Lesson Notes By Weeks and Term v4 - SHS 1
WAVES
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
WAVES
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Physics
Class: SHS 1
Term: 1st Term
Week: 11
Grade code: 1.2.2.LI.2
Strand code: 2
Sub-strand code: 2
Content standard code: 1.2.2.CS.1
Indicator code: 1.2.2.LI.2
Theme: ENERGY
Subtheme: WAVES
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.
This lesson introduces the fundamental principles of how light behaves when it strikes a surface—a phenomenon called reflection. We see the world around us primarily because of reflected light. From seeing our own image in a looking glass before school, to a trotro driver using a rear-view mirror to navigate the busy streets of Accra, to the beautiful reflection of the moon on the surface of the Volta Lake at night, reflection is a constant part of our daily experience. By understanding the laws that govern it, we can predict and explain how images are formed and how many optical instruments work.
2.1 Introduction to Reflection Reflection is the bouncing back of a wave, such as light, when it hits a boundary or surface between two different media. For light to be reflected, it must strike a surface. Surfaces that are very smooth and shiny, like a plane mirror, a polished metal sheet, or the surface of calm water, are excellent reflectors. 2.2 Essential Terminology To understand the laws of reflection, we must first understand the language used to describe it. Consider a ray of light from a source hitting a plane mirror.
(Teacher should draw this diagram on the board and label it as the explanation proceeds) Plane Mirror: A mirror with a flat reflective surface. Incident Ray (AO): The ray of light that travels from the light source and strikes the surface of the mirror. Point of Incidence (O): The exact point on the mirror where the incident ray strikes. Reflected Ray (OB): The ray of light that bounces off the mirror's surface at the point of incidence. Normal (ON): An imaginary line drawn perpendicular (at 90°) to the surface of the mirror at the point of incidence. This line is crucial for measuring our angles. Angle of Incidence (i or θi): The angle between the incident ray and the normal. (Crucial point: It is NOT the angle between the ray and the mirror surface). Angle of Reflection (r or θr): The angle between the reflected ray and the normal. 2.3 Deducing the Laws of Reflection (Experiential Activity) As the NaCCA exemplar suggests, the best way for learners to understand the laws is to discover them. This activity guides them to do just that.
Activity Title: Investigating the Laws of Reflection Objective: To find the relationship between the angle of incidence (i) and the angle of reflection (r). Materials: A drawing board or a thick piece of cardboard A sheet of plain white A4 paper A plane mirror Four optical pins A protractor A pencil and a ruler
Procedure (Teacher to guide learners in groups): Setup: Pin the sheet of paper onto the drawing board. Mirror Line: Draw a straight line, `MM'`, across the middle of the paper. This line represents the position of the mirror. Normal: Mark a point `O` near the centre of `MM'`. Using the protractor, draw a line `ON` perpendicular (90°) to `MM'` at point `O`. This is the normal. Incident Ray: Draw a line `AO` to the point `O` such that it makes an angle with the normal `ON`. Let’s start with an angle of 30°. This angle is the angle of incidence, `i`. So, `∠AON = i = 30°`. Placing Pins: Firmly stick two pins, P1 and P2, vertically along the incident ray line `AO`. Placing the Mirror: Place the plane mirror upright on the line `MM'`, with its reflective surface facing the pins. Locating the Image: Close one eye and look into the mirror from the other side of the normal. Move your head until you see the reflections of pins P1 and P2 (let's call them P1' and P2') aligned perfectly in a straight line. Placing More Pins: Now, stick two more pins, P3 and P4, on the paper between your eye and the mirror, such that P3, P4, and the images of P1 and P2 all appear to be on the same straight line. Drawing the Reflected Ray: Remove the mirror and the pins. Draw a straight line, `OB`, that passes through the pinholes left by P3 and P4, all the way to point `O`. This line `OB` is the reflected ray. Measurement: Using the protractor, carefully measure the angle between the reflected ray `OB` and the normal `ON`. This is the angle of reflection, `r` (`∠BON`). Recording and Repetition: Record the values of `i` and `r` in a table. Repeat the entire experiment for different angles of incidence, for example, 40°, 50°, and 60°.