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: 2
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 property of light called Total Internal Reflection (TIR). We have already learned about refraction, where light bends when it crosses from one medium to another. Today, we will investigate a special case where light, instead of passing through, is completely trapped and reflected back into its original medium. This principle is not just a curious scientific fact; it is the secret behind the sparkling of a diamond, the mirages we see on hot roads in Ghana, and the technology that brings high-speed internet to our homes and schools through fibre optic cables. Understanding TIR helps us appreciate how we can control and use light in powerful ways.
A. Recap: Refraction from a Denser to a Less Dense Medium
Before we discuss total internal reflection, let's remember what happens when light travels from a medium where it moves slower (optically denser) to one where it moves faster (optically less dense). A common example is light travelling from water into air, or from glass into air. Rule: When light travels from an optically denser medium to a less dense medium, it bends away from the normal. This means the angle of refraction (`r`) is always greater than the angle of incidence (`i`). B. The Journey to Total Internal Reflection: Step-by-Step
Imagine you are under the water in the Volta Lake with a waterproof torch (flashlight), shining it upwards towards the surface. We will observe what happens as you increase the angle at which you shine the light.
Scenario 1: Small Angle of Incidence (`i₁`) You shine the torch at a small angle to the normal. The light ray passes from water (denser) into air (less dense). It refracts and bends *away* from the normal. The angle of refraction `r₁` is greater than the angle of incidence `i₁`. Some light is also weakly reflected back into the water.