Focal Length (1 Viewer)

[proletariat]

I'm doing some work on Communication and there is something that is confusing me.

From what I've learned, Focal length is the length from the lens to where the light converges on the retina (right?) but I dont get this paragraph.

"But when we look at a close object the ciliary muscles tighten and
contract inwards towards the centre of the lens, so that the suspensory
ligaments become looser (Figure 1.9b). This allows the lens to bulge,
increasing its refractive power. The thicker the lens, the shorter its
focal length."

When the lens 'bulges', it bulges towards the front of the eye so focal length should not be changed, but according to this source it is changed. :S

Source is Hienemann Biology Option 1 Communication: page 12.

edit: Also, in radial keratotomy and w/e the other ones are, the retina is 'cut' with a laser or surgically. Which part of the retina is cut? Is any part of the retina made of 'useless' material?

 

[samthebear]

when the lense 'bluges' as you describe, its actually just becoming rounder in shape - it doesnt move towards the front of the eye. the position of the lens stay the same even when the cilary muscles are working. all the lens does is become rounder in shape thus increasing the refractive power resulting in a shorter focal length (that is, the distance for light to reach the retina)

the refractive power of lenses change as the density of the material through which light travels changes. so when your cilary muscles are at rest, the lens becomes more elongated and flat resulting in a lower refractive power which means the focal length increases. However when the cilary muscles contract, the lens becomes rounder resulting in a higher refractive power resulting in a shorter focal length.

i dont know who told you that the lens moves to the front of the eye but all the lens does is become rounder in shape.

This whole thing about lenses changing refractive power has to do with accomodation - i dont know if you've covered accomodation yet but once you do the way the lens changes its density by becoming rounder or more elongated will make more sense then.

 

[Spark_Coaching]

This can be sort of confusing, especially the relationship between the ciliary muscles, the suspensory ligaments and the shape of the lens. Try to make sense of the following:

If the lens becomes more rounded (greater curvature) it refracts light to a greater extent and close objects can be focused.

The ciliary muscles are responsible for adjusting the shape of the lens. When they contract, the lens becomes more rounded. In this case the suspensory ligaments are relaxed and allow the lens to bulge.

When they relax however, they pull on the suspensory ligaments which in turn pull on the lens and keep it thin. Which is used for focusing on distant objects.

 

[Lukybear]

I do not think the focal length changes as lens changes curvature in eye as image projected in focused.

Unless anyone cares to expand.