ABO NOCE Basic Opticianry Ophthalmic Optics 3 — Questions and Answers
Question 1: What is the focal length of a +2.50 D lens?
- 25 cm
- 40 cm (Correct answer)
- 50 cm
- 4 cm
Correct answer: 40 cm
Focal length (in meters) = 1 / power, so 1 / 2.50 = 0.40 m = 40 cm.
Question 2: Chromatic aberration in lenses is caused by:
- Different wavelengths of light being refracted at different angles (Correct answer)
- Imperfections in the lens surface curvature
- The thickness of the lens at the optical center
- Decentration of the optical center
Correct answer: Different wavelengths of light being refracted at different angles
Different wavelengths (colors) of light have different refractive indices in a material, causing them to focus at slightly different points — this is chromatic aberration.
Question 3: Which lens form has the least peripheral distortion for most prescriptions?
- Flat form (plano front)
- Toric form
- Best form (corrected curve) (Correct answer)
- Equiconvex form
Correct answer: Best form (corrected curve)
Best form (corrected curve or best-form) lenses are designed with specific base curves that minimize oblique astigmatism and peripheral distortion.
Question 4: The critical angle for total internal reflection depends on:
- Only the wavelength of light
- The refractive indices of both media at the interface (Correct answer)
- Only the angle of the incident beam
- The thickness of the denser medium
Correct answer: The refractive indices of both media at the interface
The critical angle is determined by sin(θc) = n2/n1, so it depends on the refractive indices of both the dense and less-dense media at the interface.
Question 5: Oblique astigmatism in ophthalmic lenses is an example of which type of lens aberration?
- Chromatic aberration
- Coma
- Monochromatic (Seidel) aberration (Correct answer)
- Diffraction error
Correct answer: Monochromatic (Seidel) aberration
Oblique astigmatism is one of the five classic Seidel (monochromatic) aberrations that affect the quality of images formed by lenses.
Question 6: A high Abbe value (V-number) in a lens material indicates:
- Greater chromatic aberration
- Less chromatic aberration (Correct answer)
- Higher refractive index
- Greater surface reflectance
Correct answer: Less chromatic aberration
A high Abbe value means the material disperses different wavelengths of light less, resulting in less chromatic aberration and better optical clarity.
Question 7: What happens to the optical path of light when it passes through the optical center of a lens?
- It is deflected toward the base
- It passes through without deviation (Correct answer)
- It is reflected back
- It is split into its spectral components
Correct answer: It passes through without deviation
Light passing through the optical center of a lens is not deviated (no prismatic effect), regardless of lens power.
ABO NOCE Basic Opticianry Ophthalmic Optics 3
What is the focal length of a +2.50 D lens?