Checkpoint: design an optical signal for a door

Question and prediction

Lesson 66 let you encode ‘door open’ as an electrical signal. Now design a paper model: an LED module points at a small mirror on a moving door, and a fixed screen shows a light spot. Turning the door changes the reflected direction. Predict which of two positions puts the spot inside a drawn target.

New words without gaps

An optical signal carries a message by light. A position sensor here is only the chosen rule ‘spot in target = door closed’; a drawing does not become a reliable security device. A controlled condition is something kept unchanged in a comparison: source, mirror, screen and distances. As in the previous block, people choose a message code; a physical quantity does not say ‘closed’ on its own.

Work through supplied data

Supplied table: with the door closed the mirror normal is vertical, and a ray incident at 30° reflects at 30° to the normal, making a spot on target. When the door opens, the mirror turns 10°. If the incoming ray stays fixed, the reflected ray rotates by 20° in the ideal plane-mirror model, moving the spot outside the target. The diagram shows angles, not exact millimetres at the screen. Mirror turn and reflected-ray turn are not equal.

Test safely

First run a paper test: draw both mirror positions, normals and rays. If desired, use a ready low-power LED module and small safe mirror with adult supervision; use no laser, sunlight or mains equipment. Repeat several closures, recording hits and false signals instead of one success.

A common mistake

One hit does not prove reliability. Repeat trials, secure the mount and check changing ambient light. Light also does not need air like sound; it crosses empty space. Without a screen distance, the spot’s shift cannot be stated in centimetres.

Support diagram

Transfer to another case

Replace the mirror with rough white card. Will a narrow reflected ray and one predictable spot remain? Name the model feature that fails and a safe way to test it.

Check your understanding

Close the text and explain the left-to-right drawing to a new learner. Name an observation, a teaching assumption and something the evidence does not prove. Then answer the transfer question without a hint and check units or direction against the diagram. If you disagree, return to the first unfamiliar word rather than memorising a sentence. A spoken explanation, a drawing and a measurement are different kinds of evidence: label each honestly.

Lab notebook page

Add a line to My World Lab Notebook: question → prediction → controlled conditions → changed condition → observation or calculation → conclusion → limit. Put a scale on a drawing or mark it ‘schematic’. If you ran no experiment, write ‘supplied calculation’, not ‘we measured’. Revisit the entry after the next lesson and note what became clearer.

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