Lesson Plan

• Describe electromagnetic wave properties and define redshift.
• Explain why light from distant galaxies is redshifted due to the expansion of space and the Doppler effect.
• Calculate redshift and recessional velocity using the given formulas.
• Apply Hubble’s Law to estimate distances to galaxies.
• Interpret observational spectra to support the expanding‑universe model.

• Projector or interactive whiteboard
• Slide deck with spectra diagrams
• Printed worksheet with data table and practice questions
• Scientific calculator or calculator app
• Graph paper or digital plotting tool
• Access to an online redshift simulation (optional)

Show an image of a galaxy spectrum with a clearly shifted absorption line to spark curiosity. Ask students what the colour shift suggests compared with a laboratory spectrum, linking to prior knowledge of wavelength and colour. Explain that they will discover why this shift occurs and how it reveals the universe’s expansion, with success measured by correctly calculating redshift and interpreting Hubble’s Law.

1. Do‑now (5'): Quick written response to “If a spectral line moves toward longer wavelengths, what does that imply about the source?” Collect answers.
2. Mini‑lecture (10'): Review EM wave basics, definition of redshift, Doppler effect, and expansion of space using slides.
3. Guided calculation (12'): Work through the sample data for Galaxy A, calculating z and v with calculators; record results on worksheet.
4. Interactive data analysis (15'): Students plot velocity versus distance from the provided table, derive an approximate Hubble constant, and discuss the trend.
5. Concept discussion (8'): Small groups answer practice question 2 on why redshift stems from space expansion, then share explanations.
6. Exit ticket (5'): Write one sentence summarising why redshift indicates an expanding universe and compute the distance for a galaxy with z = 0.05.

Recap that redshift provides direct evidence for cosmic expansion and enables distance measurement via Hubble’s Law. Reinforce the key formulas and how they were applied today. For homework, complete the remaining practice questions and bring a recent news article that mentions redshift observations.