Diffraction

Diffraction

The way waves spread out as they move through a gap

What does the amount of diffraction depend on

-Size of the gap

-Wavelength

Oscillating paddle

Continually dips into the water of a ripple tank to create regular waves with straight, parallel wavefronts

Does a larger or smaller wavelength diffract more for the same gap

Larger

Does a larger or smaller gap cause the same wavelength to diffract more

Smaller

When is the most diffraction

When the gap size is equal to the wavelength

Experiment to investigate diffraction

1- An oscillating paddle in a ripple tank creates regular, parallel wavefronts

2- Objects are placed into the ripple tank to create a barrier with a gap

3- Vary the size of the gap and view the effect on diffraction

What happens if the gap size is decreased too much

Waves cannot pass through the gap so are reflected

Explain why you can hear someone around a doorway, but can’t see them

The size of the doorway is roughly the size of a sound wavelength, so lots of diffraction occurs, allowing someone to be heard out of the line of sight. The size of the doorway is much greater than the wavelength of light, so the diffraction is tiny

What explains the diffraction of waves

Huygens’ construction

Huygens’ construction

Every point on a wavefront may be considered to be a point source of secondary wavelets that spread out in the forward direction at the speed of the wave. The new wavefront is tangential to all of these secondary wavelets

Other than gaps, what can cause diffraction

Obstacles

Shadow

The area where the light has been blocked

Explain the length of the shadow of a wider object

There will be less diffraction and so the shadow will be longer

Explain the length of the shadow of a less wide object

There will be more diffraction, so the shadow will be shorter

What condition must be met for a diffraction pattern

The wavelength is similar to the aperture

What does a diffraction pattern look like

A bright central fringe with alternating dark and bright fringes on either side of it

For the same slit width how does a larger wavelength affect the diffraction pattern

Wider pattern

For the same slit width, how does a shorter wavelength affect the diffraction pattern

Thinner pattern