Unit 7 7.6 and 7.7. 7.6 Summarize reflection and interference of both sound and light waves and the...

Unit 77.6 and 7.7

7.6Summarize reflection and interference of both sound and light waves and the refraction and diffraction of light waves.

Key Concepts

Wave behaviors: reflection, refraction, diffraction,

Interference: constructive and destructive

Concave and convex lenses

Plain mirrors

Law of reflection

InterferenceWaves interfere with each other.

Interference may be constructive A crest will interfere with another crest

constructively to produce a larger crest and a trough will interfere to produce a larger trough.

Compressions interfere constructively with each other as do rarefactions.

Interference may be destructive A crest will interfere with a trough to lessen or cancel

the displacement of each. Compressions interfere with rarefactions to lessen or

cancel the displacement of each.

Sound Waves

Sound Waves

Sound is a longitudinal mechanical wave, requires a medium, and can be produced by vibrating objects.

Sound like other waves reflects. Sound produces echoes when it

bounces off hard surfaces.

Sound Waves

Sound waves interfere with each other changing what you hear. Destructive interference makes sounds

quieter; constructive interference makes sounds louder.

Sound waves reflect in tubes or some musical instruments to produce standing waves which reinforce sound through constructive interference to make the sound louder.

Light Waves

Light waves reflect

When light rays reflect they obey the “Law of Reflection”. The angle of incidence is equal to the angle of reflection.

The angle of incidence is the angle between the incident ray and a line normal (perpendicular) to the surface at the point where the light strikes.

The angle of reflection is the angle between the reflected ray and the normal line.

ReflectionReflection from a mirror:

Incident ray

Normal

Reflected ray

Angle of incidence

Angle of reflection

Mirror

Reflection

Light waves reflect in plain mirrors to produce images.

The image appears as far behind the mirror as the object is in front of it.

The image and the object appear to be same size.

The image is upright.

Diffraction

Diffraction is the bending of a wave around a barrier or around the edges of an opening.

Waves with a longer wavelength diffract more readily so in order to observe light diffraction the barriers or openings must be small

When light waves diffract interference patterns can often be observed.

Interference

Light can interfere to produce interference patterns.

Light can interfere constructively and destructively.

When light interferes a pattern is often seen with light and dark areas created by constructive and destructive interference.

At other times interferes to produce a color pattern. When a color interferes destructively we will see the colors that are not interfered with destructively.

Interference

Light can reflect off the bottom and top surfaces of thin film such as oil on water or bubbles and produce a color pattern due to interference.

Light can diffract through small slits or around lines to produce light and dark patterns or color patterns due to the interference of light

Refraction

Light like other waves can refract.

Waves refract when they change direction upon entering another medium in which the waves travel at different speeds.

The waves must strike the new medium at an angle other than straight on in order to change direction.

Light refracts when it enters a different medium at an angle other than perpendicular and changes speed.

Refraction

Refraction

How Light Rays Bend

Light slows down when it enters the prism and bends down when it strikes at this angle.

When light exits the prism it speeds back up and bends down again.

When white light enters another medium such as a prism the colors may spread out. This is because the violet end of the spectrum slows down more than the red end and therefore bends more.

Prism

LENSESLenses can be convex (converging)

Used on people with farsightedness (can’t see up close)

Or lenses can be concave (diverging)

Used on people with nearsightedness (can’t see far away)

Lenses

7.7Explain the Doppler effect conceptually in terms of the frequency of the waves and the pitch of the sound.

SOUND WAVESRemember, they are mechanical…must have a medium.

They are longitudinal so air molecules move parallel to the direction of the wave.

Direction of Wave

Energy travels out in all directions from vibrating object.

Intensity of Sound

The amount of energy in a wave. Intensity determines the amplitude

of a wave. Intensity relates to the loudness of

a sound - the greater the intensity, the louder the sound.

IntensityDecibels (dB) - the units of intensity. Sounds over 120 dB will cause

pain in the ears. Ear damage can begin with

exposure to sounds as low as 85 dB.

Decibels of other Sounds

40—quiet conversation

60—busy street

75—vacuum cleaner

85—heavy truck traffic

110—power tools

140—singer screaming into microphone

155—jet engine—up close

PitchThis is how high or low a sound seems.

A bird makes a high pitch. A lion makes a low pitch.

Pitch has to do with frequency and amplitude.

More on PitchThe amount of frequency depends on the amplitude of the wave.

High pitch: high frequency, shorter amplitude

Low pitch: low frequency, larger amplitude

Hearing Frequencies

Humans hear between 20 Hz and 20,000 Hz.

Ultrasonic—frequencies above 20,000 Hz; dogs and bats can hear ultrasonic

Infrasonic—frequencies below 20 Hz; wind, earthquakes, heavy machinery

DOPPLER EFFECTAn apparent frequency shift due to the relative motion of an observer and a wave source.

The pitch will seem to change due to the wave moving.

As wave approaches you, you perceive a higher pitch.

As wave moves away from you, you perceive a lower pitch.

http://www.cpo.com/Weblabs/chap2/doppler4.htm

Doppler Shift

A Doppler shift occurs when a wave source is moving towards an observer or away from the observer

A Doppler shift also occurs when the observer is moving toward the wave source

There is no shift when the source and observer are not moving toward or away from each other.

As a wave source approaches an observer, the observer perceives a higher frequency than the source is producing. Wavelengths are shorter and the frequency is higher in front of a moving source.

When the wave source is moving away from the observer he/she will perceive a lower frequency than the source in producing. Wavelengths are longer and the frequency is lower behind a moving source.

What is happening here?