Chapter 6: Interactions Between Light and Matter

Question 1

What is light?

Answer 1

• electromagnetic radiation, which consists of oscillating electric and magnetic fields

Question 2

Which constant represents the speed of light in a vacuum?

Answer 2

• constant c: 3x10^8 m/s

Question 3

What happens to light travelling through adjacent small slits?

Answer 3

• undergoes diffraction and interference

Question 4

What is light described as?

Answer 4

• 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

Question 5

What is the relationship between wavelength, frequency and the speed of light?

Answer 5

c= vλ

- v is frequency and λ is wavelength

Question 6

monochromatic

Answer 6

• light that consist only of waves with the same wavelength

Question 7

coherent and what is an example?

Answer 7

• when multiple light waves have the same wavelength and are produced at the same time, the light is described as coherent
• a laser

Question 8

incoherent and what is an example?

Answer 8

• when multiple light waves have different wavelengths and/or are produced at random moments of time
• a lightbulb

Question 9

linearly polarized light and how is it formed? (3)

Answer 9

• 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

Question 10

electromagnetic spectrum

Answer 10

• illustrates the continuous range of wavelengths of light

Question 11

What are the 6 categories of electromagnetic radiation?

Answer 11

• radio, microwave, infrared, ultra-violet, x-ray, gamma rays

Question 12

What is the continuous spectrum of light and how is it made? (2)

Answer 12

• 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

Question 13

Why does light have a particle-like nature? (2)

Answer 13

• 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

Question 14

What is the energy expression for a photon?

Answer 14

• E=pc=(hc)/λ=hv

where v is frequency

Question 15

Planck relation

Answer 15

• 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)

Question 16

How do photons compare to ordinary particles? (2)

Answer 16

• 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

Question 17

How is light viewed (in terms of waves and particles? (3)

Answer 17

• 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

Question 18

What is intensity/brightness proportional to?

Answer 18

• for waves: to amplitude

- for particles: to number density of photons

Question 19

photoelectric effect (2)

Answer 19

• 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

Question 20

What does the photoelectric effect experiment consist of? (4)

Answer 20

• 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

Question 21

What is significant about the threshold frequency in the photoelectric effect? (2)

Answer 21

• 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

Question 22

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?

Answer 22

• Planck’s constant

Question 23

Where does the KE of the ejected electrons originate from? (2)

Answer 23

• from the energy of the incident light (E_light)

- some energy is consumed in removing the electron from the surface (E_binding)

Question 24

How do you calculate the KE of ejected electrons using the conservation of energy?

Answer 24

E_k = E_light - E_binding = h(v-v_0) = hv - hv_0

Question 25

What is the KE of ejected electrons proportional to?

Answer 25

• frequency of incident light

Question 26

What is the number of electrons emitted proportional to?

Answer 26

• the intensity of the incident light: more intense light contains more photons so more electrons are emitted from the surface

Question 27

What would happen if a photon strikes a surface with energy greater than the binding energy?

Answer 27

• 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

Question 28

What is the binding energy related to?

Answer 28

• the electrostatic attraction between the electron and the nuclei

Question 29

What is the only logical explanation of the photoelectric effect?

Answer 29

• there must be particles of energy that give their entire energy to an electron in the metal

Question 30

What happens when light interacts with matter?

Answer 30

• 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

Question 31

What condition needs to be met for a quantum state transition to occur

Answer 31

• energy of the photon must match the energy difference between the quantum states

Question 32

spectrum

Answer 32

• what is produced when the measure of intensity of light absorbed or emitted is plotted against frequency or wavelength

Question 33

What information does a spectrum give us?

Answer 33

• 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

Question 34

absorption spectrum

Answer 34

• 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

Question 35

emission spectrum

Answer 35

• 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

Question 36

atomic spectroscopy

Answer 36

• measure of the energy of light that is absorbed or emitted by atoms or ions as they change their electronic quantum states