Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Describe how lenses form images using ray diagrams.
• Identify the types of lenses and their characteristics.
• Calculate image or object distance and focal length using the lens formula.
• Apply knowledge of lenses to real-world scenarios.

Lesson summary

This lesson explores the fascinating world of lenses, a key application of how we control light, which behaves as a wave. Lenses are not just pieces of glass in a science lab; they are all around us. From the glasses (spectacles) many of us wear to see clearly, to the cameras in our phones we use to capture memories, and even the simple magnifying glass a watch repairer at Makola or Kejetia market uses for tiny details. Understanding how lenses work helps us understand these technologies and the very nature of light itself. By learning how lenses bend light to form images, we are learning to control and use one of the most fundamental forms of energy in our universe.

Lesson notes

2.1 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 allows it to form an image.

There are two main types of lenses:

A. Convex (or Converging) Lens: Shape: Thicker in the middle and thinner at the edges. Action: It converges, or brings together, parallel rays of light to a single point. Analogy: Think of it as a lens that "gathers" light.

B. Concave (or Diverging) Lens: Shape: Thinner in the middle and thicker at the edges. Action: It diverges, or spreads out, parallel rays of light. These rays appear to come from a single point behind the lens. Analogy: Think of it as a lens that "scatters" light. 2.2 Important Terminology for Lenses To describe how lenses work, we must understand these terms. Imagine a horizontal line passing through the centre of the lens. Principal Axis: The imaginary horizontal line that passes through the centre of the lens at a right angle to its surface. Optical Centre (O): The exact geometric centre of the lens. A special property of this point is that any ray of light passing through it does not deviate from its path. Principal Focus (F) or Focal Point: For a convex lens, it is the point on the principal axis where rays of light 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 parallel rays of light *appear* to diverge after passing through the lens. A lens has two principal foci, one on each side, at an equal distance from the optical centre. Focal Length (f): The distance between the optical centre (O) and the principal focus (F). 2.3 The Rules for Drawing Ray Diagrams (Image Formation) To find out where an image will be formed by a lens, we can draw a ray diagram. We only need to draw at least two of these three "principal rays" from the top of the object. The point where these two rays intersect (or appear to intersect) after passing through the lens is where the top of the image will be.

Evaluation guide

• Describe how images are formed by lenses.
• Collaborative learning: Let learners to watch a video/simulation to see how lenses form images
• and identify and note the rays that are involved in the ima ge formation process after watching the
• video.