Vision 1 Flashcards
What is refraction?
Bending of light when it passes from one optical medium to another
Light rays bending to form a sharp image on the retina
What are refractive errors?
Mismatch between how much we bend light rays
What parts of the eye does light pass through and therefore contribute to refraction
Cornea
Aqueous humor
Lens
Vitreous humor
All these transparent to allow light to fall on retina
How does a share image form on the retina?
Light waves from an object bend at the cornea, bend some more at the lens to form a clear image on the retina.
This bending of light waves is called Refraction.
When an object comes closer, the eye needs more bending power to focus on an object (the lens becomes thicker and hence more powerful, and a clear image is formed on the retina again)
What is better at refraction cornea or lens?
Cornea is the most powerful “bender” of light (45D) but lens (15D) has the capacity to change its “bending power”
Explain accommodation
Our eyes have the capacity to change the focus from distant objects (infinity) to close objects (20cm)
The changes occurring in both eyes as it changes focus from a distant to a close object is called accommodation.
What 3 things happen simultaneously and comprise accommodation?
- LENS CHANGES SHAPE (becomes thicker & more spherical)
- Pupil constricts
- Eyes converge
Describe the process of lens thickening
- Ciliary muscle contracts making the ciliary body bulge (parasympathetic).
- Space in the middle decreases
- Suspensory ligaments become lax
- Lens is no longer under stretch
- LENS BECOMES THICKER
Describe the process of pupil constriction
- When we are looking at an object up close, we need a sharp focus.
- To sharpen focus the pupil constricts to allow only a few rays (those from the object) to pass through.
- Pupillary constrictor (sphincter pupillae) is a concentric muscle around the border of the pupil which gets parasympathetic innervation IIIn.
Describe the process of eye convergence
When focusing on an object up close, our eyes have to turn in to look at the object (convergence)
We use our medial rectus muscles of both eyes to converge (IIIn)
Humans spend a lot of time doing “close” work, so our Medial recti muscles are thicker than our lateral recti muscles.
What are the 4 refractive errors?
- Myopia: short sightedness
- Hyperopia: long sightedness
- Astigmatism: non-spherical curvature of cornea (or lens)
- Presbyopia: long-sightedness of old age
If you are lucky enough to have perfect vision you are an emmetrope (i.e. you have emmetropia)
What is myopia?
Shortsightedness: Close objects look clear, distant objects appear hazy.
Picture focuses in front of retina
Basically the bending power of cornea + lens are too much for that eyeball
When the object is brought closer the rays coming from it are divergent; and then this “bending power” comes to use. The image is formed on the retina without needing to increase curvature of lens.
What is the most common cause of Myopia?
Eyeball too long
So the cornea and lens, when the bend rays of light, make the image to form IN FRONT of the retina. So far off objects not seen clearly.
When the object is brought closer the rays coming from it are divergent; and then this “bending power” comes to use. The image is formed on the retina without needing to increase curvature of lens.
What are the symptoms of myopia?
Headaches, Complain of not being able to see blackboard/ distant objectsInfants & preverbal children divergent squint
Infants and preverbal children -> divergent squint
Toddlers -> loss of interest in sports/people. More interest in books, pictures.
Teachers may notice child losing interest in class
How to correct myopia
Bending power needs to be decreased.
Biconcave lenses
- Spectacles
- Contact lenses
- Laser eye surgery
What is hyperopia?
Farsightedness
Image focuses behind the retina
Close objects look hazy, distant objects appear clear.
What causes hyperopia?
Eyeball too short or cornea + lens too flat
So when an image of a distant object is formed on the retina it lies BEHIND THE RETINA.
The person then automatically starts to use his accommodative power and makes the lens thicker.
This causes the image to form on the retina.
So he is using the lens power to see far off thing (that he should normally be seeing without using any power).
When seeing closer objects, he uses more and more power until ultimately, his power is all used up.
What are the symptoms of hyperopia?
Eyestrain after reading/ working on the computer in a young individual.
Convergent squint in children/ toddlers - needs immediate correction with glasses/lenses to preserve vision in both eyes and prevent lazy eye
What is the correction of hyperopia?
Biconvex glasses alleviates use of glasses for focusing distant objects and “restS” the accommodative power
Contact lenses
Laser eye surgery
What is astigmatism?
Both close and distant objects appear hazy
Multiple images are formed
What causes astigmatism?
Surface of the lens has different curvatures in different meridians
So the bending of light rays along one axis will never be the same as that of the other axis.
So image formed is always hazy, whatever the distance of the object
How do you correct astigmatism?
Special glasses - called cylindrical glasses (which are curved in only one axis)
Laser eye surgery can also be used to correct the defect.
Need special contact lenses called topic lenses
Explain presbyopia
Longsightedness of old age.
With age the lens gets less mobile/elastic.
So when the ciliary muscle contracts, it is not as capable as before to change shape.
So seeing near objects/ reading the newspaper starts to become difficult -> needs glasses to read
Usually starts in 50s
How do you correct presbyopia?
Biconvex “reading glasses”
