In HCV Q-8 (photoelectric effect) we’ve not taken into consideration the 70% light that is reflected for calculating the momentum. BUT in Q-7 we have taken it so . Is this not contradictory. explain where am I wrong.

It's a common point of confusion when studying the photoelectric effect and how we account for light reflection in different scenarios. Let's break this down to clarify why the treatment of light reflection differs between the two questions you've mentioned.

Understanding the Photoelectric Effect

The photoelectric effect describes the phenomenon where electrons are emitted from a material (usually a metal) when it absorbs light of sufficient energy. The key factor here is the energy of the incoming photons, which must exceed a certain threshold to liberate electrons from the material's surface.

Momentum and Light Reflection

When light strikes a surface, it can either be absorbed or reflected. The momentum of light is given by the equation:

• Momentum (p) = Energy (E) / c

where c is the speed of light. When light is absorbed, its momentum is transferred to the material, contributing to the momentum of the emitted electrons. However, when light is reflected, the situation changes slightly.

Analyzing the Two Questions

In Q-7, if you accounted for the 70% reflection, it likely involved a scenario where the total momentum change needed to be considered, including both absorbed and reflected light. This is crucial in calculating the net momentum imparted to the system.

In contrast, in Q-8, if the problem specifically focused on the electrons emitted due to the absorbed light, it might have simplified the situation by assuming that only the absorbed portion contributes to the momentum of the emitted electrons. This is a common approach in problems where the primary focus is on the energy transfer leading to electron emission.

Why the Difference Matters

The key takeaway is that the treatment of light reflection depends on the context of the problem. If the question emphasizes the energy and momentum of the emitted electrons, it may not require considering the reflected light. However, if the question involves a broader analysis of momentum transfer in the entire system, then accounting for reflection becomes essential.

Conclusion

In summary, the apparent contradiction arises from the specific focus of each question. In Q-8, the emphasis might be solely on the absorbed light's contribution to the photoelectric effect, while Q-7 takes a more comprehensive view of momentum transfer. Understanding the context and the specific requirements of each problem is crucial in physics, especially in topics like the photoelectric effect.