# wave interactions and interference

## wave interactions

Waves interact with matter in several ways. The interactions occur when waves pass from one medium to another. Besides bouncing back like an echo, waves may bend or spread out when they strike a new medium. These three ways that waves may interact with matter are called reflection, refraction, and diffraction. Each type of interaction is described in detail below. For animations of the three types of wave interactions, go to this URL:

## standing waves

When a wave is reflected straight back from an obstacle, the reflected wave interferes with the original wave and creates a standing wave. This is a wave that appears to be standing still. A standing wave occurs because of a combination of constructive and destructive interference between a wave and its reflected wave. You can see animations of standing waves at the URLs below. http://skullsinthestars.com/2008/05/04/classic-science-paper-otto-wieners-experiment-1890/ Its easy to generate a standing wave in a rope by tying one end to a fixed object and moving the other end up and down. When waves reach the fixed object, they are reflected back. The original wave and the reflected wave interfere to produce a standing wave. Try it yourself and see if the wave appears to stand still.

## constructive interference

Constructive interference occurs when the crests of one wave overlap the crests of the other wave. This is illustrated in Figure 19.20. As the waves pass through each other, the crests combine to produce a wave with greater amplitude. You can see an animation of constructive interference at this URL: http://phys23p.sl.psu.edu/phys_anim/waves/em

## destructive interference

Destructive interference occurs when the crests of one wave overlap the troughs of another wave. This is illustrated in Figure 19.21. As the waves pass through each other, the crests and troughs cancel each other out to produce a wave with less amplitude. You can see an animation of destructive interference at this URL: http://phys23p.sl.psu.ed

## diffraction

Did you ever notice that when youre walking down a street, you can hear sounds around the corners of buildings? Figure 19.18 shows why this happens. As you can see from the figure, sound waves spread out and travel around obstacles. This is called diffraction. It also occurs when waves pass through an opening in an obstacle. All waves may be diffracted, but it is more pronounced in some types of waves than others. For example, sound waves bend around corners much more than light does. Thats why you can hear but not see around corners. For a given type of waves, such as sound waves, how much the waves diffract depends on two factors: the size of the obstacle or opening in the obstacle and the wavelength. This is illustrated in Figure 19.19. Diffraction is minor if the length of the obstacle or opening is greater than the wavelength. Diffraction is major if the length of the obstacle or opening is less than the wavelength.

## wave interference

Waves interact not only with matter in the ways described above. Waves also interact with other waves. This is called wave interference. Wave interference may occur when two waves that are traveling in opposite directions meet. The two waves pass through each other, and this affects their amplitude. How amplitude is affected depends on the type of interference. Interference can be constructive or destructive.

## reflection

An echo is an example of wave reflection. Reflection occurs when waves bounce back from a barrier they cannot pass through. Reflection can happen with any type of waves, not just sound waves. For example, Figure 19.15 shows the reflection of ocean waves off a rocky coast. Light waves can also be reflected. In fact, thats how we see most objects. Light from a light source, such as the sun or a light bulb, shines on the object and some of the light is reflected. When the reflected light enters our eyes, we can see the object. Reflected waves have the same speed and frequency as the original waves before they were reflected. However, the direction of the reflected waves is different. When waves strike an obstacle head on, the reflected waves bounce straight back in the direction they came from. When waves strike an obstacle at any other angle, they bounce back at the same angle but in a different direction. This is illustrated in Figure 19.16.

## refraction

Refraction is another way that waves interact with matter. Refraction occurs when waves bend as they enter a new medium at an angle. You can see an example of refraction in Figure 19.17. Light bends when it passes from air to water. The bending of the light causes the pencil to appear broken. Why do waves bend as they enter a new medium? Waves usually travel at different speeds in different media. For example, light travels more slowly in water than air. This causes it to refract when it passes from air to water.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

An echo occurs because of wave

``````a. interference.

b. diffraction.

c. refraction.

-->  d. reflection.
``````

If a wave strikes a barrier at a 45 angle, what is the angle of reflection?

``````a. 180

b. 120

c. 90

-->  d. 45
``````

Light is refracted when it

``````a. strikes a barrier it cannot pass through.

b. spreads around an obstacle such as a wall.

-->  c. passes from air to water at an angle.

d. interferes with other waves.
``````

