Optics

• Are Real Images Always Inverted and Virtual Images Always Erect?
  Real images are inverted and virtual images erect for a real object imaged by one standard lens or mirror, but the rule has limits.
• What Can You Use Instead of the Time-Averaged Poynting Vector?
  For periodic fields, use the cycle-averaged Poynting vector; for pulses, integrate instantaneous flux over time. The right choice depends on what you need.
• How Does Diffraction-Grating Spectrometry Work?
  Learn how a diffraction grating separates light by wavelength and how to calculate a spectral line’s wavelength from its order and angle.
• What Is the Pole of a Spherical Mirror?
  The pole of a spherical mirror is the point where its principal axis meets the reflecting surface, usually at the center of the mirror’s aperture.
• Can Light and Darkness Coexist?
  Light and darkness can appear together as illumination varies across a scene; darkness is low or absent visible light, not a separate substance.
• Can the Focal Length of a Convex Lens Be Negative?
  A convex lens usually has positive focal length in air, but it can have negative focal length if its surroundings make it diverge light.
• How to Calculate Field of View From Focal Length
  Focal length alone does not determine field of view: use the sensor dimension and calculate 2 arctan(sensor size divided by 2 focal length).
• Are Coherent States of Light Classical or Quantum?
  Coherent states are quantum states with classical-like field behavior, Poisson photon statistics, and unavoidable quantum fluctuations in measurements.
• What Is the Color of a Mirror?
  A mirror is usually described as silver or neutral gray, but its apparent color comes from reflected light and can show a slight green tint.