Light Travels in Straight Lines

Activities to try

1. 1

   Fire a beam, block it, and discover light won't turn corners

   Why it works: Light travels in straight lines through the space between source and object; a shadow has the object's outline (as seen from the source) because light travels straight past the edges and does not turn corners into the space behind the object, unless something such as a mirror redirects it.

   1. Aim a torch across a darkened scene so the beam's straight-edged path becomes visible (use a slightly hazy/dusty or smoky room, where particles scatter some light to your eyes, or draw the beam with straight ray lines, so the travelling path shows).
   2. Place a card with a single hole in the beam's path. Slide the hole until light passes through and reaches a small 'eye' on the far side; note it only gets through when the hole sits on the straight line between torch and eye.
   3. Nudge the hole off that line and observe the light is blocked — it will not bend to find the gap.
   4. Drop an opaque shape into the beam and observe the light passing straight past its edges, throwing a shadow on a screen whose outline matches the shape as seen from the torch (it may be bigger or stretched depending on distances and how the shape and screen are angled).
   5. Try to 'make the shadow wrap round the back of the object' or 'bend the light round the corner' — and find you cannot.
2. 2

   Three holes in a line — predict, then break the alignment

   Why it works: Light reaches the eye only when the holes are lined up; the offset-hole result contradicts a 'light bends to find its way' model and supports the model that light travels in straight lines.

   1. Line up three cards, each with a small hole punched in the middle, and shine a torch so light passes through all three holes to your eye (threading string through the holes helps show the straight line).
   2. Before moving anything, predict: if you slide the MIDDLE card sideways, will you still see the light? Say why first.
   3. Slide the middle card off-line and check your prediction.
   4. Explain what the result proves about how light travels.
   5. Discuss: when you see the beam in dusty air, are you seeing the light itself or the dust it lights up?
3. 3

   Draw and use ray diagrams to predict a shadow

   Why it works: A correct source–object–screen ray diagram with ruler-straight, arrowed rays from the source both represents straight-line travel and lets you predict shadow shape.

   1. Draw rays with a ruler as straight lines starting from the light source, putting a direction arrow on every ray.
   2. Draw rays grazing the edges of an opaque object and continuing to a screen to show where the shadow falls and why its outline matches the object.
   3. Predict the shadow's outline for a given shape BEFORE revealing or checking the actual result, then compare.
   4. Explain in words why the shadow reproduces the object's outline, referring to the straight rays past its edges.