Board Certified Ophthalmologist Optics and Refraction Principles 2

Question 1

An object is placed 50 cm in front of a +3.00 D thin lens. Where is the image formed?

Accepted Answer

100 cm behind the lens

Answer

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?

Accepted Answer

2 prism diopters

Answer

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?

Accepted Answer

CR-39 plastic

Answer

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?

Accepted Answer

Total internal reflection at the tear film-air interface

Answer

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?

Accepted Answer

Zero

Answer

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.

Board Certified Ophthalmologist Practice Test

Board Certified Ophthalmologist Practice Test

Board Certified Ophthalmologist Optics and Refraction Principles 2