Introduction
Yes, you can rely solely on the NCERT books for UPSC Physics Optional preparation. The National Council of Educational Research and Training (NCERT) books are designed to provide a strong foundation in the subject and cover the basic concepts in a comprehensive manner. However, it is important to supplement your preparation with other resources to ensure a thorough understanding of the subject.

Benefits of NCERT books
1. Strong foundation: NCERT books are known for their clarity and simplicity, making it easier for students to grasp the fundamental concepts of Physics.
2. Conceptual clarity: The NCERT books provide a conceptual understanding of various topics, which is crucial for answering UPSC Physics Optional questions.
3. Comprehensive coverage: The books cover a wide range of topics in physics, ensuring that you have a holistic understanding of the subject.
4. Authenticity: NCERT books are published by the government and are considered to be reliable sources of information for UPSC exam preparation.

Supplementary resources
While NCERT books are a great starting point, it is advisable to supplement your preparation with additional resources to enhance your knowledge and problem-solving skills. Some recommended resources include:

1. Reference books: Apart from NCERT books, you can refer to standard textbooks such as "Concepts of Physics" by HC Verma, "University Physics" by Young and Freedman, and "Fundamentals of Physics" by Resnick and Halliday. These books provide a deeper understanding of the subject and include advanced topics.
2. Previous years' question papers: Solving previous years' question papers will give you an idea of the type of questions asked in the UPSC Physics Optional exam. It will also help you identify your weak areas and work on them.
3. Online resources: There are several educational websites, such as EduRev, that provide study materials, video lectures, and practice questions for UPSC Physics Optional preparation. These resources can supplement your learning and help you revise and practice effectively.

Conclusion
While NCERT books are a valuable resource for UPSC Physics Optional preparation, it is essential to supplement your learning with additional resources. This will help you gain a deeper understanding of the subject and improve your problem-solving skills. Make sure to use a combination of NCERT books, reference books, previous years' question papers, and online resources for a comprehensive and effective preparation strategy.