How light goes through transparent medium but not in opaque
When light is incident on a medium, it interacts with the matter. The matter consists of separate energy levels (atomic, molecular, vibrational, rotational, etc various different kinds), with forbidden regions between them. If energy is provided (through heat, electricity, light, etc) that is more than the energy gap between the levels, excitation of the relevant particles occur, i.e., they absorb the provided external energy and jump to a higher energy level.
Now, simply speaking the following scenarios can occur:
- The external energy is less than the energy gap, so the light is not absorbed and passes through the medium. Transparent. E.g. Glass can pass visible light but absorbs UV light, whereas Quartz can pass UV light. This is because glass has higher energy levels than what visible light provides.
- Energy is more than the energy gap so is absorbed (e.g. dipoles excitation) and then reemitted. If the emitted light is in phase only in one direction, only that light is observed. E.g. fluorescence, phosphorescence, scattering, diffraction.
- The light energy is absorbed and then dissipated in the form of heat or some other form. Opaque.
There are many other aspects and there may be the combination of the above scenarios in the real world. Also, there are sometimes metastable states in the forbidden region, or the energy levels shift due to some perturbation or modification to the medium, which can change it’s nature from transparent to opaque and vice-versa.
For example, in glass, the light is actually absorbed for a moment (levels in forbidde region, energy time uncertainty) and reemitted so there is refraction (refer point 2 above), but the energy is not absorbed because in a larger time scale (larger compared to light frequency and time of transitions, so several orders of magnitude less than 1), the light energy is less than the energy levels. This momentary absorption and reemission is actually why the speed of light is slower in mediums.
When light is incident on a medium, it interacts with the matter. The matter consists of separate energy levels (atomic, molecular, vibrational, rotational, etc various different kinds), with forbidden regions between them. If energy is provided (through heat, electricity, light, etc) that is more than the energy gap between the levels, excitation of the relevant particles occur, i.e., they absorb the provided external energy and jump to a higher energy level.
Now, simply speaking the following scenarios can occur:
- The external energy is less than the energy gap, so the light is not absorbed and passes through the medium. Transparent. E.g. Glass can pass visible light but absorbs UV light, whereas Quartz can pass UV light. This is because glass has higher energy levels than what visible light provides.
- Energy is more than the energy gap so is absorbed (e.g. dipoles excitation) and then reemitted. If the emitted light is in phase only in one direction, only that light is observed. E.g. fluorescence, phosphorescence, scattering, diffraction.
- The light energy is absorbed and then dissipated in the form of heat or some other form. Opaque.
There are many other aspects and there may be the combination of the above scenarios in the real world. Also, there are sometimes metastable states in the forbidden region, or the energy levels shift due to some perturbation or modification to the medium, which can change it’s nature from transparent to opaque and vice-versa.
For example, in glass, the light is actually absorbed for a moment (levels in forbidde region, energy time uncertainty) and reemitted so there is refraction (refer point 2 above), but the energy is not absorbed because in a larger time scale (larger compared to light frequency and time of transitions, so several orders of magnitude less than 1), the light energy is less than the energy levels. This momentary absorption and reemission is actually why the speed of light is slower in mediums.
