Lesson Notes By Weeks and Term v4 - JHS 3
ENERGY
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
ENERGY
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Science
Class: JHS 3
Term: 2nd Term
Week: 9
Grade code: B9.4.1.3.1
Strand code: 4
Sub-strand code: 1
Content standard code: B9.4.1.3
Indicator code: B9.4.1.3.1
Theme: FORCES AND ENERGY
Subtheme: ENERGY
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.
Light is a form of energy we use every day in Ghana—seeing in the classroom, reading, taking photos with phones, driving at night, and even in hospitals (eye tests). Sometimes light does not travel in a straight line: it changes direction (bends) when it moves from one material (medium) to another, such as from air to water or air to glass. This bending explains why a spoon looks “broken” in a glass of water and why a swimming pool looks shallower than it really is. This lesson is aligned to Indicator B9.4.1.3.1: Demonstrate that light changes path when it travels from one medium to another (refraction).
2.1 Light as Energy Light is a form of energy that travels as waves (and also as particles called photons). In a uniform medium (same material), light travels in a straight line. When light meets a boundary between two different media (e.g., air and water), its speed changes, and this causes the path to bend. 2.2 Medium A medium is any substance through which light travels. Examples: air, water, glass, plastic, kerosene, oil. 2.3 Refraction (Main Idea for the Indicator) Refraction is the change in direction (bending) of light when it travels from one medium to another because its speed changes. Why does refraction happen? Light travels at different speeds in different media. In air, light travels faster than in water or glass. When light enters a denser medium (like water or glass), it slows down and bends towards the normal. When light leaves a denser medium into a less dense medium (like from water to air), it speeds up and bends away from the normal. 2.4 The Normal, Incident Ray, and Refracted Ray When drawing refraction: Incident ray: the incoming light ray. Normal: an imaginary line drawn perpendicular (90°) to the surface at the point where the ray hits. Refracted ray: the ray after it enters the new medium. 2.5 Bending Towards or Away from the Normal (Rule) From less optically dense to more optically dense (e.g., air → water/glass): Bends towards the normal From more optically dense to less optically dense (e.g., water/glass → air): Bends away from the normal
> Note: “Optically dense” refers to how much the medium slows light, not how heavy it is. 2.6 Everyday Observations (Explained) (A) Pencil/Spoon looks bent in water When part of a pencil is in air and part is in water, light from the submerged part refracts at the water-air boundary before reaching your eyes. Your brain assumes light travelled straight, so the pencil appears “shifted” or “bent.” (B) Deep water appears shallower Light from the bottom of a river/pool bends away from the normal as it leaves water into air. The bottom appears raised, so the water looks shallower than it truly is.
Guided Practice (With Solutions) Practical Demonstration 1: “Bent Pencil” in Water (Core Demonstration) Task: Put a straight pencil/spoon in a transparent glass of water and observe from the side.
Question 1 (Observation): What do you see about the pencil at the water surface? Solution: The pencil appears bent or broken at the surface of the water. Commentary: This is due to refraction at the boundary between water and air.