Q U I Z No. 15
1. What does it mean to
say an atom is excited?
An electron or electrons
have more energy than their ground state, which is where they would be if they
had not gained energy in some way.
2. How does the energy of
the photon that is emitted by an electron jumping between two energy levels
compare to the energy difference of the two energy levels?
Exactly the same energy.
3. How is the energy of a
photon related to its frequency?
Higher energy means higher
frequency: E = hf.
4. Which has the higher
frequency, red or blue light?
BLUE.
5. Why does the flame of a
burning log have different colors in it?
Different gases in the air
have different emission spectra when excited, so the colors come from electrons
jumping down from different energy levels in (possibly) different atoms.
6. When a gas glows,
discrete colors are emitted. When a solid glows, the colors are smudged or
continuous. Why?
In the solid, the atoms
are all connected to each other and the individual electrons from different
atoms are interacting with each other in ways that prevent them from having
distinct energy levels like we saw in gases.
7. How does an absorption
spectrum differ in appearance from an emission spectrum?
The absorption spectrum
has dark lines which indicate which color photons were absorbed by the gas and
able to excite photons up to higher levels. There are always less dark
absorption lines than bright emission lines because the electrons can jump down
to the ground state after being excited in many ways but can only jump up to a
level from the ground state in one way.
8. How can astrophysicists
tell whether a star is receding or approaching Earth?
The light is blue or red
shifted.
9. How can atoms be
excited?
By being hit by photons of
the right energy, or through collisions.
10. How does the avalanche
of many photons in a laser beam differ from the large number of photons emitted
by an incandescent lamp?
A laser has light of a
single frequency but an incandescent bulb has a continuous spectrum of all
frequencies that it emits.
11. Green light is emitted
when electrons in a substance make a particular energy-level transition. If
blue light were instead emitted from the same substance, would it correspond to
a greater or lesser change of energy in the atom?
Greater.
12. Ultraviolet light
causes sunburns, but visible light does not. Why is this so?
Really, it’s complicated,
but a simple answer is that the UV photons have more energy and can do the
damage to the cell that causes sunburn.
13. Why doesn’t a mercury
vapor lamp “run out” of excited atoms and produce dimmer and dimmer light?
The electrons keep getting
excited by the voltage that is applied to the lamp.
14. In what specific way
does light from distant stars and galaxies tell astronomers that atoms
throughout the universe have the same properties as those on Earth?
You can see exactly thet
same emission and absorption lines in stars from everywhere!!
15. If we continue to
heating a piece of initially room-temperature metal in a dark room, it will
begin to glow visibly. What will be the first color and why?
Red – since it’s the
lowest frequency and energy.
16. How do the surface
temperatures of reddish, bluish and whitish stars compare?
Coldest, hottest, middle.
17. What is a quantum
light called?
Photon.
18. Which determines the
number of the ejected electrons?
Brightness, since this
corresponds to the number of photons striking the metal.
19. What evidence is there
that light is a particle?
Photographs can be made
such that they show individual photons striking the film/ detector.
Interference patterns can be observed being built up photon by photon.
20. Why won’t a very
bright beam of red light impart more energy to an ejected electron than a
feeble beam of violet light?
A single photon must be
absorbed by a single electron – and a single red photon has less energy than a
single UV photon.
21. Does light travel from
one place to another in a wavelike way or in a particle-like way?
Depends how you’re looking
at it, but most often in a wave-like way, diffracting and interfering with
slits and gratings.
22. Does light interact
with a detector in a wavelike way or in a particle-like way?
Depends how you look at
it, but most often as a particle since its energy is delivered in chunks or
quanta.
23. When does light behave
as a wave? Traveling through slits and gratings.
When does it behave as a
particle?
Depositing its energy at a
detector.
24. What evidence is there
that particles, like electrons, are waves?
They can form interference
patterns on screens and have a “De Broglie Wavelength” that determines the
patterns of interference that form.
25. You cannot know both a
particle’s position and its velocity arbitrarily accurately. Knowledge of one
quantity to high precision necessarily reduces your knowledge of the other
quantity.
26. Can we speak of
photons of white light? Why or why not?
No, because photons have a
specific energy and frequency, but white light is a mixture of many frequencies
of light.
28. If a beam of red light
and a beam of violet light have exactly the same energy, which beam contains
the greater number of photons?
RED
29. In the photoelectric
effect, does brightness or frequency determine the kinetic energy of the
ejected electrons?
Frequency, since this
tells you how much energy a single photon gives to the electron.
30. Why are ultraviolet
photons more effective at inducing the photoelectric effect than photons of
visible light?
They have more energy and
can dislodge more “tightly” bound electrons from the metal.
31. A hydrogen atom and a
uranium atom move at the same speed. Which possesses more momentum? Uranium
Which has the longer wavelength?
Hydrogen ( λ = h/(mv) )
32. An electron and a
proton travel at the same speed. Which has more momentum? Proton Which has the
longer wavelength?
Electron
33. One electron travels
twice as fast as another. Which has the longer wavelength?
Slower one
34. How is it possible to
take photographs in complete darkness?
There may be no visible
light, but there could be other frequencies present that our eyes cannot
detect but other detectors can. Also, “darkness” to our eyes may still mean
there are small numbers of visible wavelength photons present that sensitive
detectors may “photograph”.
35. When you look at a
distant galaxy through a telescope, how is it that you’re looking backward in
time?
The light left the galaxy a huge number of years ago (e.g. the
Andromeda Galaxy is about 3 million light years away – the light is 3 million
years old!)
