Why does light intensity decrease when light passes through a glass block?

[Zebadee]

Hi Guys,

I am new to this forum and have a "quick" question.

Say you had a laser pointer and you directed the laser through a glass block and then to a LDR why would the light intensity recorded be lower than without the glass block? What is happening to the light when it is in the glass? And why does making the glass thicker make the light intensity decrease even more?

Thanks a lot.

Oli.

 

[dst]

I'm guessing that the some of the particular photons will have energies corresponding to vibrational modes in the glass (lattice, electronic, molecular, etc). So each time that happens, some light is absorbed.

 

[Zebadee]

So the glass actually absorbs some of the light?

 

[dst]

Yes.

http://plus.maths.org/issue13/features/garbett/index.html

 

[Zebadee]

Thank you very much, I will give this a good read.

 

[Claude Bile]

Increasing the thickness of a glass layer would increase losses due to absorption and scattering since both are dependent on length.

You also get Fresnel reflections, which is about 4% of the incident light reflected per interface for glass.

Claude.

 

[Andy Resnick]

Hi Guys,

I am new to this forum and have a "quick" question.

Say you had a laser pointer and you directed the laser through a glass block and then to a LDR why would the light intensity recorded be lower than without the glass block? What is happening to the light when it is in the glass? And why does making the glass thicker make the light intensity decrease even more?

Thanks a lot.

Oli.

the glass scatters light out of the beam direction. Unless you are working with optical-quality glass, glass is fairly heterogeneous.

Even so, 'perfectly manufactured' glass is oh-so-slightly absorptive.

Plot the intensity as a function of glass thickness- it should be exponential (Beer's law)

 

[Claude Bile]

Even so, 'perfectly manufactured' glass is oh-so-slightly absorptive.

Indeed, the best glass for optical transmission (silica optical fibre) has losses of about 0.1 dB per km, most of which is due to absorption.

Claude.