Wave Behaviors. Reflection When a wave encounters a different medium, only a portion of the wave enters the new medium. Some of the initial wave energy.

Wave Behaviors

Reflection

When a wave encounters a different medium, only a portion of the wave enters the new medium. Some of the initial wave energy will "bounce back" at the boundary This phenomenon is known as reflection.

Incident wave - the initial wave

When the incident wave reaches the boundary of the two mediums, a portion of the wave will enter the second medium (transmitted wave) and the remainder will be reflected (reflected wave).

The degree of difference between the two mediums determines how much of the incident wave is reflected.

With similar mediums (fresh water and salt water), most of the incident wave will be transmitted.

With different mediums (water and molasses), most of the incident wave will be reflected.

If a wave is travelling from a less dense to a denser medium, the reflected wave is inverted.

Reflection and Inversion

A portion of the wave willbe reflected anda portion transmitted

An incident wave in a low density medium approaches a higher density medium.

After the wave has encountered the boundary, the transmitted wave carries on in an upright position, but the reflected wave is inverted.

When a wave travels from a more to a less dense medium?

Frequency

The speed of a wave changes when the medium changes, but does the frequency also change?

Initial freq. = 5 waves per second (5 Hz).

These waves travel down the loose slinky until they encounter the stiffer slinky.

The speed of the waves will definitely change as they enter the stiffer slinky material,

Will you still see five waves per second? Is it possible to see anything but five waves per second?

No If the frequency dropped to only four waves per

second, where would the fifth wave go? Waves can't just disappear. Likewise if the frequency were to increase, would a new wave appear out of thin air?

Therefore frequency does not depend on medium.

Change in Wavelength

One variable does change change when a wave enters a new medium? If the velocity changes, the frequency doesn't and if v = f λ is to be true, then the wavelength must also change.

Wave Fronts

A method of graphically representing waves.

Draw lines called rays. A ray is simply a line drawn in the direction of the wave motion and it is perpendicular to the wave fronts.

The normal, which is just another name for a perpendicular, or 90 degree, line – give a reference point

Law of Reflection

angle of incidence – the angle between normal and incident ray (symbol θi)

angle of reflection- the angle between the normal and the reflected ray (symbol θr)

Law of Reflection θi = θr

Note The angles of incidence and reflection are measured

from the normal to the ray and not from the boundary to the ray.

Superposition

When two waves pass through the same place at the same time, they will combine to form a wave that is some combination of the two. This process is called interference.

Constructive interference - the two waves combine to reinforce each other and produce a larger wave than the original waves

Destructive interference - the waves tend to cancel each other, then the interference is

Principle of superposition- the wave produced by any interference is simply the algebraic sum of the two original waves.

Principle of Superposition

Wave Phases and Standing Waves

Phase -the relative position of the crests of two different waves. Constructive interference, the crests

line up and the waves are said to be "in phase."

Destructive interference , the crests do not line up and the waves are said to be "out of phase."

Standing Waves

An interesting situation can arise if the two interfering waves have just the right frequency.

Example: A slinky has one end attached to a wall and

you are shaking the other end. As you send waves down the slinky, they will reflect from the fixed end.

These reflected waves then interfere with the incident waves being produced by your hand.

Usually the interference that takes place in this situation produces a jumble of constructive and destructive interference.

However, if the frequency of the waves is just right an interesting pattern will be seen. You will see a large amplitude wave that appears to stand still on the rope. nodes - areas where destructive interference

is always taking place antinodes - areas of constructive interference

Standing Wave

Standing Wave Frequency

Fundamental frequency – the smallest frequency that will produces a standing wave.

If you double the frequency of a standing wave, you will produce another standing wave with more nodes and antinodes.

Sample Problem

The distance between adjacent nodes in a vibrating string is 15 cm. What is the wavelength of the standing wave?

Give it a go…….

A standing wave wit ha frequency of 45.0 Hz is produced in a string. The distance between the 2nd node and the fifth node is 36 cm. What is the wavelength of the wave?

Refraction

The speed of a mechanical wave depends on the medium it is travelling in.

Refraction - when a wave changes its direction due to a change in speed.

Since the wave’s frequency remains the same, the wavelength must also change.

Diffraction

When waves encounter a barrier, they tend to bend around it and spread into the area behind the obstacle. This phenomenon is called diffraction.

The larger the wavelength of the waves as compared to the size of the obstacle, the greater the amount of diffraction.

Waves encountering a small obstacle as compared to their wavelength will refract a great deal into the area behind the obstacle.

When they encounter a larger obstacle, the amount of diffraction is less.

An increase in the wavelength increases the amount of diffraction