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: 2nd Term
Week: 1
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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Welcome, students! Today, we are moving from the general properties of light to a very special and powerful type of light: LASER light. The word LASER is an acronym that stands for Light Amplification by Stimulated Emission of Radiation. You have likely seen lasers in your daily lives, even if you didn't realise it. The red light that scans barcodes at Shoprite or Melcom, the laser pointers used in presentations, and even the technology that reads DVDs and Blu-ray discs all use lasers. In Ghana, advanced hospitals like Korle Bu use lasers for delicate eye surgery.
A. What is a LASER? A laser is a device that produces a very special, highly concentrated beam of light. Unlike a regular light bulb or a torchlight which produces light that spreads out in all directions and contains many colours (wavelengths), a laser produces light that is organized, focused, and pure.
To understand lasers, we must first compare laser light to ordinary light.
| Feature | Ordinary Light (e.g., from a Torchlight) | Laser Light | | :--- | :--- | :--- | | Source | Spontaneous emission of photons. | Stimulated emission of photons. | | Colours | Usually a mixture of many colours (wavelengths), appearing as white or yellowish light. | A single, pure colour (a single wavelength). | | Direction | Spreads out (diverges) quickly in many directions. | Travels in a narrow, straight, and parallel beam. | | Phase | Waves are out of phase (chaotic and jumbled). | All waves are in phase (organized and in step). | | Intensity | The energy is spread over a large area, so it is less intense. | The energy is concentrated in a tiny spot, making it extremely intense (bright). | B. The Four Key Properties of Lasers
The NaCCA curriculum requires us to focus on four main properties. Let's explore each one in detail. Monochromaticity Meaning: The word "mono" means one, and "chroma" means colour. Therefore, monochromatic light is light of a single colour or, more scientifically, a single wavelength. Explanation: Ordinary white light, like sunlight or light from an incandescent bulb, is a mixture of all the colours of the rainbow (Red, Orange, Yellow, Green, Blue, Indigo, Violet). Each colour corresponds to a different wavelength. A laser, however, is designed to produce light of a very specific and narrow range of wavelengths. A red laser produces only red light, and a green laser produces only green light. Analogy: Imagine a radio tuned to a single, clear station like Peace FM. You only hear that station's broadcast. This is like a laser. Now, imagine a radio that is picking up static and bits of every station at once. That is like ordinary white light – a jumble of many different frequencies (or wavelengths). Ordinary Light: Contains many wavelengths (`λ₁`, `λ₂`, `λ₃`, etc.) Laser Light: Contains only one primary wavelength (`λ`). Coherence Meaning: Coherence means that all the light waves in a laser beam are "in phase." This means their crests and troughs line up perfectly with each other as they travel. Explanation: In a laser beam, all the photons (light particles/waves) are identical and travel in perfect unison. They maintain a constant phase relationship with each other in both time and space. Ordinary light is incoherent; the photons are emitted randomly, and their waves are all jumbled up, like a noisy, disorganized crowd. Analogy: Think of a group of soldiers marching in a parade. They all step with their left foot at the same time, and their right foot at the same time. They are perfectly synchronised. This is coherent. Now, think of the crowd at Makola Market. Everyone is moving in different directions at different times. This is incoherent. Incoherent Light (Ordinary): Waves are out of step. Coherent Light (Laser): Waves are in perfect step (in phase). Collimation (or High Directionality) Meaning: Laser light is highly collimated, which means it travels as a narrow, parallel beam that spreads out very little, even over long distances. Explanation: The light rays in a laser beam are almost perfectly parallel to each other. This is why a laser pointer can create a small, bright dot on a wall far away. In contrast, light from a torchlight or a car headlamp spreads out (diverges) very quickly, becoming dimmer as it travels further. Analogy: Imagine spraying water from a garden hose. If you use a normal nozzle, the water spreads out in a wide cone. This is like a torchlight. If you use a jet-stream nozzle, the water shoots out in a single, tight stream that can travel a long way. This is like a laser beam. Divergent Beam (Torchlight): Collimated Beam (Laser): Brightness (or High Intensity) Meaning: Because all the light energy is concentrated into a very narrow, collimated beam, a laser beam is incredibly intense or bright at the point where it hits a surface. Explanation: Intensity is power per unit area (I = P/A). A simple 5-milliwatt laser pointer is not very powerful overall. However, because it focuses all that power onto a spot that might be only 1 mm² in area, the intensity at that spot is enormous. The sun is incredibly powerful, but its energy is spread all over the earth, so its intensity at any one point is much less than that of even a modest laser beam. Analogy: Think of focusing sunlight with a magnifying glass to burn a piece of paper. The magnifying glass gathers light energy from a large area and concentrates it onto a tiny spot. A laser does this automatically—it produces light that is *already* concentrated.