light: wave or particles (p259) newton was a supporter of the particle theory of light huygens...

Light: Wave or particles• (p259) Newton was a supporter of the particle

theory of light• Huygens believed light traveled as waves• Evidence gathered in the 1800’s supported the

wave theory• In the early 1900’s Hertz discovered the

photoelectric effect which could only be explained if light existed as particles (massless lumps of energy)

• We now believe light has a dual nature (acts a s both particles and waves

What are waves?

• Waves come in many forms

• Water waves, Sound waves, Radio waves, Earthquake waves, Light waves, Shock waves and of course “THE WAVE”

• Waves can be defined as a wiggle in space and time

“The Wave”

Why is wave motion important?• Waves happen all around us, all the time.• Wave motion allows us to examine the transfer

of energy from one place to another.

• (p 260) energy travels by either waves or particles

• A wave is a disturbance (vibration) that carries energy from one point to another without the transmission of matter (particles)

• Waves can be classified by what they need to travel in or through:

• (p 260) Mechanical waves require a medium to travel through. Sound is a mechanical wave because it can only travel if it is in a medium such as air, water, steel

• Electromagnetic waves can travel in a vacuum. Light is an electromagnetic wave because it can travel through a vacuum (space)

Mechanical Wave Types (p260)

• Longitudinal waves:are created by moving particles in a medium

back and forth along the same directionThe vibration of the medium is parallel to the

energy flow• Transverse waves:

are created with an up/down or side to side motion (at right angles to the direction of the

wave)vibration of the medium is perpendicular to the energy flow

Longitudinal Waves

Vibration of medium (parallel) (p 260)

Energy flow

Direction of motion

Longitudinal Waves

Longitudinal Wave

Compression Rarefaction

Energy flow

Transverse Waves

Vibration of medium (perpendicular)(p261)

Energy flow

Direction of motion

Wave Diagram #1

Wave parts (p261)

• Some parts of a wave refer only to longitudinal or transverse waves

• Transverse:

Crests are the high parts of a transverse wave

Troughs are the low parts of a transverse wave• Longitudinal:

rarefactions are stretched regions of a longitudinal wave

compressions are compressed regions of a longitudinal wave

Wave parts (p 262)

• Other parts of a wave refer to both longitudinal or transverse waves, although they may be more commonly shown on a transverse wave diagram

• Wavelength (λ) is the distance between two same points on a wave (for example, from crest to crest or from rarefaction to rarefaction)

• Amplitude maximum displacement of the medium from the equilibrium position

• Displacement is the distance from the 2 opposite parts of the wave (crest to trough, rarefaction to compression)

Wave Diagram # 2

• Frequency is the number of waves or vibrations that pass a point in a given time (number/second):

(add) frequency = # of times

second

• units of frequency are “per second” or hertz (Hz)

• Period is related to frequency

• Period (T) is the time for one event (wave)”

T = 1/f f = 1/T

Wave terms (p 262)

• (p 262) Points on a wave that are “in” or “out” of phase:

i) points in phase are at the exact same point on the wave but separated by a whole number wavelengths

ii) out of phase points are separated by more or less that a whole number of wavelengths

Diagram:

Speed of a Mechanical Wave

• (p 263) The speed of a mechanical wave depends on the medium it is traveling through

• The speed of sound in air depends on the temperature of the air:

v = 331m/s +0.6T

(T is the temperature in oC)

• The speed of sound is fastest in solids, and slowest in gases

• This means, sound travels faster in steel than in air

Universal Wave Equation

• (p 263) The speed of a wave ca also be determined from wavelength and frequency:

• This equation is really a form of: v = d/t because:

• Example problems 1 - 3 (p 264)

Electromagnetic Spectrum (p275)

• (p276) electromagnetic waves (EMR) do not requires a medium therefore they can travel through space

• There is a broad range of frequencies of electromagnetic radiation (EMR) called the electromagnetic spectrum

• (add to margin p 276) the speed of all EMR which includes light : c = 3.0 x 108m/s

• Visible light is a very small part of the electromagnetic spectrum. It has a range of wavelengths between 7 x 10-7m and 4 x 10-7m:

ROYGBIV

ROYGBIV

Low frequency high frequency

Long wavelength short wavelength

7 x 10-7m 4 x 10-7m

• (add to p 276) remember frequency depends on the source generating the wave

• Frequency is related to the energy of the wave: the higher the frequency the greater the

energy

• Energy of the wave also depends on the square of the amplitude of the wave. This means if the amplitude is doubled (2X) the energy of the wave is 22 = 4X