Elowen holds a tennis ball in a flashlight beam, a few feet from the wall. The ball's shadow has a sharp, clean edge. Nothing blurs or fades at the line between lit and dark.
In the electromagnetic waves topic, you saw the evidence that light travels as a wave. This lesson shows a simpler way to draw light, and says when the simpler drawing is enough. First, one wrong idea has to go.
You see this page because light from a lamp bounces off the page and enters your eyes.
Your eyes send out no light at all; light only comes in to them.
Elowen's shadow has a sharp edge because light from the flashlight travels in straight lines.
Light travels in straight lines through a uniform medium, a material that is the same all the way through.
Light changes direction only where it meets a different material.
In the waves topic, you drew wavefronts, with an arrow perpendicular to the fronts showing which way the wave travels.
A straight arrowed line that shows which way the light travels is called a ray.
The ray model is the wave picture simplified for a purpose: shadows and mirrors need only the direction light travels, so rays are all this topic draws.
A streetlight shines on a stop sign at night. What kind of edge does the sign's shadow have on the road?
Light from the streetlight travels in straight lines through the air.
A straight line from the light either clears the sign's edge or is blocked by it.
There is no in-between path, so the lit road ends exactly where the blocked region begins.
The shadow's edge is sharp.
Fintan checks whether a shelf's front edge is straight by crouching and sighting along it with one eye. Why does this work?
Light from every point of the edge travels to Fintan's eye in a straight line.
If the edge is straight, all of those lines lie along one line of sight.
Any bump on the edge sticks out of that line of sight, and he sees it at once.
Sighting along an edge works because light reaches the eye in straight lines.
Greer shines a laser pointer across a dusty room, and the beam shows up in the dust. What path does the beam take through the air?
Greer says: 'You see a tree because your eyes send out light that touches the tree.' Is Greer correct?
Fintan says: 'A drawn ray is a straight line that shows which way the light travels.' Is Fintan correct?
Which statement is the ray model of light?
A basketball hangs in a single floodlight's beam in a gym, and its shadow on the floor has a sharp edge. Which fact about light explains the sharp edge?
Haldis stands in a hallway outside an open doorway. A lamp sits inside the room, around the corner, and she cannot see it. Which statement explains this?