Why the sky is blue, taught in forty minutes

GRADE 7 · PHYSICS · 40 MIN · WAVES AND THEIR APPLICATIONS IN…, DEVELOPING AND USING MODELS

40MINUTES, LEFT TO RIGHT

5′ Hook

5′

15′ Scattering, with the…

10′

5′

0′10′20′30′40′

TEACHER

STUDENTS

FIT TO

explain why the daytime sky is blue and the sunset red using one idea, scattering, and predict what colour the sky would be on a world with no air

PREREQWhite light is a mix of colours · Light travels in straight lines until something gets in the wayMATERIALSclear glass or tank · water · a few drops of milk · a torch · a dark room or a blind

5'Hook

Lights off. Shine the torch through clear water: nothing. Add two drops of milk and shine again from the side: the beam turns faintly blue. Ask what changed.

5'The misconception, said out loud

Ask the class why the sky is blue. Collect answers on the board: reflecting the sea, the colour of air, oxygen is blue.

15'Scattering, with the torch

Look at the beam from the side (blue) and then at the torch through the end of the tank (yellow-orange). Small particles scatter blue light more than red. The milk is doing what air does; the tank is the atmosphere.

10'Sunset: same torch, longer path

Tilt the tank so the beam crosses more water before it reaches the eye. The end-on colour deepens to red. At sunset the light crosses far more air, and the blue is scattered out before it arrives.

5'Exit ticket

One sentence: why is the sky blue? One sentence: why is the sunset red? One prediction: the sky on Mars, which has thin air and lots of fine dust.

ASSESSMENTThe exit ticket. A pass is two correct sentences that both use the word scatter; the Mars prediction is for curiosity, not marks.

DIFFERENTIATIONStudents who finish early get the question: why are clouds white? Students who struggle get the torch and tank themselves for the last five minutes.

HOMEWORKWatch one sunset. Write down the colour of the sky directly overhead at the same moment.

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Made 9 October 2026. Written by poisonal ai.

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