OBT Principles of Optical Beam Physics 2 — Questions and Answers
Question 1: Which property of light determines its energy per photon in optical beam therapy?
- Wavelength (Correct answer)
- Beam diameter
- Pulse duration
- Spot size
Correct answer: Wavelength
Photon energy is inversely proportional to wavelength (E = hc/λ), so shorter wavelengths carry higher energy per photon.
Question 2: What term describes the phenomenon where optical beam intensity decreases as it travels through tissue?
- Refraction
- Attenuation (Correct answer)
- Diffraction
- Polarization
Correct answer: Attenuation
Attenuation refers to the reduction in beam intensity due to absorption and scattering as light propagates through tissue.
Question 3: In a Gaussian beam profile, where is the intensity at its maximum?
- At the beam periphery
- At the beam center (on-axis) (Correct answer)
- At the focal point only
- Uniformly distributed across the beam
Correct answer: At the beam center (on-axis)
A Gaussian beam has its peak intensity at the center axis, with intensity decreasing exponentially toward the edges.
Question 4: What is the Rayleigh range of a focused optical beam?
- The distance over which beam power drops by 50%
- The distance from the waist where beam area doubles (Correct answer)
- The minimum achievable spot size
- The wavelength of the optical source
Correct answer: The distance from the waist where beam area doubles
The Rayleigh range is the axial distance from the beam waist at which the beam cross-sectional area doubles, marking the boundary of the focal zone.
Question 5: Which interaction mechanism primarily accounts for therapeutic heating when near-infrared light is absorbed by water in tissue?
- Photochemical reaction
- Photoionization
- Photothermal conversion (Correct answer)
- Photomechanical disruption
Correct answer: Photothermal conversion
Photothermal conversion occurs when absorbed photons are converted to heat via molecular vibration, producing therapeutic warming effects in tissue.
Question 6: What does the term 'fluence' represent in optical beam therapy dosimetry?
- Beam power at the target
- Energy delivered per unit area (Correct answer)
- Number of pulses per second
- Beam divergence angle
Correct answer: Energy delivered per unit area
Fluence (J/cm²) is the total energy delivered per unit area, serving as the primary dose metric in optical beam therapy.
Question 7: Which optical phenomenon causes a beam to bend when it passes from one tissue layer to another with a different refractive index?
- Absorption
- Scattering
- Refraction (Correct answer)
- Fluorescence
Correct answer: Refraction
Refraction causes light to change direction at the interface between materials with different refractive indices, described by Snell's Law.
OBT Principles of Optical Beam Physics 2
Which property of light determines its energy per photon in optical beam therapy?