Lesson Notes By Weeks and Term v4 - SHS 1

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Explain the concept of total internal reflection and its conditions.
• Describe how refraction relates to total internal reflection.
• Identify real-life examples of total internal reflection.
• Solve problems related to the critical angle.

Lesson summary

This lesson explores a fascinating behaviour of light called Total Internal Reflection (TIR). We have previously learned that light bends (refracts) when it moves from one medium to another. Today, we will discover the special conditions under which light does not pass through but instead reflects completely back into its original medium. This principle is not just a scientific curiosity; it is the reason why diamonds sparkle so brightly, why we can see mirages on hot Ghanaian roads, and most importantly, it's the technology behind the high-speed fibre optic cables that bring internet to our homes and schools from companies like MTN and Vodafone.

Lesson notes

Recap: Refraction of Light Before we dive into total internal reflection, let's remember what refraction is. Refraction is the bending of light as it passes from one medium to another (e.g., from air to water). This happens because light travels at different speeds in different media. A medium's ability to bend light is measured by its refractive index (n). A higher `n` means the medium is "optically denser". For example, water (n ≈ 1.33) is optically denser than air (n ≈ 1.00). Rule of thumb: When light goes from a less dense to a denser medium (e.g., air to glass), it bends towards the normal. When light goes from a denser to a less dense medium (e.g., water to air), it bends away from the normal.

This second point is the key to understanding today's lesson. Condition 1: From Denser to Less Dense Medium Total internal reflection can only happen when light is travelling from a medium with a higher refractive index (optically denser) to a medium with a lower refractive index (optically less dense). Example: From glass to air, water to air, diamond to water. It cannot happen when light travels from air to water. The Journey to Total Internal Reflection: A Step-by-Step Explanation

Imagine you are underwater in the Volta River, shining a powerful torch up towards the surface. The light from the torch is travelling from a denser medium (water) to a less dense medium (air). Let's see what happens as you change the angle of the torch. Step 1: Small Angle of Incidence (i) You shine the light almost straight up. The angle of incidence `i` is small. The light ray passes out of the water into the air, bending away from the normal. The angle of refraction `r` is greater than `i`. Some light is refracted, and a faint amount is reflected back into the water. Step 2: Increasing the Angle of Incidence (i) You tilt your torch further. The angle of incidence `i` increases. The angle of refraction `r` also increases, moving further and further away from the normal. The refracted ray gets closer to skimming the surface of the water. The reflected ray becomes stronger. Step 3: The Critical Angle (c) You continue tilting your torch until you reach a specific angle of incidence where the refracted ray does not enter the air but skims exactly along the boundary between the water and air. This means the angle of refraction `r` is exactly 90°. This special angle of incidence is called the Critical Angle (c). Definition: The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the less dense medium is 90°. Step 4: Total Internal Reflection (TIR) What happens if you tilt the torch even further, so that the angle of incidence `i` is greater than the critical angle (i > c)? The light can no longer escape into the air. None of it is refracted. Instead, all the light is reflected back into the water, as if the surface were a perfect mirror. This phenomenon is called Total Internal Reflection (TIR). The light obeys the law of reflection (angle of incidence = angle of reflection). Summary of the Two Conditions for TIR Light must be travelling from an optically denser medium to an optically less dense medium. The angle of incidence `(i)` in the denser medium must be greater than the critical angle `(c)`. Calculating the Critical Angle (c)

We can find the critical angle using Snell's Law of refraction: `n₁ sin(θ₁) = n₂ sin(θ₂)`

Evaluation guide

• Explain total internal reflection.
• Collaborative learning: Put learners into groups of mixed abilities and let them wa tch a
• simulation of refraction. Specifically let learners consider situations where the light is moving from
• a medium of higher density into another of lower density. Learners must take note of observations.