How we see 1 Flashcards
What is refraction?
Bending of light when it passes from one optical medium to another
when those optical mediums have different refractory indexes
In what parts of the eye is light refracted?
Cornea
Aqueous humor
Lens
Vitreous humor
What is the refractive power of the cornea and lens of the eye?
Cornea = 45D (dioptres)
Lens = 15D but variable (this is what we focus with)
As an object comes closer. How must the refractive power of the eye change in order to maintain a clear image?
More bending power is needed by the eye
To do this - the lens thickens thus becoming more powerful
What is the image on the retina like compared to what we see?
Image on retina is the superimposition of the object
What is accommodation?
Our eyes can change its focus for objects 20cm to infinity
The changes occurring in both eyes as it changes focus from a distant to a close object is called as accommodation
3 factors:
- Lens shape
- Pupil constriction/dilation
- Eye convergence/divergence
What is happening here and why?
Thickening of the lens to Increase refractive power (closer focus)
How it works:
- Parasympathetic stimulation causes Ciliary body to contract
- (When it does this it weirdly bulges out)
- Suspensory ligaments become lax ∴ less stretching tension on the lens so it thickens
Describe why our pupils constrict when objects move closer to us
Object close up ∴ need sharp focus
To sharpen focus, our pupil constricts to only let a few rays (those from the object) through
Describe how our pupils constrict to focus on something close
Pupillary constrictor (sphincter pupillae) is a concentric muscle around the border of the pupil
Parasympathetic stimulation (via IIIn) causes it to constrict - decreasing the circumference of the pupil
Describe why and how our eyes converge to focus on an object that’s close up
Pretty simple. You turn your eyes in when looking at something close up
We use our medial rectus muscles of both eyes to converge. (IIIn of both sides)
These are thicker than the lateral rectus because our eyes converge often
What are the spicy names for short & long-sightedness and normal vision
Myopia = short-sightedness
Hyperopia = long-sightedness
Presbyopia = long-sightedness with old age
Emmetropia = perfect vision
What is astigmatism?
non-spherical curvature of cornea (or lens)
Describe how Myopia happens
Most common cause = eyeball is too long
Basically, the total refractive power of the cornea & lens is too much for that eyeball so the image focuses in front of the retina
When the object is brought really close up, the image is formed on the retina without the lens needing to change shape - thus the person can focus on close up objects (hence short-sighted)
How would myopia present?
Complaints of:
- Not being able to see distant objects (no shit)
- Headaches
- Lack of interest in sports
- Loss of interest in class
- Increased interest in books/pictures (stuff thats close up)
Signs:
-
Divergent squint
- ^ in Infants/pre-verbal children
What type of lens will correct myopia?
Biconcave
Spectacles or contact lenses
These reduce total refractive power
Laser eye surgery is other option
What causes Hyperopia?
Hyperopia (far-sightedness) caused by the eyeball ‘being too short’
Refractive power of eye is too low, so focal point is behind the retina
Explanation:
The person will begin using the accommodative power of their eyes at a further distance than a normal person (so will be thickening lens at a distance when normal people would not be)
As the object gets closer, the eyes will be unable to accommodate it at a much greater distance than normal
How does Hyperopia present?
Complaints of:
- Eye strain after reading/using computers etc (close up shit)
- Struggling to read and all that
Signs:
- Convergent squint (in the youngers)
- May have a lazy eye
How is hyperopia treated?
Why is swift treatment important?
Biconvex glasses / contacts
Laser eye surgery
Children with Hyperopia need immediate correction with glasses/lenses to preserve vision in both eyes and prevent a “lazy eye”.
How would astigmatism present?
What causes it?
Complaints of:
- Distant and close objects out of focus (hazy)
Cause:
- Surface of cornea (or lens) has different curvatures in different meridians (ie not spherical)
- Light rays in one axis are bent more than light rays in the other
What are the options for the treatment of astigmatism?
Glasses:
- Cylindrical glasses - which are curved in only one axis
Contact lenses:
- ‘Toric lenses’
Laser eye surgery - can also be used
What causes presbyopia?
With age the lens gets less mobile/elastic - thus won’t change shape as easily/well
Accommodation to focus on nearby objects is impacted - reading etc becomes harder
Corrected using Reading glasses (Biconvex)
