16.3 CHAPTER 16. OPTICAL PHENOMENA; PROPERTIES OF MATTER
- Explain why the skyis blue.
More practice video solutions or help at http://www.everythingscience.co.za(1.) 01mn (2.) 01mp16.3 The photoelectric effect ESCHF
Around the turn of thetwentieth century, it was observed by a number of physicists
(including Hertz, Thomson and Von Lenard) that when light was shone on a metal,
electrons were emitted by the metal. This is called the photoelectric effect. (photo- for
light, electric- for the electron.)DEFINITION: The photoelectric effect
The photoelectric effectis the process wherebyan electron is emitted
by a metal when light shines on it.At that time, light was thought to be purely a wave. Therefore, physicists thought that
if a more intense (i.e. brighter) light was shoneon a metal, then the electrons would
be knocked out with greater kinetic energies than if a faint light was shone on them.
However, Von Lenard observed that this did not happen at all. The intensity of the light
made no difference to the kinetic energy ofthe emitted electrons! Also, it was observed
that the electrons were emitted immediately when light was shone on themetal - there
was no time delay.Einstein solved this problem by proposing thatlight is made up of packets of energy
called quanta (now called photons) which interacted with theelectrons in the metal
like particles instead ofwaves. Each incident photon would transfer all its energy to
one electron in the metal. For a specific colour of light (i.e. a certain wavelength
or frequency), the energy of the photons is given by E = hf = hc/λ, where h is
Planck’s constant. Theenergy needed to knockan electron out of the metal is called
the work function (symbol φ) of the metal. Therefore, the amount of energyleft over
as the kinetic energy (Ek) of the emitted electron would be the difference between
the incoming photon’s energy and the energy needed to knock out the electron (work
function of the metal):Ek = hf− φ