What do we mean by photoelectric
effect? Observe this animation. In this animation, light strikes a metal surface
and electrons are ejected from the surface. So, the photoelectric effect phenomenon is the
emission of electrons from the surface of a body (generally metal) when exposed
to suitable electromagnetic radiation. In 1888, Hallwachs set up the fundamental
experiment that verified the photoelectric effect experimentally. Here we have an
electroscope with one movable leaf and a perfectly cleaned zinc plate. The zinc plate is
fixed to the metal cap of the electroscope. The system is charged negatively, so the leaf rises to
the left under the effect of the repulsive force. When the ultraviolet radiation emitted by the
ultraviolet lamp reaches the zinc plate, the leaf falls immediately. But, why? This is because
the electroscope loses its negative charge. When the negatively charged zinc plate is
exposed to ultraviolet radiation electrons are liberated from its surface, so the electroscope
discharges and the leaf falls down. This is the photoelectric effect phenomenon. The emitted
electrons are called photoelectrons because they are ejected from the surface of the metal
due to the energy carried by the incident light. The electroscope is charged negatively again. The
ultraviolet lamp is replaced by a red laser. When the charged zinc plate is illuminated by a red
light, the leaf does not fall whatever the sign of the charge of the zinc plate. Then, electrons
are not liberated from its surface. Why does the photoelectric effect not work for red light?
Let's see in the simulation in the next slide. Focus please, this is important. Would any light
incident on any pure metal liberate electrons from the surface of the metal or are there specific
conditions? In this PHET simulation, a sodium plate is illuminated by red light of wavelength
(λ = 750 nanometers). We noticed that there is no emission of electrons from the surface of sodium
whatever the exposure time to the incident light. Let's increase the intensity or the power of the
incident light. Also, we still have no emission of electrons. Now, let's decrease the wavelength
of the incident light to (λ = 563 nanometers). We still have no emission of
electrons whatever the power and the exposure time to the incident light.
Let's decrease λ to a value of 540 nanometers. Also, no electrons are liberated from the
surface of sodium. If λ equals 539 nanometers, now, we notice that electrons are
liberated from the surface of sodium. So, photoelectric effect takes place. If we
decrease the wavelength to another value of 309 nanometers, electrons are still liberated from
the surface of sodium. So, we can conclude that if the wavelength of an incident light is less than
or equals 539 nanometers, electrons are liberated from the surface of sodium. And, if λ is greater
than 539 nanometers, the incident light is not capable of liberating any electron whatever the
power and the exposure time to the incident light. (539 nanometers) is called the threshold
wavelength of sodium. Now, let's determine that threshold wavelength of copper. For, ( λ =
665 nanometers), electrons are not liberated from the surface of copper whatever the power or the
exposure time to the incident light. For ( λ = 264 nanometers), electrons are not liberated from
the surface of copper. For ( λ = 263 nanometers), we notice that now electrons
are liberated from the surface of copper. Therefore. (263 nanometers) is
the threshold wavelength of copper. Then, if λ is less than 263 nanometers, electrons
are liberated from the surface of copper. As we have seen in the previous slide, in order
to extract electrons from the surface of a pure metal, then the wavelength of the incident
radiation must be less than or equal to a certain threshold wavelength. Each pure metal
has its own value of the threshold wavelength. In this table, we have the threshold
wavelength for some pure metals. Now, if the wavelength of the incident radiation
is greater than the threshold wavelength, there is no emission of photoelectrons
from the surface of the metal whatever the radiation power and the exposure time to the
incident radiation. So, what is the definition of the threshold wavelength of a certain metal?
It is the maximum wavelength of the incident radiation which is capable of extracting
electrons from the surface of this metal. Now, let's study another characteristic of
a pure metal, called threshold frequency and denoted by (𝛎o). We know that the wavelength of an
electromagnetic radiation in a certain medium is given by the speed of this radiation in the same
medium over its frequency. Then, the threshold wavelength λ𝐨 of a pure metal equals (c over the
threshold frequency 𝛎o of the same metal, where c is the speed of light in vacuum and it equals
[ 3 × (10)^8 m/s ]. We have seen in the previous slide that in order to extract electrons, the
wavelength (𝛌) of the incident radiation should be less than or equal to the threshold wavelength
(𝛌o) of the pure metal. But, 𝛌 equals ( c / 𝛎 ), where (𝛎) is the frequency of the incident
radiation. Compare these two fractions. Since, they have the same numerator, then to extract
electrons from the surface of a pure metal the frequency (𝛎) of the incident radiation should be
greater than or equal to the threshold frequency of the pure metal. Then, the threshold frequency
of a pure metal is the minimum frequency of the incident radiation which is capable of extracting
electrons from the surface of this metal. What is the conclusion of this study? In
photoelectric effect, extraction of electrons from the surface of the pure metal depends only
on the nature of the metal and on the frequency or the wavelength of the incident electromagnetic
radiation. If the frequency of the incident radiation is less than the threshold the frequency
of the metal or if the wavelength of the incident radiation is greater than the threshold wavelength
of this metal, then the incident electromagnetic radiation is not capable of liberating any
electron whatever the radiation power and the exposure time to the incident radiation. In the
end, don't miss the rest of the material about the photoelectric effect explained in the next videos.
You can find the links in the description. [Music]