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What really is reflection? Is it just the reemission of EMR? I asked my teacher, he said in quantum sense, it is true. But when I read something about emissivity in Stefan Boltzmann's equation, it states that if an object reflects most light, then it absorbs and reemits less. So what it said means reflection and absorption and reemission are different concepts.

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Reflected light can be though of as originating in oscillating charges in the medium. The incident light causes the charges to oscillate, and the oscillators generate the reflected light. This process happens almost instantaneously. The atoms in the medium are oscillating coherently (in step) with the incident radiation. The frequency of the light is such that none (well, very little) energy is deposited in the medium.

But some radiation is absorbed, especially if the energy of the radiation matches energy level spacings in the medium. Absorption means (almost by definition) that the energy of the light is diverted from coherent oscillation of the atoms in the medium. How this is described classically is a somewhat long story, but if you allow for quantum mechanics, you can say that the atoms are excited to a higher energy level, removing the energy from the incident light. In reflection, the atom merely oscillates without being excited to a higher level.

The diverted energy can go almost anywhere from there. It can re-radiate in some other direction (scattering) or the energy can be transferred to other atoms by collisions (without radiation). Often the energy is ultimately distributed into vibrations of many atoms, and the temperature of the medium rises. Whatever happens, the process is not "nearly instantaneous" the way it is for reflection.

In a nutshell, reflection is coherent and nearly instantaneous. Absorption/re-emission is incoherent, and is not quite instantaneous (but it can be very fast).

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