3. Optics of the eye Flashcards
When a person looks at an arbitrary object, her or his eyes are positioned in such a way that the light rays coming from the object meet at the ___
yellow spot
(central area of the retina responsible for highest-resolution vision.)
What is yellow spot?
central area of the retina responsible for highest-resolution vision.
What happen to the eye when ciliary muscles are relaxed?
the lens remains flattened
→ light rays entering the eye from close objects are focused behind the retina.
→ NO ACCOMODATION
→ a sharp image is formed on the retina only from very distant objects.
What happen to the eye if the ciliary muscles contract?
Then the the lens becomes relaxed and protruded
→ Its increased curvature (and power) allows the focusing of light rays from close objects on the retina.
→ Accommodation for near objects
→ Image formation by refraction on a spherical surface separating two media with different refractive indices
What is POWER OF THE EYE?
Sum of the powers of all refractive surfaces of the eye
(its unit is the diopter: 1 dpt = 1/m, Deye = 59 -72 dpt).
- D is the total power of the eye in diopters (1/m)
- o and i are the object and image distances, respectively
- n is the index of refracion of the object medium (usually air, n = 1)
- n’ is the average index of refraction of the eye tissues.
What is accommodation?
adjustment of the total power of the eye by changing the curvature of the eye lens in order to focus on objects at different distances.
What is FAR POINT OF VISION (or)?
the distance from the eye to the farthest point whose image can be focused on the retina with a relaxed eye (without accommodation).
The far point of vision (or) is the distance from the eye to the farthest point whose image can be focused on the retina with a relaxed eye (without accommodation).
→ The power of the eye is the __ (smallest/largest) in this case
→ Give the formula?
SMALLEST
- D is the total power of the eye in diopters (1/m)
- o and i are the object and image distances, respectively
- n is the index of refracion of the object medium (usually air, n = 1)
- n’ is the average index of refraction of the eye tissues.
What is near point of vision (op)?
The distance from the eye to the nearest point whose image can be focused on the retina by accomodation
The distance from the eye to the nearest point whose image can be focused on the retina by accomodation is called the near point of vision (op).
→ The power of the eye is the ___ (smallest/largest) in this case
→ Give the formula
largest
- D is the total power of the eye in diopters (1/m)
- o and i are the object and image distances, respectively
- n is the index of refracion of the object medium (usually air, n = 1)
- n’ is the average index of refraction of the eye tissues.
What is ACCOMMODATION POWER?
the difference between the largest and the smallest power of the eye
List 3 COMMON DISORDERS OF THE IMAGE FORMATION SYSTEM
Nearsightedness (myopia)
Farsightedness (hyperopia)
Age-related farsightedness (presbyopia)
What is NEARSIGHTEDNESS (MYOPIA)?
parallel light rays from an infinitely distant object point are focused in front of the retina.
→ It can be corrected with diverging eyeglasses.
How to CORRECT Nearsightedness (myopia)?
It can be corrected with diverging eyeglasses.
What is FARSIGHTEDNESS (HYPEROPIA)?
light rays from close objects are focused behind the retina.
→ It can be corrected with converging eyeglasses.
How to correct FARSIGHTEDNESS (HYPEROPIA)?
It can be corrected with converging eyeglasses.
What is AGE RELATED FARSIGHTEDNESS (PRESBYOPIA)?
the accommodation power of the eye decreases due to the loss of elasticity of the lens.
→ The near point of vision recedes farther and farther from the eye.
→ It can be corrected with converging eyeglasses and by using different lenses for different object distance range.
How to correct AGE RELATED FARSIGHTEDNESS (PRESBYOPIA)?
It can be corrected with converging eyeglasses and by using different lenses for different object distance range.
Fill the gap
What happen if a combination of hyperopia and presbyopia occurs? How to correct it?
then the patient cannot see sharply far objects either.
→ In this condition eyeglasses with a bifocal lens (combination of two lenses of different focal length), or progressive lens (lenses with gradually changing focal length) may be used
What is reduced eye?
- Simplified model of the human eye with a reduced number of refracting surfaces.
- It is a homogeneous body possessing a single refracting surface.
- The nodal point K is at 17 mm distance from the yellow spot.
Fill the gap
The reduced eye is a homogeneous body possessing a single refracting surface
The index of refraction of the reduced eye is ___
The curvature of its spherical surface is ___
The center of curvature is the nodal point K, at ___
- n = 1.34
- r = 5.1 mm
- 17 mm distance from the yellow spot.
What is LIMITING ANGLE OF VIEW (α)?
the smallest angular view of separated points A and B that can be just distinguished. Normal value is 1’.
(Angle of view (or angular height of the object) means the angle between the light rays from the edges of the object to the nodal point K.)
What is VISUAL ACUITY?
Resolution of the eye, or visual acuity (visus) is expressed as the ratio of the normal limiting angle of view (1’) to the actual limiting angle of view ( ) in percents
What is the achievable resolution of the eye determined by? (factors influenced visual acuity)
It determined by..
- the optical aberrations of the eye
- diffraction on the pupil
- the anatomical density of the photoreceptor units of the retina.
Visual acuity is influenced by several factors
What can decrease the sharpness of image in eye? Why?
Image is not sharp due to diffraction.
→ Airy disks
What is Airy disk?
- Diffraction patterns
→ Even the optically perfect eye does not form points in the image
→ Light rays incident on the eye are diffracted on the pupil as the consequence of the wave character of light
→ Thus. the “image points” are formed as diffraction patterns called Airy disks.
What are PHOTORECEPTORS OF THE RETINA?
Cones are responsible for the daylight (photopic, color vision) and rods for the night (scotopic) vision.
Does fovea contain any rods?
NO
visual acuity is the greatest in the ___
fovea centralis.
How to calculate The size of the break of the image of the Landolt ring on the retina?
This size is approximately the diameter of a single cone. As the image distance of the reduced eye is i = 17 mm
- a is the size of the break of the Landolt ring
- x is the distance from the chart when the break appears and disappears.
What is the receptor density:?
Reciprocal of the area of the receptor cell
What is BLIND SPOT?
Area at the point of exit of the optic nerve, where no photoreceptors are present on the retina, and thus the detail of the image projected to this spot is lost.
What is BLIND SPOT?
Area at the point of exit of the optic nerve, where no photoreceptors are present on the retina, and thus the detail of the image projected to this spot is lost.
How to determine blind spot?
How to calculate the distance of their images d ‘ (thus the distance of the yellow spot and blind spot on the retina)
Close your right eye and focus your left eye on the cross. Move the paper until a distance x is found where the red dot disappears. When this occurs, the image of the spot has fallen on the blind spot of the retina
→ the measured distance x and known distance of the cross from the dot d the distance of their images d ‘ (thus the distance of the yellow spot and blind spot on the retina) can be calculated
If we determine the distances x1 and x2 when the red dot appears and disappears
→ How to calculate the size of blind spot?
the difference of the distances d’1 and d’2 of the left and right edge of the blind spot from the yellow spot:
