Board Certified Ophthalmologist Optics and Refraction Principles 2 — Questions and Answers
Question 1: An object is placed 50 cm in front of a +3.00 D thin lens. Where is the image formed?
- 50 cm behind the lens
- 33 cm behind the lens
- 100 cm in front of the lens
- 100 cm behind the lens (Correct answer)
Correct 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.
Question 2: 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?
- 0.8 prism diopters
- 20 prism diopters
- 2 prism diopters (Correct answer)
- 1 prism diopter
Correct answer: 2 prism diopters
By Prentice's rule, prism equals decentration in cm times lens power: 0.4 x 5 = 2 prism diopters.
Question 3: Which spectacle lens material has the highest Abbe value and therefore the least chromatic aberration?
- Polycarbonate
- Trivex
- 1.74 high-index plastic
- CR-39 plastic (Correct answer)
Correct 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).
Question 4: Why can the anterior chamber angle not be seen directly at the slit lamp without a gonioscopy lens?
- Absorption of light by the corneal stroma
- Diffraction at the limbus
- Total internal reflection at the tear film-air interface (Correct answer)
- Chromatic aberration of the crystalline lens
Correct 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.
Question 5: What is the spherical equivalent of a ±0.25 D Jackson cross cylinder?
- +0.25 D
- -0.25 D
- +0.50 D
- Zero (Correct answer)
Correct 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.
Question 6: During the duochrome (red-green) test, a patient reports the letters on the red side are clearer. What is the appropriate adjustment?
- Add plus sphere
- Increase cylinder power
- Rotate the cylinder axis
- Add minus sphere (Correct answer)
Correct answer: Add minus sphere
Red clearer means the focal points lie anterior to the retina (relative myopia), so more minus power is needed.
Question 7: 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?
- At the nodal point of the eye
- At the corneal plane
- At the anterior focal point of the eye (Correct answer)
- At the posterior principal plane
Correct 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.
