3.2

• a beam may be parallel, diverging or converging
• normal - line which is perpendicular to a surface

  angle of incidence - angle between the incident ray and the normal to a surface

  angle of reflection - angle between reflected ray and the normal to a surface

  law of reflection - the angle of incidence is equal to the angle or reflection
• a periscope consists of a tube containing two plane mirrors, fixed parallel to and facing each other. each makes an angle of 45 with line joining them. light from the object is turned through 90 at each reflection and an observer is able to see over a crowd
• if a parallel beam of light falls on a plane mirror it is reflected as a parallel beam and regular reflection occurs
• most surfaces reflect light irregularly and the rays in an incident parallel beam are reflected in many directions as diffuse reflection
• a real image is one which can be produced on a screen and is formed by rays that actually pass through the screen
• a virtual image cannot be formed on a screen and is produced by rays which seem to come from it but do not pass through it
• the image in a plane mirror is:

  • as far behind the mirror as the object is in front, with the line joining the same points on object and image being perpendicular to the mirror
  • same size as the object
  • virtual
• the bending of light when it passes from one material to another is called refraction
• angle of refraction - angle between refracted ray and the normal to a surface
• a ray of light is bent towards the normal when it enters an optically denser medium at an angle so the angle of refraction is smaller than angle of incidence
• a ray of light is bent away from the normal when it enters an optically less dense medium
• a ray emerging from a parallel sided block is parallel to the ray entering but is displaced sideways
• a ray travelling along the normal direction at a boundary is not refracted
• refractive index - the ratio of the speeds of a wave in two different regions
• n = speed of light in air/speed of light in medium

  n = sin i/sin r
• when light passes at small angles of incidence from an optically dense to a less dense medium there is a strong refracted ray and a weak ray reflected back into the denser medium
• critical angle - angle of incidence which produced an able of refraction of 90
• for angle of incidence greater than c the refracted ray disappears and all the incident light is reflected inside the denser medium and is said to undergo total internal reflection
• light can be trapped by total internal reflection inside a bent glass rod and piped along a curved path
• optical fibres are being used to carry telephone, high speed broadband internet and cable tv signals as pulses of visible or infrared light
• optical fibres allows information to be transmitted at a higher rate and the data is more secure as they are unaffected by electronic interference and can be used over long distances
• principal axis - line through the optical centre of a lens at right angles to the lens

  principal focus - point on the principal axis of a lens to which light rays parallel to the principal axis converge or diverge

  focal length - distance between the optical centre and the principal focus of a lens
• if image is between f and 2f - real, inverted, diminished

  if image is at 2f - real inverted, same size

  if image is beyond 2f - real, inverted enlarged
• converging lens gives an upright, enlarged, virtual image of an object placed between lens and f. angle made at eye by image is larger than the angle made when it is viewed directly at the near point
• short sighted person - image of a distant object is formed in front of the retina because the eyeball is too long or because the eye lens cannot be made thin enough, problem is corrected by a diverging lens
• long sighted person - image of an object is focused behind the retina because the eyeball is too short or the eye lens cannot be made thick enough, corrected by a converging lens
• when sunlight falls on a triangular glass prism, a band of colours called a spectrum is obtained and dispersion occurs
• the prism seperates the coulours because the refractive index of glass is different for each colour
• red light has the longest wavelength and lowest frequency, violet light has the shortest wavelength and the highest frequency
• red

  orange

  yellow

  green

  blue

  indigo

  violet
• light of one colour and one frequency is called monochromatic