Free Board Certified Ophthalmologist Optics and Refraction Principles Questions and Answers — Questions and Answers
Question 1: A patient has a spectacle prescription of -10.00 D sphere. The measurement from the posterior surface of the spectacle lens to the corneal apex (vertex distance) is 12 mm. To achieve the same effective optical correction with a contact lens, what power should be prescribed?
- -11.36 D
- -10.00 D
- -9.00 D
- -8.93 D (Correct answer)
Correct answer: -8.93 D
For spectacle powers greater than +/- 4.00 D, the vertex distance becomes clinically significant when converting to a contact lens prescription. The formula for the effective power at the cornea (Fc) is Fc = Fs / (1 - d * Fs), where Fs is the spectacle power and d is the vertex distance in meters. In this case, Fs = -10.00 D and d = 0.012 m. Fc = -10.00 / (1 - (0.012 * -10.00)) = -10.00 / (1 + 0.12) = -10.00 / 1.12 ≈ -8.93 D. Moving a high minus lens closer to the eye reduces its effective power, so a weaker minus contact lens is required. [4, 9, 16]
Question 2: A patient is looking through their +5.00 D spherical lens at a point 6 mm below the optical center. Using Prentice's rule, what is the magnitude and direction of the induced prism?
- 1.5 prism diopters, base down
- 3.0 prism diopters, base up (Correct answer)
- 3.0 prism diopters, base down
- 1.5 prism diopters, base up
Correct answer: 3.0 prism diopters, base up
Prentice's rule states that the prismatic effect (P) in prism diopters is equal to the lens power in diopters (D) multiplied by the decentration in centimeters (c). Here, D = +5.00 D and c = 6 mm = 0.6 cm. P = 5.00 * 0.6 = 3.0 prism diopters. For a plus lens, the base of the prism is always towards the optical center. Since the patient is looking below the optical center, the induced prism is base up. [1, 14, 23]
Question 3: An eye's refractive state is such that parallel rays of light form two focal lines. The vertical focal line is focused 1.00 D in front of the retina, while the horizontal focal line is focused directly on the retina. How is this refractive error best classified?
- Compound myopic astigmatism
- Mixed astigmatism
- Simple myopic astigmatism (Correct answer)
- Simple hyperopic astigmatism
Correct answer: Simple myopic astigmatism
This condition describes simple myopic astigmatism. In this type of astigmatism, one of the principal meridians is myopic (focuses light in front of the retina), while the other meridian is emmetropic (focuses light directly on the retina). Compound myopic astigmatism would have both focal lines in front of the retina, and simple hyperopic astigmatism would have one line on the retina and one behind it. [12, 15, 29]
Question 4: During static retinoscopy from a working distance of 67 cm, the examiner sweeps the light streak horizontally across the patient's pupil and observes a red reflex that moves in the same direction as the streak ('with' motion). Which of the following does this finding indicate?
- The patient's far point is located between the examiner and the patient.
- The patient's far point is located at the examiner's pupil.
- The patient's far point is located behind the examiner's pupil. (Correct answer)
- The patient is myopic by more than -1.50 D.
Correct answer: The patient's far point is located behind the examiner's pupil.
'With' motion during retinoscopy signifies that the patient's far point is located behind the examiner's pupil (the peephole of the retinoscope). [5, 20, 28] This occurs in hyperopia, emmetropia, or myopia of less power than the dioptric equivalent of the working distance. In this case, the working distance is 67 cm, which corresponds to 1/0.67m ≈ +1.50 D. 'With' motion means the eye is relatively hyperopic to this +1.50 D working distance lens, requiring the addition of plus-powered lenses to achieve neutrality. An 'against' motion would indicate the far point is between the examiner and the patient. [5, 20, 28]
Question 5: A biconvex lens made of crown glass (n=1.52) has a power of +5.00 D in air (n=1.00). If this same lens is fully submerged in water (n=1.33), what is its new approximate power?
- It becomes a minus-powered lens.
- +7.80 D
- +5.00 D
- +1.83 D (Correct answer)
Correct answer: +1.83 D
The power of a lens depends on the difference between the refractive index of the lens material and the surrounding medium. The simplified Lensmaker's equation is Power ∝ (n_lens - n_medium). In air, the power is proportional to (1.52 - 1.00) = 0.52. In water, the power is proportional to (1.52 - 1.33) = 0.19. The new power can be found by the ratio: P_water / P_air = (0.19 / 0.52). So, P_water = +5.00 D * (0.19 / 0.52) ≈ +1.83 D. Submerging the lens in a medium with a higher refractive index reduces the refractive power of the lens. [2, 17, 25]
Question 6: Which of the following describes positive spherical aberration in the human eye?
- Central light rays are focused posterior to peripheral light rays.
- Vertical light rays are focused at a different point than horizontal light rays.
- Off-axis light rays create a comet-shaped blur.
- Peripheral light rays are focused anterior to central (paraxial) light rays. (Correct answer)
Correct answer: Peripheral light rays are focused anterior to central (paraxial) light rays.
Positive spherical aberration occurs when peripheral light rays are refracted more strongly than central, paraxial rays, causing them to focus at a point anterior to the central rays. [3, 7, 11] This is the typical state for the average human cornea. In a young eye, the negative spherical aberration of the crystalline lens helps to offset the cornea's positive aberration, but this balance changes with age as the lens develops positive spherical aberration. [11, 21]
