Lesson Notes By Weeks and Term v4 - SHS 1
WAVES
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WAVES
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Subject: Physics
Class: SHS 1
Term: 1st Term
Week: 15
Grade code: 1.2.2.LI.1
Strand code: 2
Sub-strand code: 2
Content standard code: 1.2.2.CS.2
Indicator code: 1.2.2.LI.1
Theme: ENERGY
Subtheme: WAVES
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Welcome, future scientists and engineers! Today, we are exploring a fascinating part of Physics that we use every single day: mirrors. We are not just talking about the flat mirrors we use to check our school uniforms. We are looking at curved or spherical mirrors. Have you ever noticed the side mirror on a *trotro* or a taxi? It makes cars behind look smaller and further away. Or have you seen a security mirror in a big shop like Melcom, which allows the attendant to see a wide area? These are spherical mirrors at work. By understanding how they form images, we unlock the principles behind telescopes, car headlights, and even solar cookers.
A. Introduction to Spherical Mirrors
A spherical mirror is a mirror whose reflecting surface is a part of a hollow sphere. Imagine taking a shiny, hollow ball (like a Christmas ornament) and cutting a piece from it. If the inside is polished, it's a concave mirror. If the outside is polished, it's a convex mirror. Concave Mirror (Converging Mirror): The reflecting surface is curved inwards, like the inside of a spoon or a ladle (*kalabash*). It is called a converging mirror because it causes parallel rays of light to meet (converge) at a point. Convex Mirror (Diverging Mirror): The reflecting surface is curved outwards, like the back of a spoon. It is called a diverging mirror because it causes parallel rays of light to spread out (diverge) as if they are coming from a point behind the mirror. B. Terminology of Spherical Mirrors
To understand ray diagrams, we must know the language. Look at the diagram below as you read the definitions. Pole (P): The geometric centre of the reflecting surface of the mirror. Centre of Curvature (C): The centre of the sphere of which the mirror is a part. For a concave mirror, C is in front. For a convex mirror, C is behind. Radius of Curvature (R): The distance between the Pole (P) and the Centre of Curvature (C). It is the radius of the sphere. Principal Axis: The straight line passing through the Pole (P) and the Centre of Curvature (C). Principal Focus / Focal Point (F): For a concave mirror, it is the point on the principal axis where rays of light parallel to the principal axis *actually meet* after reflection. It is a real focus. For a convex mirror, it is the point on the principal axis from which rays of light parallel to the principal axis *appear to diverge* after reflection. It is a virtual focus. Focal Length (f): The distance between the Pole (P) and the Principal Focus (F). Important Relationship: The focal point (F) is exactly halfway between the pole (P) and the centre of curvature (C). Therefore, f = R / 2. C. Real vs. Virtual Images Real Image: Formed by the *actual intersection* of reflected light rays. It can be projected onto a screen (like a cinema projector). Real images are always inverted (upside down). Virtual Image: Formed where the reflected light rays *appear to meet* when extended backwards. It cannot be projected onto a screen. Virtual images are always erect (upright). D. The Rules for Ray Tracing
To find where an image is formed, we only need to draw two special rays from the top of the object. The point where these two reflected rays (or their extensions) intersect is where the top of the image will be. There are three main rules: