Board Certified Ophthalmologist Optics and Refraction

Question 1

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?

Accepted Answer

-2.00 D

Answer

Vergence equals 1 divided by distance in meters (1/0.5 = 2), and light diverging from a real object has negative vergence.

Question 2

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?

Accepted Answer

At the anterior focal point of the eye

Answer

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.

Question 3

A patient looks through a -4.00 D lens at a point 5 mm from its optical center. How much prism is induced?

Accepted Answer

2 prism diopters

Answer

Prentice's rule gives prism = decentration in cm x power in diopters, so 0.5 x 4 = 2 prism diopters.

Question 4

When using a Jackson cross cylinder to refine the cylinder axis, how should the cross cylinder be placed?

Accepted Answer

Its principal meridians straddle the trial cylinder axis at 45 degrees

Answer

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.

Question 5

Light passes from air into the cornea at an oblique angle. What happens to the ray?

Accepted Answer

It bends toward the normal

Answer

By Snell's law, light entering a medium with a higher refractive index slows down and bends toward the normal.

Board Certified Ophthalmologist Practice Test

Board Certified Ophthalmologist Practice Test

Board Certified Ophthalmologist Optics and Refraction