Interference

Interference of Waves• Material objects cannot occupy the

same place at the same time.• for example 2 rocks cannot be at the

same spot at the same time• But waves can exist at the same time in

the same space• Take those 2 rocks and drop them in

water, the waves can overlap and form in interference pattern

Interference Patterns

As these waves interfere with each other, one of three things will happen:

1. the waves will increase in size2. the waves will decrease in size3. the waves will cancel

Superposition Principal• (p 269) The superposition principal states

that when there are 2 sources of waves in a medium, these waves will combine to give a resultant wave that is the algebraic sum of all the waves

• (add) this can only happen with waves, not particles

• The superposition principal explains constructive and destructive interference

a. Constructive Interference• (add) When the crest of one wave overlaps

the crest of another, their individual effects add together.

• (add) Once the waves have passed through each other they continue in their original directions

b. Destructive Interference

• (add) When the crest of one wave overlaps the trough of another wave, their individual effects are reduced.

                                                                                                                       

• Destructive interference does not always result in waves completely cancelling:

• (p 270) Constructive and destructive interference of sound waves results in beats

Reflection of Waves

• If you tied a piece of rope to a wall and shook the free end up and down, you would produce a wave in the rope.

• Since the wall is too rigid to shake, the wave is reflected back along the rope.

• by shaking rope at the proper frequency you can cause the incident (original) and reflected waves to form a standing wave.

Standing Wave• (add to margin of p 271) Standing waves

are created when 2 pulses with equal and opposite amplitudes meet

• Diagram:

Diffraction• (p269) Only waves diffract (particles do not)• Diffraction is the spreading out of a wave as it

passes through a small opening or around an obstacle

• The amount of diffraction depends on:a. the size of the opening: the smaller the opening the greater the diffractionb. the wavelength: the longer the wavelength the greater the diffraction

• Huygen’s explained diffraction in his principle

Diffraction depends on the size of the opening: the smaller the opening the greater the diffraction

Diffraction depends on wavelength: the longer the wavelength the greater the diffraction

Review• A wave is a wiggle is space and time

• Waves carry energy without transferring matter

• Wavelength is the distance between two identical parts of the wave (crest-crest or trough-trough)

• Waves can be classified based on how they travel:a. In a vacuum: electromagnetic wavesb. Needs a medium: mechanical waves

Review continued• There are two main types of mechanical

waves: i) Transverse ii) Longitudinal

• In constructive interference, crests overlap • In destructive interference, a crest overlaps a

trough• In a standing wave, points of complete

destructive interference (nodes) remain stationary.