Optics and Refraction 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 flashcards as text
An object is placed 50 cm in front of a thin lens. What is the vergence of light from the object arriving at the lens?
Answer: -2.00 D
Vergence equals 1 divided by distance in meters (1/0.5 = 2), and light diverging from a real object has negative vergence.
According to Knapp's rule, where should a spectacle lens sit so that an eye with axial ametropia has a retinal image the same size as an emmetropic eye's?
Answer: At the anterior focal point of the eye
A correcting lens at the anterior focal point (about 15-17 mm in front of the cornea) gives an image the same size as in an emmetropic eye when the ametropia is axial.
A patient looks through a -4.00 D lens at a point 5 mm from its optical center. How much prism is induced?
Answer: 2 prism diopters
Prentice's rule gives prism = decentration in cm x power in diopters, so 0.5 x 4 = 2 prism diopters.
When using a Jackson cross cylinder to refine the cylinder axis, how should the cross cylinder be placed?
Answer: Its principal meridians straddle the trial cylinder axis at 45 degrees
For axis refinement, the handle lines up with the cylinder axis, which puts the cross cylinder's principal meridians 45 degrees on either side of it.
Light passes from air into the cornea at an oblique angle. What happens to the ray?
Answer: It bends toward the normal
By Snell's law, light entering a medium with a higher refractive index slows down and bends toward the normal.
Using Hofstetter's average formula, what is the expected amplitude of accommodation in a 40-year-old?
Answer: 6.5 D
Hofstetter's average amplitude is 18.5 - 0.3 x age, so 18.5 - 12 = 6.5 D.
During the duochrome (red-green) test, the patient says the letters on the red side are clearer. What should the examiner do?
Answer: Add minus power
If red is clearer, the eye is relatively myopic (focus is in front of the retina), so minus is added. This is the 'RAM-GAP' rule: red add minus, green add plus.