Lesson Notes By Weeks and Term v4 - SHS 3
WAVE
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WAVE
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Subject: Physics
Class: SHS 3
Term: 1st Term
Week: 20
Grade code: 3.2.2.LI.1
Strand code: 2
Sub-strand code: 2
Content standard code: 3.2.2.CS.2
Indicator code: 3.2.2.LI.1
Theme: ENERGY
Subtheme: WAVE
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This lesson introduces the fascinating world of lasers. We will move beyond simply knowing what a laser is and delve into the fundamental physics that explains how a laser beam is produced. We encounter lasers daily in Ghana, from barcode scanners at Melcom or Shoprite, to laser levels used in the construction of new buildings in our communities, and even in fibre optic cables that bring us the internet from MTN and Vodafone. Understanding how they work is key to appreciating modern technology. This lesson explains the process step-by-step, from the behaviour of atoms to the generation of an intense, focused beam of light.
A. What is a LASER? The word LASER is an acronym. It stands for: Light Amplification by Stimulated Emission of Radiation
This name is a perfect description of how it works. We are *amplifying* light (making it stronger) using a special process called *stimulated emission*.
B. The Three Key Processes Involving Atoms and Light
To understand how a laser works, we must first understand how light (in the form of photons) interacts with electrons in an atom. Energy Levels: Imagine electrons in an atom live in houses at different levels on a hill. The house at the bottom is the most stable and is called the ground state (E₀). Houses further up the hill are less stable and are called excited states (E₁). An electron can only live in these specific houses (energy levels), not in between. Process 1: Absorption An electron is in its ground state (E₀). A photon of light with the exact right amount of energy (Energy = E₁ - E₀) hits the atom. The electron absorbs the photon's energy and jumps up to the excited state (E₁). Analogy: You give a student a precise amount of energy (like a good meal of kenkey) to climb from the ground floor to the first floor of a building. Process 2: Spontaneous Emission (This is how normal light bulbs work) An electron is already in an excited state (E₁). It is unstable here and wants to return to the ground state. After a very short time, the electron *spontaneously* (on its own) falls back to the ground state (E₀). As it falls, it releases the extra energy as a photon of light. The direction and phase of this emitted photon are random. Result: The light produced by many atoms doing this is incoherent (jumbled up), like the light from a torchlight or a candle. Process 3: Stimulated Emission (The "Secret" of Lasers) An electron is already in an excited state (E₁). *Before* it can fall down spontaneously, an incoming "trigger" photon with the exact energy (E₁ - E₀) passes by the atom. This incoming photon *stimulates* or forces the excited electron to fall back to the ground state immediately. As it falls, it releases a *new* photon. Crucial Point: The new photon is a perfect clone of the trigger photon. It has the same energy (colour), same direction, same phase, and same polarization. Result: We now have two identical photons where we started with one. This is amplification!