Lesson Plan

• Describe the seven traditional colours of the visible spectrum and their order by wavelength and by frequency.
• Explain how dispersion in a thin convex lens separates white light into its constituent colours.
• Apply the thin‑lens formula to predict focal positions for different colours.
• Compare typical wavelength and frequency values for each colour.
• Use the mnemonic “ROY GBV” to recall the colour sequence accurately.

• Projector or interactive whiteboard
• Slide set with ray diagram of a thin convex lens
• Printed handout of the colour table (wavelength & frequency)
• Coloured filters or a prism for a quick demo
• Worksheet with ordering and calculation tasks
• Whiteboard and markers

Begin with a striking image of a rainbow projected on the screen to capture interest. Ask students what determines the order of colours they see and link this to prior knowledge of light and lenses. State that by the end of the lesson they will be able to list the colours in both wavelength and frequency order and explain why a thin lens creates a spectrum.

1. Do‑Now (5'): Quick recall – students write the mnemonic “ROY GBV” and list the colours from memory.
2. Mini‑lecture (10'): Explain dispersion in a convex lens using the ray diagram slide; connect to the rainbow phenomenon.
3. Guided Practice (12'): Show the combined colour table; students work in pairs to order the colours by wavelength and then by frequency, filling a worksheet.
4. Demonstration (8'): Use a prism or coloured filters to visibly separate white light; discuss how focal length varies slightly with colour.
5. Application (10'): Students apply the lens formula to calculate approximate focal distances for red and violet rays, reinforcing the link between wavelength and focal position.
6. Check for Understanding (5'): Whole‑class quiz using clickers – identify the colour with the longest wavelength and the highest frequency.
7. Wrap‑up (5'): Review key points and answer any lingering questions.

Summarise that the seven colours can be ordered either by decreasing wavelength or increasing frequency and that a thin lens separates them due to dispersion. Ask each student to write one “exit ticket” statement describing why violet focuses closer to the lens than red. Assign homework to create a labelled diagram of a thin lens producing a spectrum and to complete a short worksheet on wavelength‑frequency conversions.