Hi gsonu953 ,
The answer to the derivations that you have posted can be found in the NCERT-PART II text book.
1. Describe the young's double slit experiment and derive the expression for the fringe width
Kindly refer to page number 363 and 364 for your answer.It explains all the details needed for your query.
2. Verify laws of reflection and refraction by Huygens wave theory
Kindly refer to page number 355, 356 and 357 for your answer.
3. You can find the answer to the third question in the page number 380, 381.
Answers to all the questions you have asked can be found exactly in the textbook with all the proper explanations needed. Please go through them and if you have any doubt regarding the concepts, we would be glad to help you with it.