A. O$_3$ in stratosphere is not completely effective in shielding us from UV-B
B. UV-A is biologically least harmful.
C. Chapman mechanism deals with only O$_3$ formation in stratosphere.
D. Sunscreen lotions either reflect or absorb or scatter UV-B and UV-A
E. UV-B helps in dissociation of O$_3$ molecule in stratosphere.
Choose the correct answer from the options given below.
Statement A Analysis: Ozone Shielding Effectiveness
The ozone layer ($O_3$) in the stratosphere absorbs most of the harmful UV-C and a significant portion of UV-B radiation. However, it does not completely block UV-B radiation, meaning some UV-B reaches the Earth's surface. Therefore, the statement that $O_3$ is not completely effective in shielding us from UV-B is correct.
Statement B Analysis: UV-A Harmfulness
UV radiation is categorized into UV-A, UV-B, and UV-C based on wavelength. UV-A has the longest wavelength and the lowest energy among the three. While it can cause skin aging and indirect DNA damage, it is considered biologically least harmful compared to UV-B (major cause of sunburn and skin cancer) and UV-C (highly damaging but mostly absorbed by the ozone layer). Thus, this statement is correct.
Statement C Analysis: Chapman Mechanism Scope
The Chapman mechanism is a set of chemical reactions proposed to describe the formation and destruction cycles of ozone ($O_3$) in the Earth's stratosphere. It explains how ozone is created from oxygen and how it is subsequently broken down. Therefore, the statement that it deals with *only* $O_3$ formation is incorrect; it covers both formation and destruction.
Statement D Analysis: Sunscreen Function
Sunscreen lotions protect the skin by physically blocking (reflecting or scattering) or chemically absorbing harmful ultraviolet radiation, specifically UV-A and UV-B. These actions prevent the radiation from penetrating the skin. This statement accurately describes the function of sunscreens.
Statement E Analysis: UV-B and Ozone Dissociation
The energy carried by UV-B photons (as well as UV-C) is sufficient to break the chemical bonds within an ozone ($O_3$) molecule. This process, photolysis, is a key step in the ozone-oxygen cycle, leading to the destruction of ozone in the stratosphere. Hence, UV-B does help in the dissociation of $O_3$. This statement is correct.
Conclusion on Correct Statements
Based on the analysis:
- Statement A is correct.
- Statement B is correct.
- Statement C is incorrect.
- Statement D is correct.
- Statement E is correct.
The correct statements are A, B, D, and E.
Important Questions from Ozone Layer Depletion
- Which of the following is a catalytically inactive (or reservoir) molecule containing chlorine in stratosphere?
Ozone layer depletion is a major phenomenon in :
- With regard to ozone depletion over Antarctica, select the false statement :
- Atmospheric life times of CFCs is typically in the range :
- The Wavelengths that are most important in the context of stratospheric ozone depletion are :
