Why does light travel through glass

I was curious too. This is the best description I could find:

An optical property describes the way a material reacts to exposure to light. Visible light is a form of electromagnetic radiation with wavelengths in the range of 400 to 700 nm corresponding to an energy range of 3.1 to 1.8 electron volts (eV) (from E = hc/, where c = 3 x 10i¹⁷ nm/s and h = 4.13 x 10^-15 eV s).

When light strikes an object it may be transmitted, absorbed, or reflected. Materials vary in their ability to transmit light, and are usually described as transparent, translucent, or opaque. Transparent materials, such as glass, transmit light with little absorption or reflection. Materials that transmit light diffusely, such as frosted glass, are translucent. Opaque materials do not transmit light.

Two important mechanisms for the interaction of light with the particles in a solid are electronic polarizations and transitions of electrons between different energy states. The distortion of the electron cloud of an atom by an electric field, in this case the electric field of the light, is described as polarization. As a result of polarization, some of the energy may be absorbed, i.e., converted into elastic deformations (phonons), and consequently heat. On the other hand, the polarization may propagate as a material-bound electromagnetic wave with a different speed than light. When light is absorbed and reemitted from the surface at the same wavelength, it is called reflection. Metals, for example, are highly reflective, and those with a silvery appearance reflect the whole range of visible light. The energy levels of electrons are quantized, i.e., each electron transition between levels requires a certain specific amount of energy. The absorption of energy results in the shifting of electrons from the ground state to a higher, excited state. The electrons then fall back to the ground state, accompanied by the reemission of electromagnetic radiation.

In nonmetals, the lower energy bonding orbitals make up what is called the valence band, and the higher energy antibonding orbitals form the conduction band. The separation between the two bands is the band gap energy, and is generally large for nonmetals, smaller for semiconductors, and nonexistent in metals.

The energy range for visible light is from 1.8 to 3.1 eV. Materials with band gap energies in this range will absorb those corresponding colors (energies) and transmit the others. They will appear transparent and colored. For example, the band gap energy of cadmium sulfide photocells is about 2.4 eV and so it absorbs the higher energy (blue and violet) components of visible light. It has a yellow-orange color as a result of the transmitted portions of the spectrum. This type of light-induced conductivity is called photoconductivity.

Materials with band gap energies less than 1.8 eV will be opaque because all visible light will be absorbed by electron transitions from the valence to the conduction band. Dissipation of this absorbed energy may be by direct return to the valence band or by more complicated transitions involving impurities. Pure materials with band gap energies greater than 3.1 eV will not absorb light in the visible range and will appear transparent and colorless.

Light that is emitted from electron transitions in solids is called luminescence. If it occurs for a short time it is fluorescence, and if it lasts for a longer time it is phosphorescence.

Light that is transmitted from one medium into another, such as from air into glass, undergoes refraction. This is the apparent bending of light rays that results from the change in speed of the light. The index of refraction (n) of a material is the ratio of the speed of light in a vacuum (c = 3 x 10⁸ m/s) to the speed of light in that material (n = c/v). The change in speed is the result of electronic polarization. Since the effect of polarization increases with the size of the atoms, glasses which contain heavy metal ions (such as lead crystal) have higher indices of refraction than those composed of smaller atoms (such as soda-lime glass).

Figure 5: This figure represents the refraction of light as it passes from a medium with low optical density (such as air) to one of higher optical density (such as water or glass). Light maintains its frequency but its speed is changed in the more dense medium. Therefore, the wavelength must change accordingly. Snell's law (n₁ sin q₁ = n₂ sin q₂) can be used to relate the indices of refraction (n), the angles (q) of incidence and refraction, and the speed (v) of light in the two media: n₁/n₂ = q₂/q₁ = v₁/v₂)

Internal scattering of light in an inherently transparent material may render a material translucent or opaque. Such scattering occurs at density fluctuations, grain boundaries, phase boundaries, and pores.