Waves 2

Sound and Light

Describing Sound WavesSound is a mechanical

(longitudinal) wave.A ringing bell is in a bell jar that has

been evacuated with a vacuum pump-why can’t you hear the bell?

Mechanical waves require a medium-they can’t travel through a vacuum.

Sound waves are longitudinal. Often drawn as a pressure wave.

Don’t confuse the pressure-time graph with a transverse wave - sound is a longitudinal wave.

How do ears detect sound?Ears detect fluctuations in air

pressureSound waves travel fastest in

solids - particles are closer together

The human ear can hear 20 Hz to 20,000 Hz frequencies

What is resonance?One object vibrating at the

same natural frequency of a second object forces the second object to vibrate

Examples: tuning fork, opera singer and a glass, drum in band room

How can the Doppler Effect be used to explain shock waves? Source moves at same speed or faster

than the wave (speed of sound)

When the source of the sound moves faster than the sound, the wave fronts bunch up and form a cone

A sonic boom occurs when the wave fronts compress and pile up along the edge of the cone

What can happen to waves at boundaries?ReflectionTransmitted to new

medium, some may be absorbed.

Refraction

What is an echo?

Reflected sound wave reaches ear more than 0.1 seconds after original sound

How long will it take for an echo to be heard in a 1200 meter well?

What is diffraction? How does it influence sound waves? Spreading out of waves as they pass

through an opening or around a barrier You can hear someone shouting in the

hallway even if they aren’t right in front of the door

Long wavelength owl sounds diffract around trees, while high tweets of small birds don’t diffract much

What is light? Light is an electromagnetic disturbance Accelerating charged particles

(electrons) move to a lower energy level and emit energy

Remember…no medium is necessary Luminated objects generate their own

light, while illuminated objects reflect light to our eyes from other sources

What is the range of frequencies for visible light in a vacuum?

See reference table 3.84 x1014 Hz (red) to 7.69 x1014 Hz (violet)

Violet - _________ __________ f

Red - _________ __________ f

What is ROY G BV? Colors of visible light

small high

large low

How can we represent how light travels? Travels in a straight line Represented by rays (straight lines with

arrows used to show path of a narrow beam of light)

What is the speed of light in space (vacuum)?

C = 3.00 x 108 m/s = f

In a medium other than a vacuum, how are v and c related?v < cRemember - v depends on the

nature of the mediumVelocity of light in air and in a

vacuum are the same within three significant digits….use the same value

What are dispersive materials, and how are they different from nondispersive materials?

Some materials allow waves of different frequencies to pass through at different speeds - these are dispersive

Colors are separated, violet is refracted the most

What is refraction?

Refraction is the change in direction of light, and it occurs when the wave passes into a new medium. The wave changes speed (and wavelength). The amount the speed changes depends on the medium.

Diagram:

i = incident angle

r = refracted angle

v = c

v < c

Water has a greater optical density, so light slows down

What happens when…… speed DECREASES in the new medium? (from less optically dense to more optically

dense medium) Light bends toward the normal (i > r)

Speed INCREASES in the new medium? (from more optically dense to less optically

dense medium) Light bends away from the normal (i < r)

What happens when……

The incident ray is along the normal? Speed changes, but the wave doesn’t

bend (i = r = 0)

Light must enter a medium obliquely to bend (at an angle, NOT along the normal)

Absolute Index of Refraction (Refractive Index) Measure of optical density Speed in a new medium depends on its

optical density Found in reference table

n = index of refraction

c = speed of light in a vacuum (or air)

v = speed of light in medium

Example: What is the speed of light in diamond?

Snell’s Law Equation:

Example: Light in water is incident on diamond. The angle of incidence is 45°. What is the angle of refraction?

n1 = 1.33 1 = 45°

n2 = 2.42 2 = ?

n = 1.33 n = 1.5 - 1.6 n = 2.42

Which has a higher index of refraction?

As n increases (more optically dense), light bends more and the refracted angle decreases

Critical Angle Critical angle is the angle of incidence

where the angle of refraction is 90° The refractive medium must have a

lower refractive index than the incident medium (light must speed up in the refractive medium).

When critical angle is exceeded you get “total internal reflection” - light doesn’t leave the medium

Light blue - refraction

Red - critical angle

Dark blue - total internal reflection

Example: Find the critical angle of light in crown glass if the refracted medium is air.

Total Internal Reflection

Law of Reflection

How can we describe images in a plane mirror?

Image is: the same distance behind the mirror the same size erect (upright) laterally reversed

Types of reflection: Regular reflection - polished surfaces -

also called specular reflection All rays reflected back are parallel Examples: mirror, pool of water

Diffuse reflection - scattering of light - caused by reflection from irregular surfaces - normals are not parallel

Examples: paper, blue sky

Wave Nature of Light

Light as a wave Christian Huygens - explained light as a

series of wave fronts whose direction could be represented by a light ray

3 pieces of evidence light behaves as a wave:

DiffractionInterferencePolarization

Remember with the word….

DIP

What is polarization? Evidence that light is a transverse wave

rather than a longitudinal one Separates light beams by plane of vibration

Why is the intensity of light reduced when it passes through polarizing material?

Waves not in the plane of vibration of the polarizing material do not pass through.

How can two polarizing filters be used so that no light will pass through?

Place the filters perpendicular to each other.

What is diffraction?

Diffraction is the spreading of waves in all directions behind an obstacle, shown below as light passes through a narrow slit.

Slit smaller than wavelength:

Slit larger than wavelength

Diffraction around a barrier

Young’s Double Slit Apparatus When light passes through a double slit

arrangement, what happens when a wave crest meets a wave crest on the screen?

Crest meets crest - constructive interference - bright areas

Crest meets trough - destructive interference - dark areas

What happens when more slits are added?

More bands of interference

What is a monochromatic light source?

Light of only one wavelength is emitted (one color)

Sources that produce waves with a constant phase relationship are …..

Coherent

coherent not coherent

Coherent waves are “in step” with each other

Lasers

Lasers produce a beam of light with a very narrow band of frequencies in which all the waves remain in phase for a longer time than ordinary sources

What are absorption spectra, and how are they different from line spectra? Absorption spectra - certain wavelengths are

absorbed by certain materials

Low temperature gases absorb certain wavelengths

Line (emission) spectra - certain wavelengths are emitted

Produced by high temperature gases at low pressure