Speed of Sound vs. Speed of Light

[ilovemcat]

Speed of Sound vs. Speed of Light

Hey guys,

Can someone tell me if my understanding is right:

v = λ x f

• For Rope/ Sound Waves, a MEDIUM is required to travel.
• The speed of a wave is dependent on the properties of the medium.
• If the properties of the medium changes, so will it's speed (ie. if the elasticity (tension) or (the inertial component) density of medium changes --> v changes)
• The frequency of wave is dependent on the source and remains the same as it travels from one medium to another.
• Therefore, if velocity of a medium decreases and frequency remains the same, wavelength must decrease.

• Generally, the speed of sound increases in the following order: (slowest) gases (air) --> liquids --> solids (fastest)
• For air, the speed of sound equals approx. 340 m/s.

• For Light Waves NO MEDIUM is REQUIRED to travel.
• In a vacuum, the speed of light is constant and equals c = 3 x 10^8 m/s (maximum value)
• When light travels through a medium, it's value decreases from its maximum ("c" above).
• Unlike the previous waves discussed - here, Light (Wave) Speed IS affected by the wave FREQUENCY (and the MEDIUM).
• This phenomenum is known as Dispersion.
• The rule of thumb is: HIGHER frequency = LOWER Velocity (ie. Violet Light travels slower than Red Light.)
• Therefore, for a given medium - we expect different indices of refraction for different frequencies of light.
• LARGER frequency = SLOWER velocity = HIGHER index of refraction (n = c/v).

• Generally, the speed of light increases in the following order: (slowest) solids --> liquids --> gases (air) --> vacuum (fastest)
• This is because index of refraction is largest for solids and smallest for gases.

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[anhooga]

Just wanted to clarify, that the speed of sound in a vacuum is nonexistent. So it wouldn't even be the slowest out of all the mediums. No sound is produced at all.

 

[Rabolisk]

Unlike the previous waves discussed - here, Light (Wave) Speed is determined by the FREQUENCY, NOT the MEDIUM.

Certainly the medium affects light speed. Are you saying that light, when holding frequency constant (i.e. choosing only one frequency of light), always travels at the same speed?

 

[ilovemcat]

Just wanted to clarify, that the speed of sound in a vacuum is nonexistent. So it wouldn't even be the slowest out of all the mediums. No sound is produced at all.

Oops! Thanks for catching that.

 

[ilovemcat]

Unlike the previous waves discussed - here, Light (Wave) Speed is determined by the FREQUENCY, NOT the MEDIUM.

Certainly the medium affects light speed. Are you saying that light, when holding frequency constant (i.e. choosing only one frequency of light), always travels at the same speed?

Ah, I should re-clarify that:

For Sound/Rope waves, speed is determined by the characteristics of the medium - NOT the frequency.
For EM waves, BOTH the characteristics of the medium and the (light) frequency determine the speed of the wave. (Generally, speed decreases with increasing frequency).

 

[ilovemcat]

Taken from TPR. They worded it better than I did:

Rule 1: The speed of a wave is determined by the type of wave and the characteristics of the medium, not by the frequency.

(Dispersion is an exception to this rule). Light waves traveling through a material medium are an exception to this rule. When light travels through a material (not vacuum), different frequencies will have different speeds.


Rule 2: When a wave passes into another medium, its speed changes, but its frequency does not.