LIGHT QUESTIONS and ANSWERS
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? Which has the greater energy per photon,
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 has the lower energy quanta – red light or blue light? Radio waves or X-rays?
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. What is the uncertainty principle with respect motion and position?
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.
27. Which laser beam carries more energy per photon – a red beam or a green beam?
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.
Which determines the number of the ejected electrons?
Brightness, since this corresponds to the number of photons striking the metal.
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!)