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Effect of light and sound to opaque object

  • Thread starter songoku
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  • #1
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Homework Statement


An opaque object 10 cm wide casts a shadow when placed in a beam of light but has little effect on a beam of sound emitted by a small source of frequency 500 Hz. This is because
a. sound is a pressure wave whereas light is an electromagnetic waves
b. sound travels much more slowly than light
c. sound waves are longitudinal whereas light waves are transverse
d. sound waves have a much longer wavelength than light waves
e. the power per unit area in a beam of sound is much lower than that in a beam of light

Homework Equations


Don't know


The Attempt at a Solution


I don't know the relation between the choices and the question. I know that sound is longitudinal and light is transverse, I know the speed of sound and light. But I don't know how to use the info to answer the question.

Thanks
 

Answers and Replies

  • #2
ehild
Homework Helper
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Think of the difference of wavelengths and Huygens–Fresnel principle and diffraction of waves.

ehild
 
  • #3
with a huge wavelength in the case of sound wave as compared to light waves,the pressure waves can really move over the edges of the object.
 
  • #4
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Think of the difference of wavelengths and Huygens–Fresnel principle and diffraction of waves.

ehild
I read a little about Huygens-Fresnel principle at wiki.
"The Huygens–Fresnel principle is a method of analysis applied to problems of wave propagation (both in the far field limit and in near field diffraction). It recognizes that each point of an advancing wave front is in fact the center of a fresh disturbance and the source of a new train of waves"

But I still don't know how to relate it with the question. Huygens-Fresnel principle says that there will be a new center of disturbance. So when the sound hits the opaque object, there will be a new center of disturbance at the object's surface. Why don't the sound being reflected?

with a huge wavelength in the case of sound wave as compared to light waves,the pressure waves can really move over the edges of the object.
OK, based on your hint, I think the answer is (d). But why with a huge wavelength in the case of sound wave as compared to light waves,the pressure waves can really move over the edges of the object?

Thanks
 

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