How is it a substance can emit photons of visible light after absorbing ultra violet photons but not with infrared

radiation

The re-emitted photons must be of lessor energy (longer wavelength) than the incident photons.

In a one-photon absorption process, the absorbed photon must have more energy that the re-emitted visible photon. Infrared phtns have less energy thn visible ones because of their lower frequency.

When you irradiate a material with high-intensity infrared laser light, it IS possible to obtain visible emission, with negligible heating, because of multiple-photon absorption and scattering processes that can occur. You do not see this occur in everyday processes, but I have observed it in the laboratry. So, the premise of your question is not quite correct. Tell your teacher and score some extra brownie points.

To understand why a substance can emit photons of visible light after absorbing ultraviolet photons but not with infrared radiation, we need to consider the concept of energy levels and transitions in atoms or molecules.

When a substance absorbs a photon of a specific energy, it can cause an electron to transition to a higher energy level. This is known as excitation. The energy of the absorbed photon must match the energy difference between the initial and final energy levels. In the case of ultraviolet (UV) photons, they have higher energy compared to visible or infrared photons.

Once an electron is in the excited state, it will eventually return to its original energy level or a lower energy level. This return to a lower energy level is accompanied by the emission of a photon. The energy of the emitted photon will be equal to the energy difference between the excited state and the lower energy level. This emitted photon can be in the visible range if the energy difference corresponds to visible light.

Now, let's consider why substances do not emit visible light after absorbing infrared radiation. Infrared photons have lower energy compared to visible or UV photons because they have lower frequencies. Therefore, when a substance absorbs an infrared photon, it can cause an electron to transition to a higher energy level, but this higher energy level is typically not within the visible range. As a result, when the electron returns to its original or a lower energy level, it emits a photon with lower energy, which falls in the infrared range.

However, it is important to note that the scenario I just described applies to one-photon absorption processes. In certain cases, under specific conditions, multiple-photon absorption and scattering processes can occur. This means that a substance irradiated with high-intensity infrared laser light could potentially emit visible light, even though the absorbed photons were in the infrared range. However, these processes are not commonly observed in everyday processes and typically require special conditions and equipment to be observed in the laboratory.

In summary, the energy of the emitted photon after absorption depends on the energy difference between the initial and final energy levels. Ultraviolet photons can lead to visible light emission because they have enough energy to excite electrons to higher energy levels within the visible range. Infrared photons, on the other hand, have lower energy, and therefore, substances generally emit infrared photons rather than visible light after absorbing them.