Yes, it is necessary to solve previous year question papers for UPSC Physics optional subject. Here are the reasons why:

1. Understanding the Exam Pattern:
- Solving previous year question papers helps you understand the exam pattern, the type of questions asked, and the weightage given to different topics.
- It gives you an idea of the format and structure of the questions, allowing you to prepare accordingly.

2. Identifying Important Topics:
- By going through previous year question papers, you can identify the important topics that are frequently asked in the exam.
- This helps you prioritize your preparation and focus more on the topics that carry more weightage in the exam.

3. Time Management:
- Solving previous year question papers helps you develop effective time management skills.
- You can practice solving the questions within the stipulated time frame, which is crucial during the actual exam.
- It helps you improve your speed and accuracy in answering questions.

4. Familiarity with the Exam Environment:
- Solving previous year question papers gives you a feel of the actual exam environment.
- It helps you become comfortable with the exam format, reducing anxiety and nervousness on the exam day.
- This familiarity can significantly improve your performance during the exam.

5. Revision and Reinforcement:
- By solving previous year question papers, you can revise and reinforce the concepts you have studied.
- It helps in retaining the knowledge for a longer duration, as you are applying it to solve real exam questions.
- It also helps in identifying any gaps in your understanding, allowing you to focus on those areas during revision.

6. Self-Assessment:
- Solving previous year question papers allows you to assess your level of preparedness.
- It helps you gauge your strengths and weaknesses in different topics.
- You can identify the areas where you need to put in more effort and improve your performance.

Conclusion:
Solving previous year question papers for UPSC Physics optional subject is essential for effective exam preparation. It helps in understanding the exam pattern, identifying important topics, improving time management skills, getting familiar with the exam environment, revising and reinforcing concepts, and self-assessment. By incorporating this practice into your study routine, you can enhance your chances of scoring well in the UPSC exam.