#47. Refraction of the eye. Physical optics. Eye as an optical system. Physical and clinical refraction. Examination of refraction - subjective and objective methods. Flashcards
What is REFRACTION?
RATIO of
Refractive Power of Lens + Cornea
Axial Length of Globe
What is the INTERNATIONAL UNITS OF MEASURE for Refractive Power?
DIOPTERS
How is Refractive Power CALCULATED?
According to LAWS of GEOMETRIC OPTICS
Explain SNELL’s LAW
REFRACTION of INCIDENT Light Ray is DETERMINED BY :
- Angle of Incidence
- Difference in Refractive Indices of 2 Media
What is the MAXIMUM TOTAL Refractive Power of an EMMETROPIC (Normal Sighted) Eye?
63 DIOPTERS with an AXIAL
GLOBE = 23.5mm
What is the Refractive Power of the CORNEA & LENS?
Cornea = 43 Diopters
Lens = 10 - 20 Diopters
** DEPENDING on Accommodation
What is the DIFFERENCE BTW the Ratio of AXIAL LENGTH to REFRACTIVE POWER in Cornea / Lens
in EMMETROPIA (Normal Sight)
AND
in AMETROPIA (Refractive Error)
1) EMMETROPIA
- BALANCED RATIO of Axial Length : Refractive Power in Cornea / Lens
- Parallel light rays enter the eye, THERFORE MEET at a FOCAL POINT on the RETINA
2) AMETROPIA:
- MISMATCH RATIO of Axial Length : Refractive Power in Cornea / Lens
E.g. Near-Sightedness / Far-Sightedness / Astigmatism
What are the 2 METHOD OF EXAMINATION for REFRACTION?
1) REFRACTIVE TESTING
2) RETINOSCOPY
3) REFRACTOMETRY
What is REFRACTIVE TESTING?
Measuring the ADDITIONAL REFRACTIVE Power, REQUIRED to PRODUCE a SHARP IMAGE on the Retina
What are the 2 FORMS of REFRACTIVE TESTING?
Explain Them.
A) SUBJECTIVE Refractive Testing
- Placing various Combos of Lenses BEFORE Patient’s eye
- UNTIL MAXIMUM Visual Acuity is reached
B) OBJECTIVE Refractive Testing
- UNAVOIDABLE when Px is UNABLE to provide Subjective Information (Infants)
What is REFRACTOMETRY?
1) OBSERVATION of TEST IMAGE projected on Px’s Retina
2) DISTANCE BTW Test Figure AND Eye is CHANGED until image APPEARS in FOCUS of Retina
3) Refraction is CALCULATED based on Measured Values.
