Lecture 10: The Eye
Lecture aims to explain:
1. Human eye anatomy
2. Optical power of a lens
3. How eyes work
4. Correction for simple faults in human vision Human eye anatomy Human eye anatomy Light passes through transparent tissue cornea first element in the eye's focusing system
Then iris controls the amount of light entering the eye
The adjustable crystalline lens – a biconvex body helping to focus the light on retina a layer of light-sensitive tissue at the back of the eye.
The retina contains light-sensitive cells called photoreceptors, which translate the light energy into electrical signals fed into optic nerve for delivery to the brain. Photoreceptors in the retina: cones and rods Rods ~120 million on the retina, fast and sensitive “black & white” light detectors, responsible for vision in reduced light levels
Cones ~6 million on the retina, slow and less sensitive “colour sensitive” light detectors
Rods ~ 100 – 1000 times more sensitive to light than cones Optical power of a lens Optical power of a lens
Each surface bends the incoming rays: the more the bending the “stronger” the surface
The lens power is defined as the reciprocal of the focal length: 1 1 1 D = = (n −1) − f R1 R2
The unit of optical power is the inverse metre or the Diopter symbolised by D How eyes work Accommodation: fine focussing
Eye diameter ~25mm
How can the lens system be adjusted so that images of objects at different distances are always obtained on the retina?
Object far away Object near
The answer was first suggested by Hermann von Helmholtz working in Berlin in 1850s. His theory remained unchallenged until the end of the 20th century Mechanism of accommodation In the human eye accommodation is carried out by changing the focal length of the crystalline lens: this would always allow to obtain the image on the retina
Object far away: muscles Object near: muscles relaxed pulling out the lens contracted “squeezing” the making it flat (larger curvature lens making it bulge (smaller radius, less strong lens) curvature radius, stronger lens)
Near point: closest distance at which an eye can see. Also termed least distance of distinct vision ~25 cm for a “normal” eye Correction for simple faults in human vision
Eyeglasses were probably invented in the late 13th century in Italy. Mentioned in the 15th century by the German cardinal Nicholas Cusa, and ceased to be novelty in the late 15th century. Myopia or near- sightedness
Myopia occurs when the eyeball is slightly longer than usual from front Object in infinity to back. This causes light rays to focus at a point in front of the retina, rather than directly on its surface.
Far point: for unaccommodated eye, object point whose image lies Distant object on retina
For a “normal” eye far point is at infinity
No accommodation for all all for 3 examples No accommodation Myopia can be corrected with Far point negative lenses Hypermetropia or far- sightedness
This vision problem occurs when Object in infinity light rays entering the eye focus behind the retina, rather than Noaccommodation directly on it. The eyeball of a farsighted person is shorter than normal.
Many children are born with Near point hyperopia, and some of them "outgrow" it as the eyeball lengthens with normal growth.
Can be corrected with positive lenses
Maximum accommodation Maximum 25 cm (near point for “normal” eye) SUMMARY Imaging in human eyes could be understood by considering how a system of two lenses works: one fixed lens in the outer shell of the eye, and one adjustable lens inside the eye.
In humans accommodation, a process of obtaining an image on the retina, is carried out by adjusting the focal length of the adjustable crystalline lens inside the eye.
Simple faults in human vision occur because the focus of the lens system in the eye can not adjust as required to obtain images on the retina. Can be corrected with additional lenses.
The lens power is measured -1 1 1 1 in m or Dioptres (D) and is = = − − defined as the reciprocal of D (n 1) the focal length: f R1 R2