Module 1 - Optics and Refraction Flashcards
What is refraction in medicine?
Clinical test in which a phoropter/loose lenses may be used to determine the eye’s refractive error and the best corrective lenses to be described
Analogy of the camera to the eye Lens filter Adjustable diaphragm Adjustable aperture Lens Film Camera body
Lens filter - cornea Adjustable diaphragm - iris Adjustable aperture - pupil Lens - crystalline lens Film - retina Camera body - sclera
What are the 2 refracting elements of the eye?
Cornea
Crystalline lens
What refracting element acts like a converging lens that accounts for 2/3 refractile power of the eye?
Cornea
How many diopters is the cornea?
40 diopters
Describe the focal length of the cornea
Constant/fixed
What is the shape of the cornea?
Corneal curvature
What refracting element acts a converging lens that accounts for 1/3 refractile power?
Crystalline lens
What refracting element is thicker at the center than at the edges?
Crystalline lens
How many diopters do the crystalline lens have?
20 diopters
Describe the focal length of the crystalline lens
Varies by changing the shape of the lens which allows eyes to focus at different distances
What is the shape produced by the crystalline lens?
Real, inverted image on the retina
What is the central opening of the iris?
Pupil
What is the diameter of the pupil in the dark?
Bigger pupil diameter
What is the diameter of the pupil in bright light?
Smaller pupil diameter
What is the effect smaller pupil?
Reduces spherical aberration (causes image blur)
What is the process by which the eye changes in optical power to maintain clear image of an object as its distance varies?
Accommodation
What are the near reflex triad?
Accommodation
Convergence
Pupillary miosis
What are the events during accommodation?
Ciliary body contracts > slackening of the zonule fibers > lens assumes a spherical shape > lens become more positive > allows eye to have a focused image of near object
What happens to the ciliary body, lens zonules and lens when there is distance vision?
Ciliary body is relaxed
Lens zonules are taut
Lens is flat
What happens to the ciliary body, lens zonules and lens when there is close vision?
Ciliary body contracts
Lens zonules are relaxed
Lens assumes a spherical shape
Describe distance vision
Light rays from distance objects are parallel, so do not need much refraction to focus properly
Describe close vision
Light rays from closer objects diverges and need much refraction to focus properly
What term is used when parallel light rays are focused sharply on the retina?
Emmetropia
What term is used when the eye is unable to bring parallel light rays from a distant object into focus on the retina?
Ametropia
What are the 4 types of refractive errors?
Hyperopia
Myopia
Astigmatism
Presbyopia
Discuss the incidence of refractive errors
2.3 billion people worldwide suffer from poor vision due to refractive errors
670 million have uncorrected or inadequately corrected refractive errors
90% reside in rural areas and low-income countries
One of 5 priority eye diseases of “Vision 2020: Right to Sight” program (WHO)
What are the determinants of refractive errors?
Corneal curvature
Axial length
Loss of accommodation
What determinants of refractive errors determines the length of the eyeball?
Axial length
What type of refractive errors is known as far-sightedness?
Hyperopia
What are the 2 causes of hyperopia?
Insufficient convergence power (lens or cornea) to focus light rays on the retina
Short eyeballs
To where the rays focus to the retina in hyperopic patients?
Posterior to the retina
Who are those people present hyperopia?
Babies
Young children
*outgrown as they get older and the eyeball gets longer
What kind of lens corrects hyperopia?
Plus (convergent) spherical lens
What type of refractive errors is known as near-sightedness?
Myopia
What are the 2 causes of myopia?
Excessive convergence power (lens or cornea) to focus light rays on the retina
Long eyeballs
To where the rays focus to the retina in myopic patients?
Anterior to the retina
Is myopia genetically-inherited?
Genetically-inherited
Who are those people present myopia?
Children between ages 8-12 progresses during teenage years, stabilizes by age 20
What kind of lens corrects myopia?
Minus (divergent) spherical lens
What type of refractive error occurs when the cornea and crystalline lens may not have same radius of curvature in all meridians?
Astigmatism
What type of refractive error has more than 1 focal points?
Astigmatism
What type of refractive error results in distortion of vision at all distances?
Astigmatism
What kind of lens corrects astigmatism?
Cylindrical lens
What type of refractive error has progressive loss of accommodative ability of the crystalline lens caused by the natural process of aging (beginning at age of 40 years old)?
Presbyopia
What type of refractive error has progressive hardening of the lens which result to loss of ability to change its shape?
Presbyopia
What type of refractive error experiences difficulty with near visual work such as reading, “eye strain”?
Presbyopia
Presbyopia occurs in the presence of what 3 refractive errors?
Myopia
Hyperopia
Astigmatism
What kind of lens corrects presbyopia?
Remedied optically with plus spherical lens
What 3 means to correct refractive errors?
Spectacle correction
Contact lenses
Refractive surgeries
Spectacle correction is composed of:
Monofocal
Bifocal
Trifocal/multifocal
Contact lenses are composed of:
Spherical
Toric
Refractive surgeries consist of what 2 surgeries?
Corrective corneal surgeries
Lens surgeries
Corrective corneal surgeries consist of:
Corneal incision
Corneal ablative surgeries
What 3 types of surgeries are under corneal incision?
Radial keratotomy
Limbal relaxing incisions
Partial thickness peripheral incisions
What 3 types of surgeries are under corneal ablative surgeries?
Photorefractive keratotomy (PRK) Laser-in-situ-keratomileusis (LASIK) Femtosecond LASIK (Intralase)
What are the 3 types of lens surgeries?
Cataract surgery with implantation of intraocular lens
Clear lens surgery + IOL implantation
Phakic IOL
What intraocular lenses used in lens surgeries?
Monofocal Toric Multifocal Multifocal + toric Phalic IOL
Is contact lens left in place when examining the eye?
Contact lens may be left
What are the methods of direct fundoscopy?
- Patient should be seated comfortably. Instruct the patient to look at a point on the wall straight ahead, trying not to move the eyes
- Set the aperture wheel to select the large, round, white light
- Begin to look at the right eye about 1 foot from your patient. Use your right eye with the ophthalmoscope in your right hand. Shine the light to the patient’s pupil. Look for red reflex
- Slowly come close to the patient at an angle of about 15 degrees temporarily to the patient’s line of sight
- When the retinal vessels come into view, follow it as it widens to the optic disc which lies nasal to the center of the retina. If the image is not clear, turn the lens dial up or down until the image is clear
- Examine the optic disc, retinal blood vessels, retinal background and macula, in that order
- Repeat for the other eye
What is the definition of refraction in physics?
Change in the direction of a wave due to change in its transmission medium
- bending of a light wave as it enters a medium where its speed is different
