum & maximum focal length, with some fixed image length (i.e., the distance from the eye's lens to the retinal surface), so that there is an object length corresponding to that minimum & maximum focal length. The range of this object length is the accommodation. Myopic (near-sighted) folks have a maximum object length that is less than infinity whereas Hyperopic (far-sighted) folks have a maximum object length that is "greater" than infinity (and thus corresponds to a negative object length). It should be noted that folks who have very good accommodation can have a range within the standard accommodation range (i.e., infinity on the far end and some short length whose value I don't quite know off-hand) and thus not be considered as having bad eyes; such folks could actually focus a larger range of object lengths if they were to wear an Rx. I myself am myopic, but at an early age my accommodation was so good that with Rx, my vision was 20/10.

The condition of having a limited range of accommodation is terms presbyopia, and folks get it as part of the aging process. The ironic thing is that folks with presbyopia can actually see one side of the length better. With my myopia with presbyopia, and I can look at things at a very close distance without Rx, so I read without Rx, but absolutely need Rx to drive. Likewise folks who are hyperopic with presbyopia can see far without Rx, but they absolutely need Rx to read.

 

[rude man]

The eye lens and retina behave like any other lens-screen system: 1/f = 1/p + 1/q
where f = focal length of lens, p = distance from lens to object, q = distance from lens to retina.

To satisfy the equation for any p, since q can't be changed as it is in e.g. a camera, f must be variable. And so it is. The eye muscles squish or relax the flexible lens to adjust f to always satisfy the equation.