Light and Energy - Key Points

Radiation

  • Heat transfer method not needing particles.
  • Travels through a vacuum via electromagnetic waves.
  • EM waves: transverse waves with vibrating electric and magnetic fields at right angles.
  • All EM waves travel at the speed of light: 300 \times 10^6 m/s.

Electromagnetic Spectrum

  • Includes microwaves, ultraviolet, x-rays, gamma rays, TV and radio waves, infrared radiation, and visible light.
  • Travel at 300 \times 10^6 m/s.

Uses of Electromagnetic Radiation

  • Radio Waves: Transmit radio signals.
  • Microwaves: Heat foods (absorbed by water), satellite communications (penetrate haze).
  • Infrared Radiation: Heat given off by objects.
  • Visible Light: Part of sun's radiation, enables sight.
  • Ultraviolet Light: High energy, harmful in high levels.
  • X-rays: Pass through skin but not bone, harmful overexposure.
  • Gamma Rays: Highest energy radiation, used in medical treatments to kill cancerous cells, otherwise harmful.

Sources of Light

  • Luminous: Objects giving off their own light.
    • Incandescent: Light due to heat (e.g., sun, candle).
    • Non-incandescent: Light from chemical reactions (e.g., fluorescent tubes).
    • Bioluminescent: Light without heat (e.g., fireflies).
  • Non-Luminous: Objects seen by reflected light.

Properties of Light

  • Travels in straight lines.
  • A beam is a bundle of rays.

Dispersion of White Light

  • Different colors bend differently through a prism, forming the spectrum of white light.
  • Primary Colors: Red, Blue, Green (cannot be produced by mixing other colors).
  • Secondary Colors: Magenta, Cyan, Yellow.
  • White light forms when all colors of light combine.
  • Object color is determined by the color it reflects.

Reflection of Light

  • Opaque/mirrored surfaces: reflect light.
  • Substances absorb or transmit light.
  • Specular Reflection: Light bounces off shiny surfaces (angle of incidence = angle of reflection).
  • Rough surfaces: light reflects in all directions.
  • Angle of incidence: Angle between light ray and normal.
  • Angle of reflection: Angle formed as ray reflects from surface.
  • Law of Reflection: Angle of incidence = angle of reflection.

Images in Plane Mirrors

  • Upright.
  • Same distance behind as object is in front.
  • Same size as object.
  • Lateral Inversion: Image is reversed.
  • Used in optical instruments (telescopes, cameras, periscopes).

Reflection by Curved Mirrors

  • Convex Mirrors:
    • Wide field of view.
    • Images smaller.
    • Used as safety mirrors.
    • Cause light rays to diverge.
  • Concave Mirrors:
    • Smaller field of view.
    • Images larger.
    • Used as makeup mirrors.
    • Cause light rays to converge.

Refraction of Light

  • Bending of light when entering a different medium (due to speed change).
  • Slower in denser medium, faster in less dense medium.
  • Optical Density: Measure of light speed through a substance.
  • Light bends towards the normal (when slowing down) and away from the normal (when speeding up).

Lenses

  • Convex (Converging) Lenses:
    • Thicker in the middle.
    • Produce real and virtual images.
  • Concave (Diverging) Lenses:
    • Thinner in the middle.
    • Produce virtual, diminished, upright images.
  • Real vs. Virtual Images:
    • Real images: rays converge and can be obtained on a screen.
    • Virtual images: rays appear to diverge and cannot be obtained on a screen.

Light and the Eye

  • Cornea: Refracts light.
  • Iris: Controls pupil size.
  • Pupil: Allows light to enter.
  • Lens: Focuses light onto retina.
  • Ciliary Muscles: Adjust lens shape.
  • Retina: Contains light receptors (rods and cones).
  • Optic Nerve: Sends signals to the brain.
  • Myopia (Nearsightedness): Focus in front of retina, corrected with concave lens.
  • Hyperopia (Farsightedness): Focus behind retina, corrected with convex lens.