Light Travels in Straight Lines
Recognise that light appears to travel in straight lines and use this to explain how we see objects and why shadows have the same shape as the objects that cast them
What a learner can do afterwards
- State and demonstrate that light travels in straight lines (e.g. can't see around corners, laser pointer)
- Use straight-line light to explain why shadows have the same outline shape as the object
- Draw ray diagrams showing light travelling from source, being blocked by object, creating shadow on screen
The lesson
You already know that light bounces off things and into your eyes, and that shadows form when something blocks the light. Now look closer at how light actually moves. Light does not curve, wiggle, or bend on its own. It travels in straight lines called rays. That is why you cannot see around a corner without a mirror, and why a laser pointer makes one straight, thin line across a dark room.
Mia shines a torch at a triangular block. The rays that hit the block stop right there, while the rays around it keep going straight to the wall. The blocked patch on the wall matches the triangle, edge for edge, because none of the rays bent to fill in the gap.
To draw a ray diagram: start at the light source, use a ruler to draw straight lines out to the edges of the object, then keep those lines going until they reach the screen. The gap between the lines is the shadow, and it always matches the object's outline.
Light always travels in straight lines, so whatever blocks it casts a shadow shaped just like itself.
Watch it
Where it sits
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.