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
Why can't the anterior chamber angle be seen without a gonioscopy lens?
Answer: Light from the angle is totally internally reflected at the tear-air interface
Light from the angle hits the tear-air interface at more than the critical angle (about 46 degrees), so it is totally internally reflected. A goniolens removes that interface.
Where is the circle of least confusion in an astigmatic conoid of Sturm?
Answer: At the dioptric midpoint between the two focal lines
The circle of least confusion is at the dioptric midpoint of the interval of Sturm, which corresponds to the spherical equivalent.
Which refraction represents mixed astigmatism?
Answer: +1.00 -2.00 x 180
In +1.00 -2.00 x 180, one meridian is +1.00 and the other is -1.00, so one focal line is in front of the retina and the other is behind it.
A myope wears -10.00 D spectacles at a 12 mm vertex distance. About what contact lens power is needed?
Answer: -9.00 D
Effective power = F/(1 - dF) = -10/(1 + 0.12), which is about -8.93 D, so roughly -9.00 D.
A patient looks at an object through a prism. Which way does the image appear to move?
Answer: Toward the apex of the prism
A prism bends light toward its base, so the image appears shifted toward the apex.
A Galilean telescope uses a +5.00 D objective and a -20.00 D eyepiece. What is its magnification?
Answer: 4x, upright image
Telescope magnification is -Feyepiece/Fobjective = 20/5 = 4x, and the minus eyepiece of a Galilean design gives an upright image.
With the SRK formula, about how large an IOL power error results from a 1 mm error in axial length?
Answer: About 2.5 D
In P = A - 2.5L - 0.9K, the axial length coefficient is 2.5, so a 1 mm error changes the IOL power by about 2.5 D.