Lesson Notes By Weeks and Term v5 - Grade 12
Matter and Materials: optical phenomena and photoelectric effect – Week 6 focus
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Matter and Materials: optical phenomena and photoelectric effect – Week 6 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Physical Sciences
Class: Grade 12
Term: 2nd Term
Week: 6
Theme: General lesson support
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The interaction of light and matter is fundamental to understanding our world. From the vibrant colours of a Cape Town sunset to the solar panels powering a rural school, optical phenomena govern how we perceive and utilize light. This week, we delve into optical phenomena, focusing particularly on the photoelectric effect. The photoelectric effect demonstrates the particle nature of light and its interaction with materials at the atomic level. Understanding this phenomenon is crucial as it forms the basis of many modern technologies, including light sensors, digital cameras, and solar cells, all of which have increasingly important applications in South Africa.
2.1 The Photoelectric Effect The photoelectric effect is the emission of electrons from a metal surface when light of a sufficiently high frequency shines on it. These emitted electrons are called photoelectrons. Classical physics, which treated light as a wave, could not explain several key observations of the photoelectric effect.
Threshold Frequency (f0): For each metal, there is a minimum frequency of light, called the threshold frequency, below which no photoelectrons are emitted, no matter how intense the light is.
Work Function (W0): The work function is the minimum energy required to remove an electron from the surface of a metal. It is a property of the metal itself and is related to the threshold frequency by the equation: W0 = hf0, where h is Planck's constant (6.63 x 10-34 Js).
Maximum Kinetic Energy (KEmax): The maximum kinetic energy of the emitted photoelectrons is independent of the intensity of the light but depends on the frequency of the light.
Stopping Potential (Vs): The stopping potential is the potential difference required to stop the most energetic photoelectrons from reaching the anode. The maximum kinetic energy of the photoelectrons is related to the stopping potential by: KEmax = eVs, where e is the elementary charge (1.6 x 10-19 C). 2.2 Einstein's Photoelectric Equation Einstein explained the photoelectric effect by proposing that light consists of discrete packets of energy called photons.
The energy of a photon is given by: E = hf where: E is the energy of the photon (in Joules, J) h is Planck's constant (6.63 x 10-34 Js) f is the frequency of the light (in Hertz, Hz) When a photon strikes the metal surface, it transfers all its energy to a single electron. If the photon's energy (hf) is greater than or equal to the work function (W0) of the metal, an electron is emitted. The excess energy becomes the kinetic energy of the emitted electron. This is summarized by the photoelectric equation: E = hf = W0 + KEmax or KEmax = hf - W0 = hf - hf0 2.3 Intensity and Number of Photoelectrons The intensity of light is the amount of energy per unit area per unit time. Increasing the intensity of light does not increase the energy of individual photons. Instead, it increases the number of photons striking the metal surface.
Therefore, increasing the intensity of light increases the number of photoelectrons emitted, but it does not affect the maximum kinetic energy of the emitted photoelectrons. 2.4 The Dual Nature of Light The photoelectric effect provides strong evidence for the particle nature of light.
However, other phenomena, such as diffraction and interference, demonstrate the wave nature of light.
Therefore, light exhibits a dual nature: it can behave as both a wave and a particle.
Example 1:
A metal has a work function of 4.0 x 10-19 J.
(a) Calculate the threshold frequency for this metal.
(b) Light with a frequency of 1.5 x 1015 Hz shines on the metal. Calculate the maximum kinetic energy of the emitted photoelectrons.