Lesson Plan

• Describe the definition of monochromatic light and its relevance to lens experiments.
• Apply the thin‑lens formula and sign conventions to calculate image distances for convex and concave lenses.
• Construct accurate ray diagrams for thin lenses using parallel, focal, and central rays.
• Explain how monochromatic illumination eliminates chromatic aberration and improves measurement accuracy.
• Analyse common misconceptions about lens image formation.

• Projector and screen
• Laser pointer with diffuser (monochromatic source)
• Set of convex and concave thin lenses
• White screen or sheet of paper
• Ruler / measuring tape
• Worksheet with ray‑diagram and calculation tasks
• Printed handout of sign conventions and lens formula

Imagine seeing a perfectly sharp image of a distant object—no colour fringes, just a crisp spot. Students already know the basic shapes of lenses and the lens formula, so we will connect that knowledge to why a single‑frequency (monochromatic) light source is essential for accurate measurements. Success will be demonstrated when pupils can predict image location and draw correct ray diagrams using monochromatic light.

1. Do‑now (5'): Quick quiz on lens types and sign conventions.
2. Mini‑lecture (10'): Review monochromatic light, lens formula, and sign convention with projector slides.
3. Demonstration (10'): Set up laser‑diffuser, convex lens, and screen; measure focal length and discuss image sharpness.
4. Guided practice (15'): Pairs construct ray diagrams for given object distances (convex and concave) on worksheets; teacher circulates.
5. Concept check (5'): Exit ticket – one calculation using the lens formula and one short explanation of why monochromatic light is used.
6. Homework briefing (5'): Assign the three practice questions from the source.

We recap that monochromatic light removes chromatic aberration, allowing precise use of the lens formula and clear ray diagrams. Students submit an exit ticket summarising one calculation and one conceptual explanation, and they are reminded to complete the practice questions for homework.