Light and Matter

• Light is a form of energy.
• The unit of light is Watts, 1Watt = 1Joule per second.
• White light is made up of different colours.
• How do light and matter interact?

  • Emission
  • Absorption
  • Transmission
  • Reflection / scattering
• Transparent objects transmit light.
• Opaque objects block (absorb) light.
• Mirror reflects light in a particular direction.
• Light can act either like a wave or like a particle.
• Particles of light are called photons.
• A wave is a pattern of motion that can carry energy without carrying matter along with it.

  A) wavelength

  B) crest

  C) trough

  
• An electron moves when light passes by, showing that light carries a vibrating electric field.
• A light wave is a vibration of electric and magnetic field.
• Light interacts with charged particles through these electric and magnetic fields.
• Wavelength is the distance between two wave peaks.
• Frequency is the number of times per second that a wave vibrates up and down.

  wave speed = wavelength x frequency
• High frequency:

  Wavelength is inversely proportional to frequency.

  A) short wavelength

  
• Low frequency:

  A) long wavelength

  
• The energy of a photon depends on its frequency.
• A hertz is one cycle per second.
• The entire range of wavelengths of light is known as the electromagnetic spectrum.
• Phases of matter : Solid, Liquid, Gas (water vapour)

  • Phases of some material behave differently because of differences in chemical bonds.
• Phases of matter and phase changes:

  A) solid

  B) liquid

  C) gas

  D) plasma

  
• Melting: The process of changing from a solid to a liquid.
• Evaporation: The process of water evaporating from a liquid to a gas.
• Dissociation: breaking of molecules into atoms.
• Ionization: stripping of electrons, changing atoms into plasma.
• Atoms gain and lose energy only in amounts corresponding to particular changes in energy levels
• 3 types of spectra:

  1. Continous spectrum
  2. Emission spectrum
  3. Absorption spectrum
• Continuous spectrum: spectrum that contains all wavelengths emitted by a hot, dense light source

  
• Emission spectrum: shows colored lines of light emitted by glowing gas

  
• Absorption spectrum: shows dark lines or gaps in light after the light passes through a gas.

  
• All large, dense object emits thermal radiation.
• An object's thermal radiation spectrum depends only on 1 property: its temperature.
• Temperature is in Kelvin

  T(in C) + 273
• Hotter objects emit more light at all frequencies (per unit area).


  Hotter objects emit more photons with a higher average energy.
• The wavelength of emission peak varies with the temperature of an object.

  1. Higher temperature black bodies are brighter and have shorter wavelength.
  2. Blue hot is hotter than orange hot which is hotter than red hot
• Stefan-Boltzmann Law: The total energy emitted by a black body is directly proportional to its absolute temperature.

  F = σT^4
• Luminosity is given by

  L = 4πR ^2 × F = 4πR ^2σT ^4
• Emission Line Spectrum: A thin or low-density cloud of gas emits light only at specific wavelength that depend on its composition and temperature, producing a spectrum with bright emission lines.
• Each type of atom has a unique spectral fingerprint.