7. Photons

Question 1

Based on Tim’s notes o7

Answer 1

o7

Question 2

State the new model of radiation

Answer 2

Planck’s theory suggests that light streams away as tiny photons each having a discrete amount of radiation energy determined by the frequency and therefore wavelength of light

E = hf (Energy = h x frequency) not given in data booklet
c = fλ (speed = frequency x wavelength)

Question 3

How to convert J into eV?

Answer 3

/ by 1.6 x 10^-19
(÷ by charge of an electron)

Question 4

How to convert eV into J?

Answer 4

x by 1.6 x 10^-19
(x by charge of an electron)

Question 5

Laws of Photoelectric Emission
(I want to space this somehow)

Answer 5

• Number of electrons released from surface is proportional to the intensity (more photons) of light
• Emission occurs with no delay
• KE of photoelectrons (electrons) depends on the frequency (not intensity) of the light
• There exists a threshold frequency f₀. If the frequency is below f₀, no photoelectrons are emitted. f₀ depends on metal
• Photons penetrate the surface and put all it’s energy into one electron
• Only 1 photon can interact with 1 electron

Question 6

Explain electrons in a ‘potential energy well’

Answer 6

• Electrons sit in a ‘potential energy well’
• Electrons need a certain amount of energy to be released from metal and will have 0 energy:
• This is the work function ϕ of the metal
  Display image
• If photon has more than the minimum energy (work function ϕ)
• Then the difference is seen as KE of the photoelectron

Photo Energy = work function + Max KE
E = ϕ + KE^max

Question 7

Explain photoelectric effect

DISPLAY IMAGE

Answer 7

• The excess electrons placed on the zinc top will cause the gold leaf to repel
• UV light on a short enough wavelength shines upon the zinc and will cause the gold leaf to go down
• UV incident upon a metal surface results an electron loss
• As the wavelength increases, no electrons are lost and the gold leaf remains repelled
• Electrons can escape from the surface of certain metals if the surface of the metal is illuminated by radiation of sufficient frequency

Question 8

Explain photocell experiments
(I want to space this somehow)
(In addition, most visualised one ¬.¬)

Answer 8

1. Varying the frequency of light but keeping the intensity of light constant:
   - Saturation current is constant
   - V = KE, higher the frequency, greater KE for electrons
   Display image
2. Varying intensity of light but keeping frequency:
   - Saturation current increases as intensity increases
   - Stopping potential V_s is not affected by intensity of light
   Also Display image

Question 9

Explain matter waves

Answer 9

A wave behaves like a particle so matter could behave like a wave

λ = h/momentum = h/mv -> ρ = h/λ (given in data booklet)
m = mass
ρ = momentum

Question 10

Describe an absorption spectra?

Answer 10

Black lines on a brightly coloured background. Only certain wavelengths have been absorbed

Question 11

How is an absorption spectra obtained?

Answer 11

Passing continuous spectra though a cold gas

Question 12

Describe an emission spectra?

Answer 12

Coloured lines on a black background. Only certain wavelengths have been emitted

Question 13

How is an emission spectra obtained?

Answer 13

Heating a gas so that it emits light

Question 14

Explain emission spectra?

Answer 14

When an electron in a high energy orbit, the difference in energy is emitted as a photon (E₁ - E₂ = hf)

Question 15

Explain absorption spectra?

Answer 15

• When an incoming photon of energy E equals the energy difference between a lower energy and higher energy level
• The photon is absorbed and promotes an electron from a low energy level to a high energy level:
  Energy of a photon = hf = hc/λ = E₁ - E₂

Question 16

Explain electron diffraction
(refer to book ¬.¬ for diagram)

Answer 16

• Electrons are evaporated off a heated cathode and accelerate a thin carbon target (disc)
• electrons pass through the carbon and hit a screen to give a pattern
• Spacing between carbon atoms behave like a diffraction grating
• The diffraction probes they act like wave

Question 17

Describe the EM spectrum in terms of the orders of magnitude for each part of the spectrum

Answer 17

Radio waves - 10³(km) & 10⁰(m)
Microwaves - 10^-2(cm)
IR (infrared) - 10^-3(mm)
Visible light - 10^-6(µm):
Red = 750nm
Orange = 700nm
Yellow = 600nm
Green = 550nm
Blue = 500nm
Purple = 400nm
UV - 10^-8(µm)
X-rays - 10^-9(nm)
Gamma-rays - 10^-10(nm) & 10^-12(pm)

Question 18

Not the best thing btw, but at least it’s something to look at… for now?

Answer 18

Is there even time