Waves and Electromagnetic Radiation

The Sciences chapter 6

Maxwell, in 1867, proposed that light is an electromagnetic wave.

“The spectrum of visible light, from red to violet, is only an octave or so in the range of invisible radiations. There is a whole keyboard of information all the way from the longest wavelengths of radiowaves (the low notes) to the shortest wavelengths of X-rays and beyond (the highest notes)” (Bronowski, p. 353).

Figure 6-11

The electromagnetic spectrum includes all kinds of waves that travel at the speed of light, including radio, microwave, infrared, visible light, ultraviolet, X-rays, and gamma rays. Note that sound waves, water waves, seismic waves, and other kinds of waves that require matter in order to move travel much slower than light speed.

Waves transfer energy without transferring mass.

Figure 6-1

You can use a domino to knock over other dominoes in two different ways: (a) you can throw a domino, or (b) you can trigger a wave of dominoes.

Energy Transfer by Waves

A wave…

• Is a traveling disturbance

• It carries energy from place to place without requiring matter to travel across the intervening distance.

Properties of Waves

• Wavelengths

• Frequency

• Velocity

• Amplitude

Figure 6-2

A cross section of a wave reveals the characteristics of wavelength, velocity, and amplitude. Successive wave crests are numbered 1, 2, 3, and 4. An observer at the position of the clock records the number of crests that pass by in a second. This is the frequency, which is measured in cycles per second, or hertz.

Relationship among wavelength, frequency, and velocity

• The velocity of a wave is equal to the length of each wave times the number of waves that pass by each second.

• Wave velocity (m/s) = wavelength (m) x frequency (Hz)

• Sample Problem: One tube of a wind chime produces sound at a frequency of 440 Hz. Assuming the speed of sound is 340 m/s, what is the wavelength of sound produced by this chime?

• Wavelength = velocity / frequency

• 340 meters per second / 440 Hertz = 0.77 m

• Also see Example 6-1, p. 119.

Figure 6-4

Transverse (a) and longitudinal (b) waves differ in the motion of the wave relative to the motion of individual particles.

Two kinds of waves

Speed of Light Speed of Sound

• 300,000,000 m/s

• 186,000 miles/s

• 340 m/s

• 760 miles/hr

Example problem 6-2:The human ear can hear sounds at frequencies from 20 to 20,000 Hz. Organ pipes producing these notes would be about half the wavelength.

• Lowest note wavelength:

• Velocity / frequency• 340 m/s / 20 Hz =• 17 m• Organ pipe 8.5 m

• Highest note wavelength:

• Velocity / frequency• 340 m/s / 20,000 Hz =• 0.017 m• Organ pipe 0.009 m

How Sound is Produced…

• Sound is a form of energy caused by vibrating matter.

How Sounds Travel

Vibrations push against molecules in the air and press them closer together

Molecules pushed = compression Molecules spread = rarefaction

Sound Travel in Different States

• The speed of sound in air is about 336 meters/sec or 1100 ft/sec

• In liquids, the molecules are closer together and carry the vibrations more easily and quickly (4X >).

• In solids, the atoms are very close together and carry the vibrations very easily and quickly (9-15X >).

Characteristics of Sound

• Intensity = the “loudness” or “softness” of a sound

• Pitch = how “high” or “low” the sound seems

Characteristics of Sound

• Pitch - how “high” or “low” sound seems

• Intensity - “loudness” or “softness”

• Quality - the mix of frequencies that allow us to distinguish between different sounds

• Interference –waves from two different sources come together (constructive or destructive)

Echoes

• An echo is a sound wave that bounces back

• must be at least 17 meters (or 56 ft) away from the reflecting surface (closer and reflected sound wave blends with original)

The Human Voice

• 2 strong bands of tissue called vocal cords

--The tighter the vocal cords, faster

they vibrate, the higher the pitch.

--Increase volume by

increasing force of air

blown between the

vocal cords.

Speed of Light Speed of Sound

• 300,000,000 m/s

• 186,000 miles/s

• 331 m/s

• 760 miles/hr

Doppler Effect

• If a listener or the source of a sound is moving, the listener may hear a pitch that is different from the frequency of the source.

• The change in pitch heard when the source of sound is moving relative to the listener is the Doppler effect.

Figure 6-9

The Doppler effect occurs whenever a source of waves is moving relative to the observer of the waves. (a) Sound waves spread out from a source in all directions; stationary listeners hear the same pitch. (b) Sound waves from a moving source seem to increase or decrease in pitch, depending on whether the sound is approaching or receding. (c) The Doppler shift for light waves cause a blueshift for approaching sources, and a redshift for receding sources.

Electromagnetic “waves”

Electromagnetic spectrum

Figure 6-11

The electromagnetic spectrum includes all kinds of waves that travel at the speed of light, including radio, microwave, infrared, visible light, ultraviolet, X-rays, and gamma rays. Note that sound waves, water waves, seismic waves, and other kinds of waves that require matter in order to move travel much slower than light speed.

Anatomy of the Electromagnetic Wave

• Electrical and magnetic fields arranged at right angles to each other

• Perpendicular to the direction the wave is moving

The nature of light

 Light is a form of radiant energy given out by the Sun and other light producing bodies in the form of waves.

 Light is the one part of a group of the em spectrum that we can see.

--Consists of transverse waves, --move up and down as they travel forward

Same Speed across the whole em spectrum!

• Velocity of em waves depends on electrical and magnetic interactions, not on the properties of the wave itself.

• All em waves move at the speed of light!

• Wavelength x frequency= 300,000 km/sor 186,000 mi/s

The energy of em wavesOscillating comb andglowing embers – p.125-6

Light travels in straight lines

• Light travels at a speed of 186,000 miles/sec (300,000 km/sec)

(in 8 minutes, light to travels Sun to Earth)

• Thus we see things happen at the exact moment they are happening.

With regard to light, materials are…

• Transparent

• Translucent

• Opaque

Law of Refection:

      angle of incidence is equal to the angle of reflection (holds true for all smooth, polished surfaces)

The Plane Mirror

• light striking the mirror passes thru the transparent glass, and then almost all the light is reflected back by the shiny, opaque silver

Mirror Convex Concave

Refraction of Light & lenses

• convex lens • light rays passing thru

a convex lens are bent toward the thicker middle

• concave lens

• light rays passing thru a concave lens are bent toward the thicker ends

Spectrum of visible light

Mixing Colors of Light

Electromagnetic Spectrum

• Radio Waves• Microwaves• Infrared Radiation• Visible Light

• Ultraviolet Radiation• X-rays• Gamma Rays

Study Guide, end of chapter 6

• Discussion Questions 1, 2, 3, 4, 6, 8, 9.

• Problems 1, 5, 6, 7.

• Chemistry Reading, push on to read chapters 8 & 10.