Waves through a narrow gap

In the picture

1. 1A crest of the incoming plane wave, running straight across the tank
2. 2The narrow gap in the barrier, about one wavelength wide
3. 3Wavefronts curving outward from the gap into broad semicircles
4. 4The thin slit in the screen placed across the laser beam
5. 5The wide bright band on the wall, brightest at its centre

Examples

• Music heard through an open doorway spreads through the room instead of coming only from the door.
• Street light passing a narrow gap between curtains lights a wide patch of the wall.

Quiz: Waves through a narrow gap (Age 17)

1. 1

   As an opening becomes narrower relative to the wavelength, what happens to the spreading of the wave?

   • It spreads more widely.
   • It stops spreading and stays straight.
   • It spreads less widely.
2. 2

   When is the spreading of a wave through a gap strongest?

   • When the gap width is much larger than the wavelength.
   • When the gap width is exactly twice the wavelength.
   • When the gap width is equal to or smaller than the wavelength.
3. 3

   What happens to light after it passes through a thin slit?

   • It forms a wide bright band with weaker bands beside it.
   • It stays as a narrow beam with no spreading.
   • It becomes completely dark on the far side.

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.