How would you do this question?
What happens to the speed of the waves in a
ripple tank if the frequency of the wave source is
halved?
One of the most important things to remember about waves is their speed depends
only on the medium they travel though. If you have red and green light passing through a vacuum, for example, they still have the same speed:
. A consequence of this is that when the frequency of the wave source is doubled the wavelength must halve to keep the speed constant in accordance with
. So to answer your question, the speed does not change, because the medium (water) does not change.
I'm having a bit of trouble with VCAA 2014 Exam Question 22 (If anyone has time to help). With part a) is it's first excited state at 4.9 eV? It can absorb a 1.8eV photon because that is the exact difference between the 1st and 2nd excited states but it can't emit a photon from the first excited state because there is no energy level at (4.9eV - 1.8eV) 3.1 eV? I got really confused on this when i did it today. I read the diagram from top to bottom instead of bottom to top. Just wanted to clear it all up.
For part b I'm really lost with what to do. 0.9, 1.5 and 2.2 eV are all between the first excited state. I don't know how to use them at all to find out the unknown excited state.
Thanks
You are essentially correct for part
a. Remember that in general atoms can only take on specific discrete energy levels. So absorbing a
photon would give it an energy of
which is the second excited state but emitting a
photon would bring its energy down to
which is not one of its discrete energy states.
For part
b, we just need to use some simple observation. Since a
photon being emitted does not correspond to a drop in any of the known energy states, we have two possibilities for
, one where the energy of the atom drops to
and one where it drops from
: namely,
or
. Now we substitute another given value, an emission of a
photon, to find that
cannot be
as if
there would be no drop emitting a
photon. So
must be
, and we can verify this by seeing that
.