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: 20
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 delve into the fascinating world of lenses. Lenses are not just pieces of glass; they are the heart of many devices we use every day. From the spectacles that help our grandparents read the Bible, to the camera on your phone used to capture moments at the Black Star Square, and even the projectors used in our school assembly hall, lenses shape how we see the world. By understanding how they work, you are not just learning physics; you are learning the principles behind technologies that are vital to communication, health, and entertainment in Ghana and beyond.
A. What is a Lens? A lens is a carefully shaped piece of transparent material (like glass or plastic) that refracts (bends) light rays that pass through it. The way it bends light depends on its shape. There are two main types of lenses: Converging (Convex) Lens: Thicker in the middle and thinner at the edges. It brings parallel rays of light together (converges them) at a single point. Think of it as a "gathering" lens. Diverging (Concave) Lens: Thinner in the middle and thicker at the edges. It spreads parallel rays of light out (diverges them) so they appear to come from a single point. Think of it as a "spreading" lens. B. Essential Terminology for Lenses To describe how lenses work, we must agree on some key terms. Imagine a horizontal line passing through the centre of the lens. Principal Axis: The imaginary horizontal line passing through the centre of the lens. Optical Centre (O): The exact geometric centre of the lens. A special property of this point is that any light ray passing through it does not deviate from its path. Principal Focus (F) or Focal Point: For a converging lens, it is the point on the principal axis where rays initially parallel to the principal axis meet after refraction. For a diverging lens, it is the point on the principal axis from which parallel rays *appear* to diverge after refraction. Focal Length (f): The distance from the optical centre (O) to the principal focus (F). Centre of Curvature (2F): A point on the principal axis that is twice the distance of the focal length from the optical centre (f). It is an important reference point for placing objects. C. The Rules of the Game: Ray Tracing To find out where an image will be formed and what it will look like, we don't need to trace every single ray of light. We only need to draw two of the following three special rays, called principal rays. The point where these rays (or their extensions) intersect is where the top of the image is formed.
Principal Rays for a Converging (Convex) Lens: The Parallel Ray: A ray travelling parallel to the principal axis is refracted by the lens and passes through the principal focus (F) on the other side. The Central Ray: A ray passing through the optical centre (O) continues in a straight line without any deviation. The Focal Ray: A ray passing through the principal focus (F) on its way to the lens is refracted to become parallel to the principal axis.
Principal Rays for a Diverging (Concave) Lens: The Parallel Ray: A ray travelling parallel to the principal axis is refracted by the lens and diverges *as if* it came from the principal focus (F) on the same side of the lens. The Central Ray: A ray passing through the optical centre (O) continues in a straight line without any deviation. (This rule is the same for both lenses!) The Focal Ray: A ray that is directed towards the principal focus (F) on the opposite side of the lens is refracted to become parallel to the principal axis.
 D. Describing the Image: L.O.S.T. Once you've drawn your ray diagram and located the image, you must describe it using four characteristics. A useful acronym is L.O.S.T. Location: Where is the image formed? (e.g., between F and 2F, beyond 2F). Orientation: Is the image upright (erect) or inverted (upside down)? Size: Is the image magnified (larger), diminished (smaller), or the same size as the object? Type: Is the image Real or Virtual? Real Image: Formed by the *actual intersection* of refracted rays. It can be projected onto a screen (like an image from a cinema projector). Real images are always inverted. Virtual Image: Formed by the *apparent intersection* of the backward extensions of refracted rays. It cannot be projected onto a screen. You must look *through* the lens to see it (like a magnifying glass). Virtual images are always upright. E. Worked Example: Step-by-step Image Formation by a Converging Lens