Spectrum

• Angle of deviation


  • Depends on angle of incidence at first surface
  • Angle of prism
  • Refractive index of material
• Refractive index of glass increases with the decrease in the wavelength of light
• Violet (wavelength = 4000 A) deviates most, red (wavelength 8000 A) deviates the least
• Dispersion


  The phenomenon of splitting of white light by a prism into its constituent colours
• Spectrum


  The band of colours seen on a screen when white light passes through a prism
• The cause of dispersion of white light is that light of different wavelengths travels with different speeds in a medium
• Invisible spectrum


  The part of spectrum beyond the red extreme and the violet extreme
• Complete electromagnetic spectrum in increasing order of wavelengths

  • Gamma rays
  • X rays
  • UV rays
  • Visible light
  • Infrared rays
  • Microwaves
  • Radio Waves
• Speed


  Speed = frequency x wavelength
• Gamma rays


  • Frequency: above 10^19 Hz
  • Wavelength: Shorter than 0.1 A
• X rays


  • Frequency: between 3 x 10^19 to 3 x 10^16 Hz
  • Wavelength: 0.1 to 100 A
• UV rays


  • Frequency: between 3 x 10^16 to 7.5 x 10^14 Hz
  • Wavelength: 100 A to 4000 A
• Visible light


  • Frequency: between 7.5 x 10^14 to 3.75 x 10^14 Hz
  • Wavelength: 4000 A to 8000 A
• Infrared rays


  • Frequency: between 3.75 x 10^14 to 3 x 10^11 Hz
  • Wavelength: 8000 to 10^7 A
• Microwaves


  • Frequency: between 3 x 10^11 to 3 x 10^8 Hz
  • Wavelength: 10^7 A to 10^10 A
• Radiowaves


  • Frequency: Below 3 x 10^8 Hz
  • Wavelength: Above 10^10 A
• Properties common to all electromagnetic rays

  • Do not require medium for propagation
  • Travel with same speed in vacuum and air
  • Exhibit properties of reflection and refraction
  • In refraction, when an em ray passes from one medium to another, there is change int its direction of travel, speed and wavelength, but its frequency remains unchanged
  • Not deflected by electric and magnetic fields
  • Transverse in nature
• Gamma Rays


  • Most energetic electromagnetic radiations
  • Obtained in radioactive emissions when the nuclei of radioactive atoms pass from the excited state to the ground state. Also found in cosmic radiations
  • Cause fluorescence when in contact with fluorescent material(zinc sulphide). Easily penetrate through thick metallic sheets and pass through human body and cause great biological damage
  • Used in medical science for radiotherapy
• X rays


  • Obtained from heavy metal target of high melting point when highly energetic electron beam(cathode rays) are stopped by it
  • Chemically more active than UV radiation
  • Strongly affect photographic plate
  • Cause fluorescence when in contact with fluorescent materials
  • Pass through human flesh, don't pass through bones
  • Used in detection of fractures in bones and teeth, CAT scans, atomic arrangement in crystals, detection of concealed precious metals
• UV radiation


  • First detected by Prof. J. Ritter in 1801
  • Detected when silver chloride solution is exposed to electromagnetic rays starting from the red to the violet end and then beyond it
  • UV rays can be detected by their chemical activity on dyes and photographic plates
  • The spectrum of UV radiations is obtained by passing the radiations through a quartz prism instead of a glass prism because glass absorbs radiations
  • The electric arc and sparks give uv radiation. Mercury vapour lamps emit UV radiation. Sun is a main source of uv radiation
• Properties of UV


  • UV bulbs have an envelope made of quartz instead of glass, because glass absorbs radiation
  • They travel in a straight line with a speed of 3 x 10^8 m/s in air/vacuum
  • Scattered by dust particles
  • Obey laws of reflection and refraction
  • Affect photographic plate
  • Produce fluorescence on striking zinc sulphide screen
• Harmful effects of UV

  • Health hazards and skin cancer
• Useful UV


  • Sterilising air, surgical equipments
  • Detecting purity of germs, eggs, ghee
  • Producing vitamin d in food of plants and animals
• Infrared radiation

  • First detected by William Hershel in 1800
  • The spectrum of ir radiation is obtained by using a rock salt prism, because a rock salt prism does not absorb infrared radiations
  • Sources of ir radiation: red hot objects
• Uses of infrared radiation

  • Therapeutic purposes
  • Night photography and in foggy/misty conditions where it can penetrate easily
  • Dark rooms for photograph developing
  • War signal
  • Remote control
• Microwaves


  • Wavelength 10^7 A to 10^10 A (1mm to 1m)
  • Sources: electronic devices like crystal oscillators
  • Uses: satellite communication, analysis of molecular and atomic structures, radar communication, microwave ovens
• Radio waves


  • Longest wavelength (above 1 m/10^10 A)
  • Frequency below 3 x 10^8 Hz
  • Uses: radar communication, television transmission
• Scattering


  1. Absorption and re-emission of light energy by dust particles and air molecules of atmosphere

  2. Air molecules of size smaller than wavelength of incident light absorb energy of incident light and re-emit without change in wavelength

  3. I ∝ 1/wavelength^4

  4. Violet - least wavelength, most scattering

  5. Red - most wavelength, least scattering

  6. Violet scattered 16 times more than red

  7. Air molecules of size bigger than the wavelength scatter all light to same extent
• Applications of scattering


  • Red colour of sunrise and sunset (scattered blue, unscattered red)
  • White colour of sky at noon (directly overhead, no distance or time to scatter)
  • Blue colour of sky (blue scattered more than red)
  • White colour of clouds (particles bigger than wavelength of visible light)
  • Red for danger signals (less deviation)