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.3
Strand code: 2
Sub-strand code: 2
Content standard code: 1.2.2.CS.2
Indicator code: 1.2.2.LI.3
Theme: ENERGY
Subtheme: WAVES
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Mirrors are part of our everyday lives in Ghana. From the small mirror we use to get ready for school in the morning, to the side mirrors on a *trotro* that help the driver see the traffic, to the security mirrors in supermarkets at the Accra Mall or Kejetia Market. These are not all flat (plane) mirrors. Many are curved, or *spherical*, and they change how we see things. Today, we will move beyond just drawing diagrams and learn how to use mathematics—specifically, the Mirror Formula—to precisely calculate where an image will be formed and what it will look like. This skill is crucial for understanding optics in technologies like telescopes, car headlights, and even solar cookers.
A. Recap of Key Terms for Spherical Mirrors Before we use the formula, let's remember the language of mirrors: Concave Mirror (Converging): Curls inwards, like the inside of a spoon. It can form both real and virtual images. Convex Mirror (Diverging): Bulges outwards, like the back of a spoon. It *always* forms a virtual, erect, and diminished image. Pole (P): The geometric centre of the mirror's surface. Centre of Curvature (C): The centre of the sphere from which the mirror was cut. Principal Axis: The line passing through the Pole (P) and the Centre of Curvature (C). Principal Focus (F): The point on the principal axis where rays parallel to the axis converge (concave) or appear to diverge from (convex) after reflection. Focal Length (f): The distance from the Pole (P) to the Principal Focus (F). Note: `f = R/2`, where R is the radius of curvature (distance from P to C). Object Distance (u): The distance from the object to the Pole (P) of the mirror. Image Distance (v): The distance from the image to the Pole (P) of the mirror. B. The Sign Convention: "Real is Positive" To use the formulas correctly, we must follow a set of rules for positive (+) and negative (-) signs. This is the most important part! We will use the "Real is Positive" convention, which is very intuitive.
| Quantity | When it is POSITIVE (+) | When it is NEGATIVE (-) | | :--- | :--- | :--- | | Object Distance (u) | Always positive for a single real object. | (Negative for virtual objects, which is rare at SHS1 level). | | Image Distance (v) | For a Real Image (can be formed on a screen, in front of the mirror). | For a Virtual Image (cannot be formed on a screen, behind the mirror). | | Focal Length (f) | For a Concave Mirror (a real focus). | For a Convex Mirror (a virtual focus). | | Magnification (m) | For an Erect/Upright Image. | For an Inverted/Upside-down Image. |
Teacher's Tip: A simple way to remember: Real things are positive. A real image has a positive `v`. A real focus (concave mirror) has a positive `f`. A virtual image has a negative `v`. A virtual focus (convex mirror) has a negative `f`. C. The Mirror Formula This formula is a mathematical relationship connecting the object distance (`u`), the image distance (`v`), and the focal length (`f`).
> 1/f = 1/u + 1/v