What is happening at a atomic level when light hits an object?

Question

I just began learning light and optics and I am just so confused about what reflection, refraction and absorption is. I think a large part of my confusion stems from exactly what happens at an microscopic level when light hits an objects that causes it to be either reflected, refraction and absorbed? Like during reflection, does the object not absorb and transmit light at all, causing it to have no choice but to bounce back?

Answer 1

I just began learning light and optics and I am just so confused about what reflection, refraction and absorption is.

Below I will assume there are two media with different indices of refraction: the first medium is the one from which the incident light arrives and the second medium is that which the incident light either reflects off of or refracts and propagates through.

• Reflection is the process by which a fraction of photons incident upon a second medium are returned to the medium from which they originated. Reflectance is the term used to describe the efficiency/effectiveness of reflecting electromagnetic radiation by a given material.
• Transmission is the process by which a fraction of photons incident upon a second medium continue to propagate into the second medium across the boundary. Transmittance is the term used to describe the efficiency/effectiveness of allowing incident radiation to propagate.
• Refraction is the term used to imply a change in wave propagation direction due to a change in the medium through which the radiation propagates (e.g., light crossing from air-to-water). The refractive index or index of refraction is a vector that describes how the radiation propagates through a medium, i.e., $\mathbf{n} = \tfrac{\mathbf{k} \ c}{\omega}$, where $\mathbf{k}$ is the wave vector, $c$ is the speed of light in vacuum, and $\omega$ is the wave frequency.
• Absorption is the process by which a fraction of photons incident upon a second medium are converted to kinetic energy of atoms (typically, the electrons orbiting the nucleus of an atom are the beneficiaries of this new mechanical energy). The energy can be converted to electron kinetic energy (e.g., altering electron orbitals) or internal energy of the absorbing medium, e.g., increased thermal energy. Absorbance is the term used to describe the loss of radiation to the medium during transmission. This is often synonymous to attenuation for linear media (i.e., the superposition principle applies to any given set of electromagnetic waves)

I think a large part of my confusion stems from exactly what happens at an microscopic level when light hits an objects that causes it to be either reflected, refraction and absorbed?

On a classical level, some of the incident radiation is almost always reflected, transmitted, and absorbed/attenuated (e.g., wave amplitude decreases as it propagates through the medium due to absorbed photons converting to internal energy of the medium). In most cases during your first introduction to optics, you can entirely ignore absorption/attenuation. Thus, you need only calculate the reflection and transmission coefficients to determine the ratios of reflected and transmitted to the incident radiation.

Like during reflection, does the object not absorb and transmit light at all, causing it to have no choice but to bounce back?

At a quantum level, things are a little different but here is the sugar-coated version using the example of sunlight hitting a typical, green leaf. Let's ignore the sun's complex light spectrum for the moment and just assume nice black-body radiation.

When you see a green leaf on a tree/plant, you are seeing the specific range of frequencies/wavelengths reflected by the material in the leaf (i.e., chlorophyll). You see only green because the medium absorbed and/or transmitted the other frequencies/wavelengths. If you held a leaf over a white background next to a completely opaque object (e.g., steel disk), you may be able to notice that the shadow cast by the leaf is not as dark as that of the opaque object (one needs to limit light so it only hits the leaf prior to the white background otherwise scattering from adjacent locations can cause similar effects). This is due to some of the light being transmitted through the leaf. The rest is absorbed and converted to internal energy, e.g., heat (basically, the particles jostle/oscillate faster resulting in a higher mean random kinetic energy).

At an atomic level, when photons are incident on a new medium, they are absorbed by the atoms/electrons and then re-emitted or converted to internal energy.

The re-emitted photons can be at the same frequency/wavelength as when they were absorbed or at a different frequency/wavelength. In the former case, the absorbing atom/electron is effectively unchanged by the interaction. In the latter case, the absorbing electron needs to change its energy to account for the different energy of the re-emitted photon (e.g., change orbital level).

The absorption and re-emission process is stochastic, thus, the direction of the re-emitted photon can be random relative to its incident direction. Statistically, some photons are re-emitted propagating back into the medium from which they originated while others try to go further into the material. The ensemble average of all these absorptions and re-emissions leads to the macroscopic, classical approximations we call optics.