Two steady sources send ripples across the same stretch of water. What appears where the two sets of ripples overlap?
Jocasta aims a red laser at a card with two narrow slits and looks at the wall beyond. She expects two thin red stripes, one behind each slit. Instead, the wall shows a whole row of bright red bands with dark gaps between them.
In the waves topic, you saw interference: ripples from two steady sources overlap and make a stable pattern of reinforced regions and canceled regions. You also saw diffraction, a wave spreading out after a narrow gap. And the electromagnetic waves topic gave the evidence that light travels as a wave. This lesson puts those pieces together: some light behaviors, only a wave can produce.
Light leaves the laser and passes through the two slits. From each slit it spreads out, the way a wave spreads after a narrow gap. The two spreading waves overlap on their way to the wall.
Where the two waves arrive in step, they reinforce each other, and the wall is bright. Each bright band is called a bright fringe.
Where the two waves arrive out of step, they cancel each other out, and the wall is dark. Each dark band is called a dark fringe.
You have not seen optical fringes before, and you did not need to. Bright means reinforced, dark means canceled: the fringe pattern is the waves topic's interference pattern, drawn in light.
What would a stream of separate particles do at two slits? Each particle would pass through one slit or the other and land behind it. Particles would give two bright strips and no dark gaps, because separate particles do not cancel each other out.
The wall shows fringes, not two strips. So interference is a light behavior that only the wave model explains.
Diffraction tells the same story. A stream of separate particles passing a narrow gap would carry straight on in a narrow beam. A wave spreads out. Light spreads out.
Soraya shines a laser through a single very narrow slit. The wall shows a band of light wider than the slit, with faint bands at its edges. Which model of light explains the spreading?
The light passes through a narrow gap and spreads out beyond it.
Spreading through a narrow gap is diffraction, a wave behavior.
A stream of separate particles would carry straight on and make a strip no wider than the slit.
The wave model explains the spreading.
Kenji finds a stable pattern of bright and dark bands on a screen. What does the pattern tell him about how the light reached the screen?
Dark bands are places where light arriving by two routes cancels out.
Only overlapping waves can cancel each other out.
So the light reached the screen as two overlapping waves.
The pattern tells Kenji the light behaved as a wave on its way to the screen.
Marisol shines a green laser through two narrow slits onto a screen. What appears on the screen?
Which pair names the two light behaviors that only the wave model explains?
Marisol says: 'Bright fringes sit where waves from the two slits reinforce each other, and dark fringes sit where they cancel out.' Is Marisol correct?
A thin beam of light passes through a single narrow slit and spreads into a band wider than the slit. Which wave behavior is this?
On the screen of a two-slit experiment, Kenji points at a dark fringe. What do the waves from the two slits do at that spot?
Light passing the straight edge of a razor blade makes faint bright and dark bands just outside the shadow's edge. Which model of light explains the bands?