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: 2nd Term
Week: 1
Grade code: 1.2.2.LI.3
Strand code: 2
Sub-strand code: 2
Content standard code: 1.2.2.CS.2
Indicator code: 1.2.2.LI.3
Theme: ENERGY
Subtheme: WAVES
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This lesson explores a fascinating behaviour of light called Total Internal Reflection (TIR). We have previously learned about refraction—how light bends when it moves from one material to another. Today, we will investigate a special case of refraction that has profound applications, from the super-fast internet cables that connect Ghana to the world (fibre optics) to the shimmering mirages we sometimes see on hot roads in Accra or Tamale. Understanding TIR is key to understanding how much of our modern technology works.
This topic builds directly on our knowledge of refraction. Let's build the concept step-by-step. Recap: Refraction from Denser to Less Dense Medium Remember Snell's Law: n₁ sin(θ₁) = n₂ sin(θ₂) n₁ and n₂ are the refractive indices of the first and second media. θ₁ is the angle of incidence. θ₂ is the angle of refraction.
When light travels from a more optically dense medium (like water or glass) to a less optically dense medium (like air), it bends AWAY from the normal. This means the angle of refraction (θ₂) is GREATER than the angle of incidence (θ₁). *Example:* Light from a fish in the Odaw River looking up at a person on the bank. The light from the fish travels from water (denser) to air (less dense). Concept 1: The Critical Angle (c)
Since the angle of refraction (θ₂) is always greater than the angle of incidence (θ₁) when going from denser to less dense, what happens as we slowly increase θ₁? Small Angle of Incidence (Diagram A): The light ray refracts and bends away from the normal. A faint, reflected ray also exists. Increasing the Angle of Incidence: As we increase θ₁, the angle of refraction θ₂ also increases, but faster. The Critical Point (Diagram B): There is a specific angle of incidence for which the angle of refraction becomes exactly 90°. At this point, the refracted ray skims along the boundary between the two media. This specific angle of incidence is called the critical angle (c).
Definition: The critical angle is the angle of incidence in a denser medium for which the angle of refraction in the less dense medium is 90°. Calculating the Critical Angle We can derive a formula for the critical angle using Snell's Law. Let's say light goes from a medium with refractive index n₁ (denser) to a medium with refractive index n₂ (less dense).