Chapter 6: Interactions Between Light and Matter Flashcards
What is light?
- electromagnetic radiation, which consists of oscillating electric and magnetic fields
Which constant represents the speed of light in a vacuum?
- constant c: 3x10^8 m/s
What happens to light travelling through adjacent small slits?
- undergoes diffraction and interference
What is light described as?
- oscillating electric and magnetic fields or electromagnetic waves; electric and magnetic field components of a electromagnetic wave are always at the same wavelength and frequency, and are oriented at 90 degrees to each other
What is the relationship between wavelength, frequency and the speed of light?
c= vλ
- v is frequency and λ is wavelength
monochromatic
- light that consist only of waves with the same wavelength
coherent and what is an example?
- when multiple light waves have the same wavelength and are produced at the same time, the light is described as coherent
- a laser
incoherent and what is an example?
- when multiple light waves have different wavelengths and/or are produced at random moments of time
- a lightbulb
linearly polarized light and how is it formed? (3)
- has waves that have their electric (and perpendicular magnetic) field components vibrating in the same plane
- formed by passing unpolarized light through a polarizer, which only allows light with waves in a particular orientation through
- assume light is unpolarized unless stated otherwise
electromagnetic spectrum
- illustrates the continuous range of wavelengths of light
What are the 6 categories of electromagnetic radiation?
- radio, microwave, infrared, ultra-violet, x-ray, gamma rays
What is the continuous spectrum of light and how is it made? (2)
- When white light is passed through a prism, we see a visible rainbow of light created by the incident light being dispersed by refraction, meaning the light comes out with an angle depending on its original wavelength, thus lights of different wavelengths are separated and so we see variation in colour
- is all wavelengths from red to violet, with no gaps, are present, then the white light was dispersed into a continuous spectrum of light
Why does light have a particle-like nature? (2)
- incoming light acts like particles, called photons, and are discrete packets of energy and momentum, collide with an electron and deflect in the process
- wavelength of light changes after the collision with the electron which indicates that the momentum of a photon is related to its wavelength
What is the energy expression for a photon?
- E=pc=(hc)/λ=hv
where v is frequency
Planck relation
- E=hv: this important equation tells us that the energy of a photon is directly proportional to its frequency (of the inverse of its wavelength)
How do photons compare to ordinary particles? (2)
- photons carry energy and momentum like particles
- photons are massless, whereas other particles have mass
- photons are easily annihilated and created when they interact with matter, whereas most particles don’t disappear in collisions
How is light viewed (in terms of waves and particles? (3)
- diffraction and interference can be explained by viewing light as a wave
- Compton scattering can be explained by viewing light as a particle
- scientist have accepted the view of wave-particle duality for light
What is intensity/brightness proportional to?
- for waves: to amplitude
- for particles: to number density of photons
photoelectric effect (2)
- electrons ejected when light is incident on a solid substance
- forms the basis of an important method used to identify unknown substances in forensic science
What does the photoelectric effect experiment consist of? (4)
- consist of a metal plate toward which the ejected electrons travel
- electrons in metals are in orbitals, which can be excited with a certain amount of energy from light
- the plate is connected to the source metal to complete the circuit
- the metal can be ionized only if a certain minimum energy is used, meaning there is a minimum frequency of light needed
What is significant about the threshold frequency in the photoelectric effect? (2)
- the values below a certain frequency where no electrons are ejected regardless of the intensity of the incident light
- above the threshold, the KE of ejected electrons increase linearly with the frequency (not intensity) of the incident light
What is the slope of the line in the plot of KE of ejected electrons (E_k) versus frequency of the incident light (v) in the photoelectric effect?
- Planck’s constant
Where does the KE of the ejected electrons originate from? (2)
- from the energy of the incident light (E_light)
- some energy is consumed in removing the electron from the surface (E_binding)
How do you calculate the KE of ejected electrons using the conservation of energy?
E_k = E_light - E_binding = h(v-v_0) = hv - hv_0
What is the KE of ejected electrons proportional to?
- frequency of incident light
What is the number of electrons emitted proportional to?
- the intensity of the incident light: more intense light contains more photons so more electrons are emitted from the surface
What would happen if a photon strikes a surface with energy greater than the binding energy?
- an electron can be emitted and excess energy is transferred to the ejected electrons that are observed to be proportional to the number of photons that strike the surface
What is the binding energy related to?
- the electrostatic attraction between the electron and the nuclei
What is the only logical explanation of the photoelectric effect?
- there must be particles of energy that give their entire energy to an electron in the metal
What happens when light interacts with matter?
- light can be absorbed: transition from a lower energy to a higher energy quantum state
- light can be emitted: transition from a higher energy to a lower energy state
What condition needs to be met for a quantum state transition to occur
- energy of the photon must match the energy difference between the quantum states
spectrum
- what is produced when the measure of intensity of light absorbed or emitted is plotted against frequency or wavelength
What information does a spectrum give us?
- information about transitions between quantum states in matter because energy difference between these states equals the energy of the photons, E=hv, absorbed or emitted
absorption spectrum
- irradiating a sample with electromagnetic radiation and measuring the amount of light absorbed at each specific wavelength: if light is absorbed, a signal (line) at the wavelength of absorption is observed
emission spectrum
- exciting a sample to higher energy quantum states by first treating it with an energy source and then measuring the specific wavelengths of light emitted as the sample relaxes to lower energy quantum states: if light is emitted at a particular
atomic spectroscopy
- measure of the energy of light that is absorbed or emitted by atoms or ions as they change their electronic quantum states
