Rucete ✏ AP Physics 2 In a Nutshell
4. Physical Optics
This chapter explores electromagnetic waves, reflection, refraction, applications of light, interference, diffraction, and polarization. It explains the wave properties of light and how they lead to phenomena such as interference patterns, refraction through media, and polarization effects.
Electromagnetic Waves
• Electromagnetic waves are produced by oscillating electric and magnetic fields and can travel through a vacuum at the speed of light (c ≈ 3 × 10⁸ m/s).
• Light is a form of electromagnetic radiation and travels as a transverse wave.
• Each color of visible light corresponds to a different wavelength and frequency (ROYGBV).
• The wave relationship: c = λf
Reflection
• Reflection occurs when light bounces off a surface.
• Specular reflection: smooth surfaces (like mirrors); angles preserved.
• Diffuse reflection: rough surfaces; light scatters in many directions.
• Law of reflection: Angle of incidence = Angle of reflection (measured relative to the normal line).
• Laser light is both monochromatic and coherent, enhancing its intensity and directionality.
Refraction
• Refraction is the bending of light as it passes between different media due to change in speed.
• Snell’s Law: N₁ sin θ₁ = N₂ sin θ₂
• Index of refraction: N = c/v
• When entering a denser medium (higher N), light bends toward the normal; when entering a less dense medium, light bends away.
• Frequency remains constant; wavelength and speed change when entering a new medium.
• Dispersive media cause different wavelengths to refract differently, producing spectra (e.g., prism dispersion).
Total Internal Reflection
• Occurs when light tries to move from a denser to a less dense medium at a large angle.
• Critical angle (θc): the angle of incidence for which θ₂ = 90°:
sin θc = N₂/N₁
• If θ > θc, all light reflects internally — no refraction.
• Basis for fiber optics and diamond sparkle.
Interference and Diffraction
Double-Slit Interference
• Two coherent light sources create an interference pattern of bright and dark fringes.
• Bright fringes (constructive interference) occur when path difference = mλ.
• Dark fringes (destructive interference) occur when path difference = (m + ½)λ.
• Fringe spacing: Δy = (λL)/d, where L = screen distance, d = slit separation.
Single-Slit Diffraction
• Light spreads after passing through a narrow slit.
• Central maximum is wide and bright; minima occur where: a sin θ = mλ
• Diffraction is more noticeable when slit width a ≈ λ.
Polarization
• Polarization restricts the vibration of light waves to a single plane.
• Unpolarized light has waves vibrating in all planes perpendicular to the direction of travel.
• Polarizing filters allow only light oscillating in one plane to pass through.
• Applications include glare reduction (polarized sunglasses), LCD screens, and stress analysis in materials.
In a Nutshell
Light behaves as a wave, exhibiting reflection, refraction, interference, diffraction, and polarization. Understanding these properties explains everyday phenomena like rainbows, the colors seen in soap bubbles, and the functioning of fiber optics. Physical optics shows how light’s wave nature complements its particle behavior studied in modern physics.