chapter 14

• luminous objects


  emit light
• non luminous objects


  can be seen when they reflect light from a luminous object into our eyes
• laws of reflection


  light ray going towards the surface is called incident ray

  light ray going away from the surface is called reflected ray

  line perpendicular to the surface at the point of incidence is called the normal

  angle between incident ray and the normal is called the angle of incidence

  angle between reflected ray and the normal is called angle of reflection
• first law of reflection

  incident ray, reflected ray and the normal at the point of incidence lie in the same plane
• second law of reflection


  angle of incidence is equal to angle of reflection
• 2 types of reflection

  regular reflection

  diffuse reflection
• characteristics of mirror images

  mirror images and objects are equally far away from one another

  mirror image is virtual

  mirror image and object are of the same size

  mirror image is laterally inverted

  mirror image is upright
• refraction


  bending of light as it passes from one optical medium to another
• refracted ray


  a light ray that enter a medium and undergoes a change in direction

  angle of refraction is the angle between the normal and the refracted ray

  the light ray bends towards the normal as it travels from an optically less dense medium to an optically denser medium

  the light ray bends away from the normal as it travels from an optically denser medium to an optically less dense medium

  an example is air being the optically less dense medium compared to glass
• first law of refraction

  incident ray, refracted ray and the normal at the point of incidence all lie in the same plane
• second law of refraction

  for two given media, the ratio of sine of the angle of incidence to the sine of angle of refraction is a constant, that is sin i over sin r = constant
• refractive index


  defined as the ratio of the speed of light in a vacuum to the speed of light in that medium

  speed of lights varies with frequencies of light hence value of refractive index is different for different media

  the higher the refractive index, the slower the speed of light in the medium, light bends more towards the normal
• principle of reversibility of rays

  regardless of how many times a light ray has been reflected or refracted, it will follow the same path when its direction is reversed
• emergent ray


  the ray that emerges from the glass block
• effects of refraction


  "bent" objects

  misperception of depth
• when a light ray crosses both boundaries perpendicularly, there is no change in ray direction
• as the light ray moves clockwise..

  • there will be refraction and partial reflection (angle of incidence<critical angle)
  • angle of refraction becomes 90 degrees (angle of incidence=critical angle)
  • ray will be reflected off the surface of the block, no refraction (angle of incidence>critical angle)
• critical angle


  the angle of incidence in an optically denser medium for which angle of refraction in the less dense medium is 90
• total internal reflection


  complete reflection of a light ray in an optically denser medium at the boundary with an optically less dense medium
• conditions for total internal reflection

  incident ray must travel from an optically denser medium to an optically less dense medium

  angle of incidence must be greater than critical angle
• the medium with a lower refractive index has an optically less dense medium
• principal axis

  the line which passes through the centre of the lens and which is perpendicular to the plane of the lens
• optical centre


  the point on the principal axis that is the midpoint between the surfaces of the lens
• principal focal point f


  the point on the principal axis on which parallel rays meet after passing through the lens
• focal plane

  the plane perpendicular to the principal axis on which all parallel rays meet after passing through the lens
• focal length

  distance between the optical centre and the principal focus point
• ray diagram for thin converging lens

  ray 1 - passes through optical centre

  ray 2 - parallel to principal axis and pass through focal point

  ray 3 - passing through focal point and parallel to principal axis
• real image


  can be formed on a screen placed at the image plane
• virtual image


  cannot be formed on the screen placed on its image plane