Problem 22 - Chemistry: The Central Science in SI Units, Global Edition

Question

The ultraviolet spectrum can be divided into three regions based on wavelength: UV-A (315-400 nm), UV-B (280-315 \(\mathrm{nm})\), and UV-C \((100-280 \mathrm{nm})\). (a) Photons from which region have the highest energy and therefore are the most harmful to living tissue? (b) In the absence of ozone, which of these three regions, if any, are absorbed by the atmosphere? (c) When appropriate concentrations of ozone are present in the stratosphere, is all of the UV light absorbed before reaching the Earth's surface? If not, which region or regions are not filtered out?

Step-by-Step Solution

Answer

(a) The UV-C region has the highest energy photons and is the most harmful to living tissue. (b) In the absence of ozone, the atmosphere absorbs the UV-C region. (c) When appropriate concentrations of ozone are present in the stratosphere, not all of the UV light is absorbed before reaching the Earth's surface. The UV-A region is not fully filtered out and can still reach the Earth's surface.

Step 1: Recall the photon energy formula

First, let's recall the formula for photon energy. The energy of a photon is given by the equation \(E = \dfrac{hc}{λ}\), where E is the energy, h is the Planck's constant (\(6.63 x 10^{-34} Js\)), c is the speed of light (\(3 x 10^8 m/s\)), and λ is the wavelength.

Step 2: Identify the region with the highest energy photons

We are given the wavelength range for each UV region: - UV-A (315-400 nm) - UV-B (280-315 nm) - UV-C (100-280 nm) Since energy and wavelength are inversely proportional, the shorter the wavelength, the higher the energy of photons. Therefore, by comparing the wavelength ranges, we can conclude that the UV-C region has the shortest wavelengths and hence the highest energy photons, making it the most harmful to living tissue. Answer to part (a): The UV-C region has the highest energy photons and is the most harmful to living tissue.

Step 3: Identify the regions absorbed by the atmosphere

Part (b) of the problem asks us to determine which of these three regions, if any, are absorbed by the atmosphere in the absence of ozone. The atmosphere is composed of different gases and particles, which primarily absorb wavelengths in the UV-C region. So, in the absence of ozone, the atmosphere absorbs UV-C region. Answer to part (b): In the absence of ozone, the atmosphere absorbs the UV-C region.

Step 4: Determine the effect of ozone on the UV light absorption

Now, we need to analyze the effect of having appropriate concentrations of ozone in the stratosphere (part c). The ozone layer absorbs most of the harmful UV radiation, especially in the UV-C and UV-B regions since these have shorter wavelengths and higher energies. However, the ozone layer is not able to completely absorb all UV radiation, and some UV-A radiation can still reach the Earth's surface, as its wavelengths are relatively longer and have lower energy. Answer to part (c): When appropriate concentrations of ozone are present in the stratosphere, not all of the UV light is absorbed before reaching the Earth's surface. The UV-A region is not fully filtered out and can still reach the Earth's surface.

Key Concepts

Photon Energy

Photon energy is a fundamental concept in understanding how light interacts with matter. It refers to the energy carried by a single photon, which is a quantum of light. This energy determines various properties like its ability to affect biological tissues, chemical structures, or other forms of matter. The formula for calculating the energy of a photon is \[ E = \frac{hc}{\lambda} \] where:

• \(E\) is the photon energy,
• \(h\) is Planck's constant (\(6.63 \times 10^{-34} \text{J}\cdot\text{s}\)),
• \(c\) is the speed of light (\(3 \times 10^8 \text{m/s}\)),
• \(\lambda\) is the wavelength of the photon.

In essence, the energy of a photon is inversely related to its wavelength. The shorter the wavelength, the higher the energy. This is critical in explaining why certain types of radiation are more harmful than others. For example, UV-C radiation has shorter wavelengths, and hence, more potent photon energy capable of causing significant biological damage.

Wavelength

The wavelength of light is the distance between consecutive peaks in a wave of electromagnetic radiation. It is a crucial property that defines different types of light, including ultraviolet (UV) radiation. Wavelength is typically measured in nanometers (nm) for UV light, and it fundamentally influences how we perceive light and how it interacts with matter.

• UV-A ranges from 315 to 400 nm.
• UV-B spans from 280 to 315 nm.
• UV-C extends from 100 to 280 nm.

Because photon energy is inversely proportional to wavelength, shorter wavelengths correspond to more energetic and potentially more harmful photons. For instance, UV-C has the shortest wavelengths, which is why it carries the highest energy among UV types, making it most hazardous to living tissues.

Atmospheric Absorption

Atmospheric absorption refers to the process by which atmospheric gases and particles intercept and absorb electromagnetic radiation, preventing it from reaching the Earth's surface. Various components of the atmosphere, such as oxygen and ozone, play different roles in absorbing UV radiation.

In the absence of ozone, natural atmospheric gases still absorb a significant portion of UV-C radiation due to its higher energy and shorter wavelengths. This means that even without the ozone layer, some dangerous levels of UV-C are mitigated by the atmosphere. On the other hand, UV-A and UV-B, with longer wavelengths and lower energies, are less likely to be absorbed by the atmosphere alone and can reach the surface more readily.

Ozone Layer

The ozone layer is a critical component of Earth's stratosphere, located approximately 10 to 30 kilometers above the Earth's surface. It contains a high concentration of ozone (O3) molecules, making it an essential shield against potentially harmful solar radiation.

• It absorbs the majority of the Sun's UV-C and UV-B radiation, preventing these high-energy waves from reaching the surface.
• This protective function helps mitigate skin cancer risk, cataract formation, and other health issues in living organisms.
• Despite its effectiveness, the ozone layer cannot completely block UV-A radiation, allowing some of it to reach the ground.

Protecting the ozone layer through environmental policies and practices is vital for reducing UV-related hazards and preserving ecological balance.

UV Spectrum Regions

The ultraviolet (UV) spectrum is divided into three main regions: UV-A, UV-B, and UV-C, based on their wavelengths and energy levels. Each of these regions interacts differently with Earth's atmosphere and biota.

• UV-A (315-400 nm): This region has the longest wavelengths with the lowest energy level, making it less harmful compared to others. It often reaches Earth's surface and contributes to tanning and premature skin aging.
• UV-B (280-315 nm): This region has moderate energy levels and can harm living cells, causing sunburns and DNA damage. It is partially absorbed by the ozone layer.
• UV-C (100-280 nm): The shortest wavelength region, with the highest energy, is mostly absorbed by the ozone layer and atmosphere, preventing it from reaching Earth's surface.

Understanding these regions is important for gauging the potential impact of UV radiation on health and the environment. Measures like monitoring ozone concentration and applying sun protection can help mitigate UV risks.