[amalia (OP)]

Charlie got in contact with us to ask a good question:
Since everything is made up of electrons, neutrons and protons, why are some things transparent (like glass) and other things completely opaque?
Do you know the answer?

[Bored chemist]

This may help
https://www.thenakedscientists.com/forum/index.php?topic=21306.msg238377#msg238377

[hamdani yusuf]

This may help
https://www.thenakedscientists.com/forum/index.php?topic=21306.msg238377#msg238377

The most important feature about copper that makes it opaque is that it is a conductor of electricity.  This is because the atoms in the solid are bonded together in such a way to allow the outer electrons freedom of movement through the material.  Because it is a conductor it effectively "shorts out" the electrostatic part of electromagnetic waves (light)and this reflects the energy from the surface and prevents it from flowing through the body of the copper in a coherent way. 

Now glass is an insulator and electromagnetic energy can enter the material but that's not enough to make it transparent The next requirement is that the material itself does not absorb the frequencies of light that are being used.  Many insulators are coloured because they absorb light of different frequencies.

Finally there is one important additional requirement.  Glass is a uniform material and is often prepared with a smooth surface this ensures that the light is not dispersed through the material and glass is transparent.  Only a very slight disturbance of the surface will make the glass frosted or obscured.

The answer above still leaves open questions on why transparent materials like glass don't absorb visible light.

When light interacts with matter, it can be :
- reflected
- refracted
- scattered to different directions. The scattering could be predominantly reflective or refractive.
- absorbed, which can then be :
  - reemitted in different frequency (fluorescent, fosforescence)
  - converted to different form of energy, such as heat, chemical, mechanical, electrical energy.

[Bored chemist]

This may help
https://www.thenakedscientists.com/forum/index.php?topic=21306.msg238377#msg238377

The most important feature about copper that makes it opaque is that it is a conductor of electricity.  This is because the atoms in the solid are bonded together in such a way to allow the outer electrons freedom of movement through the material.  Because it is a conductor it effectively "shorts out" the electrostatic part of electromagnetic waves (light)and this reflects the energy from the surface and prevents it from flowing through the body of the copper in a coherent way. 

Now glass is an insulator and electromagnetic energy can enter the material but that's not enough to make it transparent The next requirement is that the material itself does not absorb the frequencies of light that are being used.  Many insulators are coloured because they absorb light of different frequencies.

Finally there is one important additional requirement.  Glass is a uniform material and is often prepared with a smooth surface this ensures that the light is not dispersed through the material and glass is transparent.  Only a very slight disturbance of the surface will make the glass frosted or obscured.

The answer above still leaves open questions on why transparent materials like glass don't absorb visible light.

When light interacts with matter, it can be :
- reflected
- refracted
- scattered to different directions. The scattering could be predominantly reflective or refractive.
- absorbed, which can then be :
  - reemitted in different frequency (fluorescent, fosforescence)
  - converted to different form of energy, such as heat, chemical, mechanical, electrical energy.

The answer above still leaves open questions on why transparent materials like glass don't absorb visible light.

[hamdani yusuf]

This may help
https://www.thenakedscientists.com/forum/index.php?topic=21306.msg238377#msg238377

The most important feature about copper that makes it opaque is that it is a conductor of electricity.  This is because the atoms in the solid are bonded together in such a way to allow the outer electrons freedom of movement through the material.  Because it is a conductor it effectively "shorts out" the electrostatic part of electromagnetic waves (light)and this reflects the energy from the surface and prevents it from flowing through the body of the copper in a coherent way. 

Now glass is an insulator and electromagnetic energy can enter the material but that's not enough to make it transparent The next requirement is that the material itself does not absorb the frequencies of light that are being used.  Many insulators are coloured because they absorb light of different frequencies.

Finally there is one important additional requirement.  Glass is a uniform material and is often prepared with a smooth surface this ensures that the light is not dispersed through the material and glass is transparent.  Only a very slight disturbance of the surface will make the glass frosted or obscured.

The answer above still leaves open questions on why transparent materials like glass don't absorb visible light.

When light interacts with matter, it can be :
- reflected
- refracted
- scattered to different directions. The scattering could be predominantly reflective or refractive.
- absorbed, which can then be :
  - reemitted in different frequency (fluorescent, fosforescence)
  - converted to different form of energy, such as heat, chemical, mechanical, electrical energy.

The answer above still leaves open questions on why transparent materials like glass don't absorb visible light.

In short, electric charges in glass don't resonate well in visible light spectra. Different kind of transparent materials have different response to incoming light. Some type of glass are transparent in infrared range, while some other in ultraviolet.
Their response depend on the arrangement of the electric charges in the materials. AFAIK there is no simple way to predict how they will response to incoming light since many factors can influence them such as temperature, pressure, phase, electric and magnetic field.