Light, vision, and photography

Waves

• An electro-magnetic (EM) wave is a signal carrying energy through space/time, with a physical value at each space-time point given by the strength of its electrical and magnetic fields. These fields oscillate perpendicular to each other, and perpendicular to the path of the wave (EM waves are transverse).
• EM waves travel through a vacuum (no medium)
• EM waves travel at a fixed speed c = approximately 3 x 10^8 (300 million) meters/second, or 186,000 miles/sec.
• Like periodic sound waves, periodic EM waves are characterized by their:
  • Period: T
  • Frequency: f = 1/T
  • Wavelength: L = c*T = v/f, i.e. c = f*L, f=v/L (higher frequency implies lower wavelength)
  • Amplitude: A
  • Power: proportional to A^2
  • Here is an animation, showing EM wave propagation
• Like sound waves, EM waves can be analyzed as the sum of sine and cosine waves at different frequencies and amplitudes, using the Fourier theorem
• Light comprises a frequency range within the EM spectrum in the visible range: 750 - 390 nanometers (billionths of a meter), i.e. frequencies in the range 400 to 790 terahertz (tera = trillion = 10^12), corresponding to colors ROYGBIV (red-orange-yellow-green-blue-indigo-violet)
  • Waves (transverse, longitudinal)(1D, 2D, 3D).
  • Inverse square law: the spread of wave energy is proportional to the square of the distance from the source. If the power (energy per unit time) is constant, then intensity (power per unit area) is inversely proportional to the square of the distance. What this means: a flashlight's intensity at 8 feet is only a quarter its intensity at 4 feet.
  • Reflection: a wave will bounce off a reflective boundary, partially (glass) or fully (mirror).
  • Diffraction: while light shadows appear relatively sharp (compared to sound shadows), a light wave can travel around an obstacle significantly smaller than its wavelength (see periodic waves, below), and will spread after passing through a hole.
  • Refraction: a wave changes direction when the medium changes.
  • Light waves are tranverse traveling electromagnetic waves through space (no medium) in 3D.

Terms for light and optics

• Watt: unit of power, 1 joule/second
  • a joule is the energy required to exert a force of 1 newton over one meter
  • a newton is the force required to accelerate a kilogram 1 m/s each second
• Radiant flux: measure of power in EM radiation
• Luminous flux: measure of power of light source, as perceived by the eye (= radiant flux adjusted for wavelength)
• Lumen: unit of luminous flux
• Luminous intensity: measure of power in EM radiation per solid angle unit
• Candela: unit of luminous intensity
• Lux: measure of luminous intensity = one lumen per square meter

Photography

• pinhole camera
• SLR camera
  • f-stop
  • focal plane
  • shutter speed