Vision 1 Flashcards
Define refraction accommodation and refractive errors?
Refraction - Bending of light rays as they move from one medium to another
Accommodation - Altering lens shape, pupil size, eye convergence to alter how much we refract the light
Refractive Errors - Mismatches in light ray refraction onto the retina
What kind of lens causes conversion and what kind causes divergence?
A biconvex lens converges light rays
A biconcave lens diverges light rays
What refractive media are in the eye?
- Cornea
- Aqueous Humor
- Lens
- Vitreous Humor
The cornea is the biggest bender but the lens is capable of varying its refractive power (accomadation)
At what distance will all light rays reaching the eyes be parallel?
6m and beyond
How does lens thickness change as the object moves closer to the eye?
It gets thicker because:
- The close the object the more divergent the rays reaching the eyes
- So the greater the refractive power required
How does the lens thicken?
- > Parasympathetic stimulus
- > Ciliary muscle contracts
- > Makes ciliary muscle bulge
- > Causes suspensory ligaments to go lax
- > Lens stretch decreases and lens gets thicker
How does pupil constriction affect focus?
The closer the object the more the pupil constricts to minimize the rays getting through
How does the pupil constrict?
Cranial Nerve III (Oculomotor)
- > Parasympathetic fibres
- > Sphincter Pupillae Muscle Contracts
What causes our eyes to converge to focus on closer objects?
Cranial Nerve 3 (Oculomotor)
-> Stimulates Both Medial Recti
Because we do so much close work our medial recti have become thicker than our lateral recti
What do we call the condition of having perfect eyesite?
Emmetropia i.e. you are an emmetrope
List some common refractive errors?
Myopia - Short Sightedness
Hyperopia - Long Sightedness
Astygmatism - Non-spherical cornea/lens
Presbyopia - Age related long-sightedness
What causes myopia?
The persons eyeball is slightly longer than normal
Causes the refractive power of the eye to be excessive and the rays converge before hitting the retina
How does myopia affect close vision?
It doesn’t because close up the excess refractive power of the eye does the job without needing to use your lens
How does myopia affect distance vision?
Further away objects are seen with less divergent rays so the over-refractive nature of the myopic eyeball means you cant focus the image on the retina even with the ciliary muscle fully relaxed
How would you detect a myopia in kids?
- Infants may show a divergent squint
- Toddlers may lose interest in sports/people and instead focus on books/pictures
- Difficulty seeing or losing interest in class
- Headahces