What happens when a wave passes around a barrier that is shorter than its wavelength?

``````a. The wave has a large angle of incidence.

-->  b. The wave spreads out around the barrier.

c. The wave is refracted.

d. none of the above
``````

Constructive interference occurs when two waves pass through each other and the

``````a. crests of both waves cancel each other out.

b. crests of both waves have a smaller amplitude.

-->  c. crests of one wave overlap crests of the other wave.

d. crests of one wave cancel out troughs of the other wave.
``````

Reflection occurs only with sound waves.

``````a. true

-->  b. false
``````

All reflected waves appear to be standing still.

``````a. true

-->  b. false
``````

The angle of incidence is always greater than the angle of reflection.

``````a. true

-->  b. false
``````

Reflected waves have the same speed as the original waves before they were reflected.

``````-->  a. true

b. false
``````

Diffraction is more pronounced with sound waves than light waves.

``````-->  a. true

b. false
``````

Diffraction occurs because waves travel at different speeds in different media.

``````a. true

-->  b. false
``````

Wave interference occurs whenever waves enter a new medium.

``````a. true

-->  b. false
``````

Wave interference occurs only when a wave is reflected.

``````a. true

-->  b. false
``````

You can hear sounds around the corner of a building because the sound waves are refracted.

``````a. true

-->  b. false
``````

Light waves refract when they pass from air to water.

``````-->  a. true

b. false
``````

Destructive interference decreases the amplitude of waves.

``````-->  a. true

b. false
``````

A standing wave forms when a wave is refracted.

``````a. true

-->  b. false
``````

Interference occurs only when the crests of one wave overlap with the troughs of another wave.

``````a. true

-->  b. false
``````

A standing wave occurs when a wave is reflected straight back from an obstacle.

``````-->  a. true

b. false
``````

Wave interference always changes the speed of a wave.

``````a. true

-->  b. false
``````

change in direction of waves as they enter a new medium at an angle

``````a. diffraction

b. wave interaction

c. reflection

d. constructive interference

-->  e. refraction

f. destructive interference

g. wave interference
``````

bouncing back of waves from a barrier

``````a. diffraction

b. wave interaction

-->  c. reflection

d. constructive interference

e. refraction

f. destructive interference

g. wave interference
``````

any interaction of waves with other waves

``````a. diffraction

b. wave interaction

c. reflection

d. constructive interference

e. refraction

f. destructive interference

-->  g. wave interference
``````

situation in which crests of one wave overlap crests of another wave

``````a. diffraction

b. wave interaction

c. reflection

-->  d. constructive interference

e. refraction

f. destructive interference

g. wave interference
``````

any interaction of waves with matter

``````a. diffraction

-->  b. wave interaction

c. reflection

d. constructive interference

e. refraction

f. destructive interference

g. wave interference
``````

spreading out of waves as they pass around a barrier

``````-->  a. diffraction

b. wave interaction

c. reflection

d. constructive interference

e. refraction

f. destructive interference

g. wave interference
``````

situation in which crests of one wave overlap troughs of another wave

``````a. diffraction

b. wave interaction

c. reflection

d. constructive interference

e. refraction

-->  f. destructive interference

g. wave interference
``````

Ways that waves may interact with matter include

``````-->  a. diffraction.

b. destructive interference.

c. constructive interference.

d. all of the above
``````

Reflected waves differ from the original waves before they were reflected in their

``````a. speed.

-->  b. direction.

c. frequency.

d. wavelength.
``````

Refraction occurs because waves

``````a. cannot travel through an obstacle such as a wall.

-->  b. travel at different speeds in different media.

c. interfere with their reflected waves.

d. none of the above
``````

If the length of an obstacle is greater than the wavelength of a wave, you would expect to see

``````a. no diffraction.

-->  b. very little diffraction.

c. a lot of diffraction.

d. wave interference.
``````

A standing wave is a wave that

``````a. is not moving.

b. has an upright direction.

c. is taller than other waves.

-->  d. appears to be standing still.
``````

A standing wave occurs because of a combination of

``````a. incidence and reflection.

b. refraction and diffraction.

c. refraction and interference.

-->  d. constructive and destructive interference.
``````

Which statement about destructive interference is true?

``````-->  a. It occurs when waves pass through each other.

b. It results in a wave with a higher frequency.

c. It occurs when waves interact with matter.

d. It always produces a standing wave.
``````

## diagram questions

No diagram questions associated with this lesson