photoelectric effect........?

Question

According to experiments concerned with the photoelectric effect, which of the following will increase the kinetic energy of an electron ejected from a metal surface?

1.increasing the wavelength of the light striking the surface
2.increasing the frequency of the light striking the surface
3.increasing the number of photons of light striking the surface

a.1 only
b.3 only
c.2 only
d.2 and 3
e.1 and 2

Answer 1

a) Photoelectric law postulate 3

ref: http://en.wikipedia.org/wiki/Photoelectric_effect

Answer 2

When light is incident on a metallic surface, electrons at the surface gain energy from the light. If this energy is greater than the work function of the metal (the energy required to free an electron from the surface), the electron will be ejected from the surface, acquiring some kinetic energy.

From a wave picture of light, we would expect that the number of electrons ejected and their kinetic energy would increase when the light intensity is increased. We would also expect that the frequency of the light would not affect the kinetic energy of the ejected electrons.

The particle or Einstein (he won the Nobel prize for explaining the photoelectric effect, after all) picture of light says that an increase in intensity ejects more electrons, but since the energy of each photon is unchanged, the kinetic energy of the ejected photons does not depend on the intensity. Furthermore, since the energy of a photon is h*f, where h is Planck's constant and f is the frequency of the light, the kinetic energy of an ejected electron is h*f - W, where W is the work function of the metal. Therefore, if h*f < W, no electrons will be ejected, regardless of the intensity.

It turns out that Einstein was right. So, in summary, we have KE = h*f - W, which is increased when frequency is increased (this is equivalent to decreasing the wavelength of the light, but this is not an answer choice), so the answer is 2 only, or C. (Note that the number of photons incident on the surface does not matter. That is, only one photon may eject one electron. So, 3 is wrong.)

Answer 3

The kinetic energy gained by the electron through absorbing a photon of light is the difference between the photon’s energy and the work function (some constant energy value which needs to be overcome) of the surface the electron resides on.

The energy of a photon can be found as,
E = hν
Where E is the energy, h is Planck’s constant, and ν is the frequency of the light.
As you can see, an increase in frequency means a higher energy contained by the photon.

The Kinetic Energy (KE) of the electron after absorbing an electron is,
KE = E – W
Where E is the energy of the photon which was absorbed and W is the work function of the surface. The electron will only be ejected if the KE is greater than zero. If KE is zero, the electron will be ejected, but with zero KE and will be recaptured soon thereafter.

Frequency and wavelength are inversely proportional to each other.
ν * λ = c
where c is the speed of light (a constant value), so
ν = c / λ
and,
λ = c / ν
An increase in the wavelength means a decrease in the frequency and thus a decrease in the energy of the photon.
So “1” is not correct.

An electron absorbs all of the energy of only one photon. If the photon’s energy is not sufficient to overcome the work function then it gains no Kinetic Energy.
An increase in the number of photons hitting the surface of the material will not affect the energy gained by a single electron, it only affects how many electrons get ejected from the surface.
So “3” is not correct.


The correct answer is “2” which corresponds to response c.

Answer 4

b.3... Because photons are highly energetic particles and they can kick electrons out of their atoms. Secondly, electrons are never stable in their atoms outter shells. So photons will be your answer.

Answer 5

c.2 only

Answer 6

c.2 only

Answer 7

d