# February 15 You will take a few notes about sound Then you will have some time to finish up the work from yesterday and/or the wave calculations.

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February 15 You will take a few notes about sound Then you will have some time to finish up the work from yesterday and/or the wave calculations.

Constructive Interference – 2 waves combine to form a new wave with a larger amplitude. Destructive Interference – 2 waves combine to produce a wave with a smaller amplitude. Interference When two or more waves exist in the same medium, each wave moves as though the other were absent. When they meet, they interfere with one another and make a new wave.

SOUND

Sound is energy transferred by the compression & rarefaction of matter: sound waves are compressional waves Collisions between molecules transfer energy from molecule to molecule So… can sound travel in a vacuum?

The Speed of Sound Sound waves move faster for two reasons. When the molecules in the medium are closer together (solids & liquids) When the temperature of the medium increases

Properties of Sound

LOUDNESS is the human perception of sound energy. So… the “loudness” of a sound measures its AMPLITUDE. Loudness is measured in decibels (dB)

LOUDNESS SCALE The decibel scale is logarithmic… that means that, for every 10-dB increase, the sound is 10 TIMES as loud

PITCH is the human perception of frequency. How high or how low a sound seems to be High notes have high frequencies, and low notes have low frequencies.

Human ears hear frequencies from 20 to 20,000 Hz What does this mean for wavelength?

If a frequency isn’t in the human hearing range, does it still make a sound? ULTRASONIC ~Frequencies over 20,000 Hz ~Medical & Science uses SUBSONIC or INFRASONIC ~Frequencies under 20 Hz ~feel like a “rumble”

The DOPPLER EFFECT refers to the apparent change in frequency of a sound when there is relative motion of the source and listener. Right person hears a higher f due to shorter λ. Left person hears lower f due to longer λ. Sound source moving with v s Apparent f 0 is affected by motion.

LIGHT Light energy is carried by transverse waves. Light can travel through matter, OR through a vacuum. When light strikes the boundary between two media, three things may happen: reflection, refraction, or absorption. reflection refraction absorption Water Air

Reflection, Refraction, and Absorption REFLECTION occurs when a ray from one medium strikes the boundary and bounces off. REFRACTION occurs when a ray enters the second medium and bends away or toward the normal line. INDEX OF REFRACTION: the ratio between the speed of light in a vacuum and the speed of light in a medium ABSORPTION occurs when a ray is absorbed atomically by the water and does not reappear. Water Air reflection refraction absorption

Refraction Distorts Vision Water Air Water Air Your eyes (and brain) believe that light always travels in straight lines. So, when looking through a boundary, you see objects closer to the surface due to refraction.

Away or toward? Light bends… Light bends… AWAY from the normal when going from a medium with a higher index to one with a lower index (water to air) AWAY from the normal when going from a medium with a higher index to one with a lower index (water to air) TOWARD the normal when going from a medium with lower index to one with a higher index (air to water) TOWARD the normal when going from a medium with lower index to one with a higher index (air to water) OPTICAL DENSITY: the tendency for materials to absorb electromagnetic energy. This is related by the index of refraction – higher index = more density OPTICAL DENSITY: the tendency for materials to absorb electromagnetic energy. This is related by the index of refraction – higher index = more density

A Special Case of Reflection: Total Internal Reflection When light passes at an angle from a medium of higher index to one of lower index, the emerging ray bends away from the normal. When the angle reaches a certain maximum, called the CRITIAL ANGLE ( θ C ), all of the light will be reflected internally. i = r Critical angle cccc

Diffraction Also known as “scattering” Also known as “scattering” Diffraction produces rainbows… Diffraction produces rainbows… Different colors of light have different wavelengths As the waves travel through the water droplets, each color bends by a different amount based on its wavelength

Interference Appears as a series of dots or lines Appears as a series of dots or lines First discovered by Young, who studied the nature of light… wave or particle First discovered by Young, who studied the nature of light… wave or particle

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