2020–2021 BCSC Basic and Clinical Science Course™
3 Clinical Optics
Chapter 2: Physical Optics
2.1. d. In general, it suffices to consider the wave-like behavior of light for phenomena at the macroscopic level and the simple quantum-view of light at the microscopic level. However, the quantum theory of the interaction of light and matter—quantum electrodynamics—explains all the properties of light we know, resolving the wave-particle confusion.
2.2. c. The Airy disc is the central portion of a pattern of light and dark rings formed when light from a point source passes through a circular aperture and is diffracted. The size of the Airy disc increases with smaller pupil size (especially pupil diameter <2.5 mm), longer wavelengths of light, and longer focal lengths. Retinal conditions such as macular degeneration have no effect on the size of the Airy disc.
2.3. b. Light scattering occurs when small particles suspended in a transparent medium interfere with the transmittance of light and cause photons to deviate from a straight path. Short wavelengths of light are scattered more strongly than are longer wavelengths. Larger particles scatter light more intensely than do smaller particles. In a healthy cornea, the tightly arranged and regularly spaced collagen molecules minimize the effects of scattering. When a cornea becomes edematous, excess fluid in the stroma disrupts the regular collagen structure, causing light scattering.
2.4. b. Optical coherence tomography (OCT) is an optical analogue to ultrasound imaging, using infrared light instead of sound. The much higher speed of light compared with sound allows for finer resolution, but direct electronic measurement of the shorter “echo” times it takes light to travel from different structures at axial distances within the eye is not feasible. Interferometry enables us to overcome this difficulty. More precisely, OCT uses interference of broadband or tunable coherent light to generate optical sections of the retina and cornea.
2.5. d. Laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK), common refractive surgeries of the cornea, are based on photoablation using excimer lasers generating photons with wavelengths in the ultraviolet (UV) range that are absorbed within the corneal tissue.
2.6. c. In the retina, natural chromophores strongly absorb wavelengths from about 400 nm to 580 nm, which makes the retina susceptible to photochemical injury in that region, especially from blue light. This susceptibility to damage is increased in an aphakic eye, extending to below 400 (to about 315 nm), without the UV-A absorption capability of the natural lens, and is the basis for incorporating UV-blocking and blue-blocking chromophores in some intraocular lenses.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.