How does attenuation happen? Very confused

[Clara Chung]

How can light flux decrease after traveling in a medium without scattering it? Does attenuation mean scattering?
Also in Infrared spectroscopy, why does the intensity of light decrease when the molecule is vibrating in resonant frequency? Does the electron produce an E field opposite to the incoming E field of the EM wave so that they cancel out? How can we explain it in terms of the particle nature of light?

 

[DrClaude]

How can light flux decrease after traveling in a medium without scattering it?

Absorption.

Also in Infrared spectroscopy, why does the intensity of light decrease when the molecule is vibrating in resonant frequency? Does the electron produce an E field opposite to the incoming E field of the EM wave so that they cancel out? How can we explain it in terms of the particle nature of light?

A molecule will absorb light with which it is resonant. Some of that light will be reemitted (the rest will eventually become thermal energy), but looking in the direction of the incoming light will show a great reduction in intensity. In most cases, you can treat the light as completely classical. The absorption by the molecule will correspond to the disappearance of one photon from the light field.

 

[Andy Resnick]

How can light flux decrease after traveling in a medium without scattering it? Does attenuation mean scattering?
Also in Infrared spectroscopy, why does the intensity of light decrease when the molecule is vibrating in resonant frequency? Does the electron produce an E field opposite to the incoming E field of the EM wave so that they cancel out? How can we explain it in terms of the particle nature of light?

To supplement DrClaude's response, 'attenuation' can also refer to the process by which light (or sound) is scattered out of the initial propagation direction. Beer's law is an example of this.