The Photoelectric Effect

Size: px

Start display at page:

Download "The Photoelectric Effect"

Lilian Shelton
5 years ago
Views:

1 Test metal The Photoelectric Effect Electrons Two metal plates in vacuum, adjustable voltage between them, shine light on one plate. Measure current and adjust voltage to reduce current to zero. Repeat for many wavelengths of light. 1

2 Supply monochromatic light of constant intensity at A, vary battery voltage What is the current vs battery voltage? A Current 0 Voltage B Current 0 Voltage C Current e s 0 Voltage Current Current D 0 Voltage E 0 Voltage 2

3 f lambda < threshold, C; f lambda > threshold, E C 0 Voltage Current Current E 0 Voltage 3

4 Summary of photoelectric experiment results. Current appears with no delay Electrons emitted only if frequency of light exceeds a threshold (wavelength below a threshold) color matters! Threshold frequency depends on type of metal Maximum KE of released electrons increases linearly with frequency. Max. KE= - V stopping. Stopping potential independent of intensity Current linearly proportional to intensity how do these compare with classical wave predictions? 4

5 Experiment that shows that light is also a particle. Light energy comes in particle-like chunks - basics of quantum physics. Planck s constant: h = 6.6 x Js = 4.13 x evs E photon = hf = hc/λ E beam = N hf The Photoelectric Effect What is energy of red photon? Blue photon? E photon = 1240 ev nm = 1.91 ev 650 nm E photon = 1240 ev nm = 3.1 ev 400 nm 5

6 Photoelectric effect experiment: Most loosely bound electrons One quantum of light (photon) excites one electron and one only! Electron Energy nside metal Work function Outside metal nstantaneous emission Energy in = Energy out Energy of photon = energy needed to kick + nitial KE of electron electron out of metal as it exits metal 6

7 Photoelectric effect experiment: Most loosely bound electrons Electron Energy nside metal Work function = W (also φ) Outside metal Energy of photon = energy needed to kick + nitial KE of electron electron out of metal as it exits metal hf = W + KE 7

8 KE 300 V CQ: A photon at 300 nm will kick out an electron with an amount of kinetic energy, KE 300. f the wavelength is halved to 150 nm and the photon hits the same electron, the kinetic energy KE 150 of the electron coming out is a. less than ½ KE 300. b. ½ KE 300 c. = KE 300 d. 2 x KE 300 e. more than 2 x KE 300 8

9 Summary of photoelectric experiment results. Current appears with no delay Electrons emitted only if frequency of light exceeds a threshold (wavelength below a threshold) color matters! Threshold frequency depends on type of metal Maximum KE of released electrons increases linearly with frequency. Max. KE = - V stopping. Stopping potential independent of intensity Current linearly proportional to intensity how do these compare with classical wave predictions? 9

10 Photoelectric effect experiment: Most loosely bound electrons V emit = 0 V s < 0 A negative voltage on the collector plate decreases the kinetic energy of the emitted electrons. The stopping voltage, V s, stops even the most energetic electrons and reduces the current to zero. ev s = KE max 10

11 CQ: Light of 400 nm is incident on a metal plate. The most energetic electrons come out with a certain kinetic energy, and a voltage difference of 1.8 V is required to stop these electrons. What is the work function for the metal plate? a. 1.3 ev b. 4.9 ev c. 5.8 ev d. 2.2 ev e. none of the above V 11

12 What s happening here? Each electron that pops out is accelerated more so hits far plate with higher velocity, BUT # of electrons = constant sec So current is constant! C Current NOT V=R!! 0 Battery Voltage reverse V, no electrons flow. 12

13 What s happening here? More tightly bound electrons have lower KE when they pop out. They are stopped by less negative voltage, so the current starts to drop. C Current NOT V=R!! 0 Battery Voltage reverse V, no electrons flow. 13

14 Photoelectric effect experiment: tightly bound electrons Energy in = Energy out Energy of photon = energy needed to kick + nitial KE of electron electron out of metal as exits metal Electron Energy nside metal Work function Outside metal Tightly bound electron -> needs more energy than W to escape; less initial KE, less voltage needed to repel e -. f e - is too tightly bound, it won t be released! 14

15 f lambda > threshold, E Current E 0 Voltage 15

16 Photon Energies: Typical energies Each photon has: Energy = h * f (Energy in Joules) (Energy in ev) E=hf=(6.626*10-34 J-s)*(f s -1 ) E=hf= (4.14*10-15 ev-s)*(f s -1 ) E=hc/λ = (1.99*10-25 J-m)/(λ m) E= hc/λ = (1240 ev-nm)/(λ nm) Red Photon: 650 nm Purple photon: 400 nm E photon = 1240 ev nm = 1.91 ev 650 nm E photon = 1240 ev nm = 3.1 ev 400 nm Work functions of metals (in ev): Aluminum 4.08 ev Cesium 2.1 Lead 4.14 Potassium 2.3 Beryllium 5.0 ev Cobalt 5.0 Magnesium 3.68 Platinum 6.35 Cadmium 4.07 ev Copper 4.7 Mercury 4.5 Selenium 5.11 Calcium 2.9 Gold 5.1 Nickel 5.01 Silver Carbon 4.81 ron 4.5 Niobium 4.3 Sodium 2.28

17 HGH intensity e s voltage to turn around most energetic electron stopping potential do low exper Battery Voltage

18 Which graph represents low and high intensity curves? A B 0 Batt. V 0 Batt. V C D 0 Batt. V F 0 Batt. V 0 Batt. V 18

19 HGH intensity LOW intensity e s Fewer electrons pop off metal Current decreases. Current proportional to light intensity. ans. B 19 0 Battery Voltage

20 HGH intensity LOW intensity e s Same KE electrons popping off metal. So same stopping potential Battery Voltage

21 look at sim for few different colors, small forward V Predict what happens to the initial KE of the electrons as the frequency of light changes? (Light intensity is constant) e s Predict shape of the graph nitial KE 0 Frequency of light 21

22 A nitial KE B nitial KE 0 Frequency 0 Frequency C nitial KE D nitial KE 0 Frequency 0 Frequency E. something different 22

23 Correct answer is D. do sim showing graph There is a minimum frequency below which the light cannot kick out electrons even if we wait a long time e s nitial KE 0 Frequency of light As the frequency of light increases (shorter λ!), the KE of emitted electrons increases. (it is a linear relationship) what happens if change the metal? do experiment 23

PE summary from last class:

PE summary from last class: PH300 Modern Physics SP11 Today: Finish photoelectric effect Photons Next Week: Atomic spectra/balmer series Lasers learned very early the difference between knowing the name of something and knowing something.!