Lesson Notes By Weeks and Term v4 - SHS 3
WAVE
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WAVE
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Subject: Physics
Class: SHS 3
Term: 1st Term
Week: 19
Grade code: 3.2.2.LI.2
Strand code: 2
Sub-strand code: 2
Content standard code: 3.2.2.CS.1
Indicator code: 3.2.2.LI.2
Theme: ENERGY
Subtheme: WAVE
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Welcome, future scientists and engineers! Today, we are exploring a fascinating topic that is all around us: lenses. Light, which we know behaves as a wave, can be bent and controlled. Lenses are the tools we use to do this. Think about the spectacles your grandfather wears to read his Bible or the Daily Graphic newspaper, the camera on your phone that you use to take pictures at the Black Star Square, or the projector used in church on Sunday. All of these work because of the principles of image formation by lenses. By understanding how lenses work, we are unlocking the secrets behind many technologies that shape our modern Ghanaian life.
A. What is a Lens?
A lens is a piece of a transparent material, like glass or plastic, with at least one curved surface, that refracts (bends) light rays passing through it to either converge (come together) or diverge (spread apart). B. Types of Lenses
There are two main types of lenses: Converging Lens (Convex Lens): It is thicker in the middle and thinner at the edges. It brings parallel rays of light together at a single point called the principal focus. Think of it as a "collector" of light. *Diagram:* ``` / \ | | \ / (Biconvex) ``` Diverging Lens (Concave Lens): It is thinner in the middle and thicker at the edges. It spreads parallel rays of light out as if they are coming from a single point behind the lens. Think of it as a "scatterer" of light. *Diagram:* ``` \ / | | / \ (Biconcave) ``` C. Important Terminology for Lenses
To describe how lenses form images, we must understand these key terms. Imagine a straight line passing through the centre of the lens. Principal Axis: A horizontal line passing through the optical centre and perpendicular to the lens. Optical Centre (O): The geometric centre of the lens. A ray of light passing through the optical centre does not get deviated. Principal Focus (F) or Focal Point: For a convex lens, it is the point on the principal axis where rays initially parallel to the principal axis converge after passing through the lens. For a concave lens, it is the point on the principal axis from which rays initially parallel to the principal axis appear to diverge after passing through the lens. Focal Length (f): The distance between the optical centre (O) and the principal focus (F). 2F: A point on the principal axis that is twice the distance of the focal length from the optical centre. It is sometimes called the centre of curvature. D. The Rules for Drawing Ray Diagrams (The Three Principal Rays)