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Name___________________Per____ IB/AP Physics I – Spectral Lines and Atoms
NOTES: E=hf E=hc/λ 1 eV = 1.6 x 10-19 J
E = -13.6/n2 eV
1/λ = R(1/22 – 1/n2)
c = f λ = 3.0 x 10-8 m/s
h=6.6 x 10-34 Js
SEE ALSO: Text pages 924-944
1. What were the main features of the Rutherford Model of the atom? How was it deficient?
2. When atoms emit light, there are only certain wavelengths in that light (spectral lines). The light is
not a continuous spectrum. What does this observation tell us about the nature of electrons in atoms?
Neon lasers produce red light with a wavelength of 633nm. Based on this information, what is the
energy of a one electron transition in neon? Report your answer in eV.
3. Calculate the first 5 energy levels above the ground state of hydrogen and show them in an energy
diagram (the n=1 energy level is at -13.6eV). What is the minimum amount of energy released when
an electron reaches the ground state of hydrogen? What type of energy is this?
4. Find the wavelength of the red Hα line (the lowest energy line in the Balmer Series) in the hydrogen
spectrum. You must first calculate the values of the appropriate energy levels.
5. Calculate the deBroglie wavelength of a 5eV electron. What is the significance of this deBroglie
wavelength? (λ = h/mv)
6. What type of radiation would result from transitions ending on the 100th energy level of hydrogen?
(Hint: What is the highest energy these photons could have?)
7. A hydrogen atom is in the state n=2. An energy of 10eV is provided to the atom. What is the energy
and speed of the electron after it leaves the atom? BE CAREFUL WITH UNITS.
PHOTOELECTRIC EFFECT: See text pages 903-909
1. For each type of surface, there is a threshold energy of light that causes electrons to be ejected.
Any lower energy light will not work, and all higher energy light will work.
2. If the light is below the threshold energy and does not eject electrons, then increasing the
intensity of the light does NOT help. This energy of light will NOT eject electrons.
3. If the energy of the light is above the threshold and does eject electrons, then increasing the
intensity of the light causes a greater photocurrent.
4. If the energy of the light is above the threshold and does eject electrons, then using a higher
energy of light will cause electrons to be ejected with more energy, which leads to a higher
photovoltage.
Sketch the photoelectric effect experiment!
PHOTOELECTRIC ASSIGNMENT: In Class
Explain the following aspects of the Photoelectric Effect in pictures (with a few words, as necessary).
Provide one set of pictures for each group of THREE students!
A. How was the wave model initially successful in explaining the Photoelectric Effect?
B. How did the wave model fail?
C. What determines the threshold frequency for different surfaces?
D. Why doesn’t red light (650nm) create the photoelectric effect on cesium, but blue light (450nm)
does?
E. How does the particle model explain the effect of light intensity? Include light below the threshold
and light above.
F. What is the difference between shining a green light (540 nm) on cesium and shining UV light (300
nm) on cesium?
THE PHOTOELECTRIC EFFECT PROBLEM:
A point source of light runs on 6V and 0.5amps and is 5% efficient at producing white light. A filter
around the source blocks out 98% of the white light, allowing only violet light with a wavelength of
400nm to pass through. Some of the violet light then falls on a 2cm x 2cm lithium plate located 10cm
away from the source.
Sketch this system:
(a) Find the violet source strength in Watts.
(b) How much energy is carried by one violet photon? Find this in Joules and eV.
(c) Find the violet source strength in photons per second.
(d) How many photons per second fall on the lithium plate?
(e) Find the photocurrent, in amps, if 0.1% of the incident photons produce a photoelectron.
(f) Find (look up) the binding energy for lithium.
(g) What is the maximum energy of the photoelectrons?
(h) At what speed do the fastest electrons leave the plate?
(i) What electrical power is generated by the lithium plate (thinking of it as a “solar cell”).
Name________________________Per___ IB/AP Physics I – Analysis of Light
1. The Sun’s intensity on Earth above the atmosphere is 1380 W/m2.
(a) If the Sun-Earth distance is 1.5 X 1011 m, determine the luminosity of the Sun.
(b) Astronomers classify the Sun as a yellow star, meaning that most of the light leaving the Sun is
near a yellow wavelength (500nm). Estimate the number of photons per second leaving the Sun.
(c) Suppose an astronomer points her telescope at a distant star and determines that the star’s intensity
is 2.0 X 10-11 W/m2. Spectral analysis of the light shows that most of the light emitted from the star is
at about 500nm, and thus the star is much like the Sun. How far away is this star?
2. The wave model of light and the photoelectric effect:
(a) What is the difference between high intensity red light and low intensity red light in the
electromagnetic wave model?
(b) What incorrect prediction does the wave model make about the effect of light intensity on the
ejection of electrons from atoms?
3. A small light bulb runs on 0.4 amps and 1.5V, and the light it produces is filtered so that only green
light of 550nm is emitted. The energy carried by the green light is 0.1% of the total energy consumed
by the bulb. A 2cm X 2cm cesium plate is placed 30 cm from the bulb so that the green light falls
directly onto it.
(a) How much energy is carried by one of the green photons?
(b) What is the intensity of the green light 30 cm away from the source?
(c) How many photons per second leave the filtered source?
(d) How many photons per second strike the cesium plate?
(e) Find the resulting photocurrent in amperes if 1% of the incident photons produce a photoelectron.
(f) Assuming a binding energy of 1.9 eV for Cesium, what potential would be needed to stop all of the
electrons? (What is the energy of the photoelectrons?)
(g) At what speed do the fastest electrons leave the plate?
4. So, what is the TRUE nature of light?