Photons (Unit 2) Flashcards
What is light consisted as?
discrete packets of energy
How can the photoelectric effect be demonstrated?
-Clean a zinc plate with fine emery paper
-Attach the plate to the top disc on a gold leaf electroscope, so there is
good electrical contact.
- Charge the zinc plate and inner assembly of the electroscope
negatively, by rubbing the zinc plate with a polythene rod which has been rubbed with a duster. The gold leaf should now be raised, because the leaf and the BACK PLATE are both charged negatively and repel each other.
- Place an ultraviolet lamp near the zinc plate. Switch it on. The leaf
should be seen to fall. Clearly the plate is losing charge.
- Repeat the procedure, but charging the zinc plate and inner assembly of the electroscope positively, e.g. by rubbing the plate with a charged
perspex rod. This time the ultraviolet does not affect the leaf. Charge is not lost
Ev to J?
Ev and multiply by 1.6 x 10^-19
J to Ev?
J and divide by 1.6 x 10^-19 to get Ev
Key observations from experiment to show photoelectric effect? FREQUENCY is low
-When frequency of light is too low no electrons were released no matter how bright (intensity) of the light was
Key observations from experiment to show photoelectric effect? When you increase frequency of light?
-results in a rise in the energy of the electrons released
What is f 0
The minimum frequency that causes emission of a metal
-threshold frequency
What do we conclude from the photocell experiment?
One packet of light interacts with one electron, meaning that the light wave is not a continuous stream of energy; instead, it is made up of photons
- validates Plank’s constant
What is work function equal to?
H f0 - planks constant x threshold frequency
the graph of Ek max against frequency of illuminating radiation? What can we work out?
- X intercept = F 0 (we can work out work function)
- gradient planks constant
- y intercept = work function
- y = mx +c → KE max = hf - work function
Equation for energy of photon?
-E = hf
-c = fλ
What is released from metal in photoelectric effect?
Electron not photon
What is meant by photoelectric effect?
When light or ultraviolet radiation of short enough wavelength falls on a surface, electrons are emitted from the surface.
What is meant by work function?
minimum energy needed to remove an electron from the surface.
Experiment to show how a vacuum photocell can be used to measure the maximum kinetic energy, Ek max, of emitted electrons in eV and hence in J? (Photocell experiment)
-Set up a circuit including a vacuum photocell ,microammeter and a variable d.c power supply
-Set the variable d.c power supply to 0v
-Shine a light of known frequency on the vacuum photocell
-Increase the P.D on the power supply until the current on the microammeter reads 0 μ A
-Note the p.d on power supply V and calculate Ek Max= eV stop
-Repeat with different frequencies
Wavelength of visible light?
750 (nM) (red) to 400 (nM) (purple)
EM SPECTRUM WAVELENGTHS?
Gamma ray - 10^ -12
X ray 10^-10
Ultraviolet 10^-7 or 10^-8
Visible light 7x10^-7 to 4 x10^-7
Infrared 10^-5
Microwave 10^-2
Radio 10^2
How to work out photon energy?
E = hc/λ
Determination of h using LEDs experiment?
-By slowly increasing the p.d., across an LED, the minimum p.d. required for light to be emitted can be measured. This is known as the , V min , and can be used to calculate the energy of the photons emitted by the LED, E.
E=eV min
Where e=1.60×10 −19
-As photon energy can also be calculated using E= hc/ λ
-these equations can be combined to give an expression linking V
min and λ.
V min = hc/ e λ
-Plotting a graph of V min against 1/ λ
should give a straight line through the origin, with a gradient equal to hc/e
- rearrange to work out h
Explanations from goldleaf experiment?
The current produced due to the electrons being released
was unaffected by changing the brightness of the light.
• Changing the frequency of the light, changed the current.
• If the frequency of the light was too low, no electrons were
released no matter how bright the light was
How is emission spectra produced?
-when electrons move down an energy level, then it must emit a photon of energy , photon energy must be equal to energy gap (coloured lines on black background)
How is absorption spectra produced?
-If an electron wants to move up an energy level then it must absorb a photon (black lines on coloured background)
Relationship between absorption and emission spectra?
-dark and bright fringes will have same wavelengths and frequency
What is ionisation energy?
energy required to remove an electron from the ground state
How to show particles have a wave like aspect and results?
- Through electron diffraction
-When electrons are accelerated towards graphite crystals (ACT AS DIFFRACTION GRATING), a diffraction pattern is produced. Shows electrons behave like waves, shows particles have wave like aspects and particle like aspects
Momentum change equation?
p (momentum) = h/λ
Radiation pressure equation?
Momentum change per second / area of beam (units in pascal)
Power equation?
energy/ time
Significance of photocell experiment?
-proves EM spectrum
-waves are quantisised into packets of energy
Number of electrons emitted per second?
Power/ Energy gap of photon (J)
Current / Charge of an electron
Photon momentum?
H / Wavelength
Momentum of light per second?
Photon momentum x number of photons emitted per second
Number of photons striking metal disc (from light)
- Energy / Total ENERGY (WORK FUNCTION + MAX KE)
Photon energy and electron energy?
Photon - Energy hitting/ total energy
Electron - energy / 1.6 x 1^-19
