module 7

• Wave model of light

  Light is a disturbance that travels through a medium and carries energy from one location to another without transporting matter

  
• Particle model of light

  Light is composed of particles (photons) that are travelling in a straight line

  
• Newton proposed that light is composed of particles travelling in a straight line

  
• The wave theory of light cannot explain the photoelectric effect

  
• The particle theory of light cannot explain wave-like phenomena such as diffraction and interference

  
• Propagation of light


  1. As a wave

  2. As a particle

  
• Light exhibits both wave-like and particle-like properties


  This has been a long-standing debate among scientists
• This lesson will help understand how light behaves as a wave and as a particle, and the evidence for each model

  
• This lesson will also help understand reflection and refraction of light using the wave and particle models

  
• Rene Descartes


  First who studied and explained the concept of refraction
• Descartes' experiment


  1. Used a spherical glass filled with water and sunlight to produce a rainbow

  2. Explained that refraction brought about the formation of rainbows

  3. Used a prism to observe the emergence of colors of light
• Descartes' explanation of emergence of colors of light

  • Through the concept of the plenum, the invisible substance that permeated the universe
  • The plenum was made of tiny rotating balls with the same speed
  • As the plenum reached the edge of a prism, the balls changed their rotational speeds resulting to the emergence of colors

  
• Sir Isaac Newton


  • Also studied the emergence of colors of light through a prism
  • Stated that the difference in refraction was due to the differences in the mass of the colors of light
• Particles of matter


  • Exert equal force to the particles of light
  • Colors of light with different mass and inertia will be deflected at varying degrees
  • Light particles with great mass and inertia are deflected less when acted upon by the same force of matter

  
• Descartes' very fine substance where light travels

  Composed of tiny rotating balls
• Light can behave both as a wave and as a particle
• As particles, light travels in straight lines, producing shadows when hitting an obstruction
• Reflection and refraction are brought about by light particles
• Photo-electric effect is evidence that light behaves as particles
• Light also acts as a wave, with the ability to diffract or bend around an object

  
• Refraction happens when light waves change direction as they travel through materials of different refractive indices

  
• Light waves also undergo interference
• When light as waves impact a smooth, specular surface like a mirror, they bounce off or reflect according to the arrival angles

  
• When light arrives at a mirror surface as a stream of particles, the particles bounce off in different points so their order in the beam is reversed resulting to a reversed image

  
• A beam of light undergoes refraction when it travels between two media with different refractive indices

  
• As light waves pass from one medium to another with different refractive index, they change direction and bend

  
• Refracting particles of light also change direction upon passing between two media due to a force that changes their speed

  
• Reflection can produce different types of images depending on the type of reflecting surface
• Refraction of light


  1. Angled wavefront impacts second medium before rest of front reaches interface

  2. Part travels along second medium while rest still in first medium

  3. Movement slower through second medium due to higher refractive index

  4. Wavefronts travel at different speeds, causing light to bend into second medium

  
• Refracting particles of light

  • Change direction upon passing between two media
  • Special force perpendicular to interface acts to change speed of particles as they enter second medium, resulting in bending of light particles

  
• Reflection


  • Produces different types of images depending on surface light strikes
  • Study of images usually carried out using mirrors as reflecting surface
  • Mirrors can be planar, concave, or convex
• Plane mirrors


  • Perfectly flat surface with no distortions
  • Reflect 100% of light that strikes them back at a predictable angle
• Concave mirrors


  • Converging mirrors, light is focused at a point as it strikes and reflects back from surface
• Convex mirrors


  • Diverging mirrors, light spreads over a required region as it strikes and bounces back
• Images in plane mirrors

  • Same size as object
  • Laterally inverted (left becomes right and right becomes left)
  • Upright
  • Virtual
  • As far behind mirror as object is in front
• Concave and convex mirrors


  • Parts of spherical mirrors with reflecting surfaces going inward and outward respectively
• Convex mirror images


  • Located behind mirror
  • Virtual
  • Upright
  • Reduced in size
• As object distance from mirror decreases


  Image distance also decreases and image size increases
• Concave mirror images


  • Depend on location of object
  • Can be located between center of curvature (C) and focal point (F), at C, beyond C, or on opposite side of mirror
  • Can be inverted or upright, smaller, bigger, or same size as object, and real or virtual
• Light


  • Electromagnetic radiation that travels through space as vibrating or oscillating waves
  • Composed of alternating electric and magnetic fields oscillating perpendicular to direction of propagation
  • Travels at 3.0x10^8 m/s through vacuum