Lesson Notes By Weeks and Term v4 - SHS 1
WAVES
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WAVES
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Subject: Physics
Class: SHS 1
Term: 1st Term
Week: 18
Grade code: 1.2.2.LI.1
Strand code: 2
Sub-strand code: 2
Content standard code: 1.2.2.CS.2
Indicator code: 1.2.2.LI.1
Theme: ENERGY
Subtheme: WAVES
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This lesson introduces the fascinating phenomenon of refraction, which is the bending of light. We see the effects of refraction every day, often without realising it. Have you ever put a pencil in a glass of water and seen it look "broken"? Or looked into a bucket of water and thought the bottom was closer than it really was? These are all caused by refraction. Understanding this concept is fundamental to optics and helps explain how lenses, our eyes, and even rainbows work. This lesson will move from simple observations to the scientific laws that govern this behaviour.
Starter Activity: The "Bent" Pencil *(As suggested in the NaCCA exemplar)* Take a clear basin or a transparent glass and fill it about three-quarters full with water. Place a pencil or a straight stick obliquely (at an angle) into the water. Look at the pencil from the side and from above. Observation: The pencil appears to be bent or broken at the point where it enters the water. Discussion Question: Why does our eye see a straight pencil as bent? This is the central question we will answer today. The answer is refraction. A. What is Refraction?
Refraction is the bending of a wave (in this case, light) when it passes from one medium to another medium of different density. Medium: A substance through which a wave can travel. Examples: air, water, glass, kerosene, diamond. Why does light bend? The key reason is that light travels at different speeds in different media. Light travels fastest in a vacuum (or air, which is very close). It slows down when it enters denser substances like water or glass. This change in speed causes the light ray to change direction, or bend.
Analogy: Imagine you are running fast across a smooth, paved area (like a school compound) and you run into a patch of thick sand at an angle. As your first foot hits the sand, it slows down, while your other foot is still moving fast on the pavement. This will cause your body to pivot or turn. This is exactly what happens to a wavefront of light. B. Optical Density
Optical density is a measure of how much a material slows down light. It is NOT the same as physical density (mass/volume). A medium with a high optical density slows light down more (e.g., glass, diamond). A medium with a low optical density slows light down less (e.g., air, vacuum).