Diffraction and Interference

Light

•Has wave properties.• Can diffract.Can diffract.• Can constructively or Can constructively or

destructively interfere.destructively interfere.

Wave Fronts

• Lines that are perpendicular to the motion of the wave.

• Indicate the location of the crests in the waves that are traveling together.

Huygen’s Principle

• Wave fronts are made up of tinier wave fronts.

• Every point on any wave front is a new source for a secondary wave front.

Huygen’s Principle

• You can explain reflection and refraction using Huygen’s Principle.

Huygen’s PrincipleAs the straight waves passed through a narrow

hole, they spread out in a circular pattern. Giving proof to the fact that every point on a

wave front is a new source for a new set of wavelets.

Diffraction

Any bending of a wave around an obstacle or edges of an opening by means other than reflection or refraction.

Diffraction• The amount of diffraction (bending)

depends on the size of the wavelength compared with the size of the obstruction.

• The longer the wavelength is compared to the obstruction, the greater the diffraction.

Demo

DiffractionLong AM radio waves can diffract around

hills and buildings and can be received better in more places than short waves

that don’t diffract as much.

Is Diffraction a Good Thing?

Diffraction is bad when we want to see very small objects with microscopes.

If the size of the small object is the same as the wavelength of light, the image will be blurred by diffraction.

Interference

Young’s Interference Experiment

1801, Thomas Young discovered that when light of a single color (monochromatic) was directed through two closely spaced pinholes, fringes of brightness and darkness were produced on a screen.

Young’s Interference Experiment• Bright fringes

= constructive interference– Waves arrive

at the screen in phase

• Dark fringes = destructive interference– Waves arrive

at the screen out of phase

Single-Color Interference from Thin Films

• Interference fringes can be produced by the reflection of light from two surfaces that are very close together.

• If you shine a single-color (monochromatic) light onto stacked (with an air wedge) plates of glass, you’ll see dark and bright bands.