Optics and Refraction Principles Flashcards
7 cards from real Board Certified Ophthalmologist practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Optics and Refraction Principles flashcards as text
An object is placed 50 cm in front of a +3.00 D thin lens. Where is the image formed?
Answer: 100 cm behind the lens
Incoming vergence of -2.00 D plus lens power of +3.00 D gives outgoing vergence of +1.00 D, so the image forms 1 m behind the lens.
A patient looks through a point 4 mm away from the optical center of a -5.00 D spherical lens. How much prism is induced?
Answer: 2 prism diopters
By Prentice's rule, prism equals decentration in cm times lens power: 0.4 x 5 = 2 prism diopters.
Which spectacle lens material has the highest Abbe value and therefore the least chromatic aberration?
Answer: CR-39 plastic
CR-39 has an Abbe value of about 58, much higher than polycarbonate (about 30) or 1.74 high-index (about 33).
Why can the anterior chamber angle not be seen directly at the slit lamp without a gonioscopy lens?
Answer: Total internal reflection at the tear film-air interface
Light from the angle strikes the cornea-air interface beyond the critical angle and is totally internally reflected; a goniolens replaces the air interface to overcome this.
What is the spherical equivalent of a ±0.25 D Jackson cross cylinder?
Answer: Zero
A JCC has equal and opposite powers in perpendicular meridians, so its spherical equivalent is zero and it does not shift the circle of least confusion.
During the duochrome (red-green) test, a patient reports the letters on the red side are clearer. What is the appropriate adjustment?
Answer: Add minus sphere
Red clearer means the focal points lie anterior to the retina (relative myopia), so more minus power is needed.
According to Knapp's rule, where should a correcting lens be placed to give an axial ametrope the same retinal image size as an emmetrope?
Answer: At the anterior focal point of the eye
Knapp's rule states that a lens at the anterior focal point (about 15-16 mm in front of the cornea) equalizes retinal image size in axial ametropia.