After B.Sc Physics, you can apply for the post of Technician Assistant. This again depends on the vacant positions available. ISRO also accomodates promotions to Candidates who joined with B.Sc Physics qualification provided if they improve their educational qualification through Correspondence courses. For example, if you join ISRO as their Technician Assistant with B.Sc qualification but complete your Masters or Ph.D through Correspondence, should there be a vacant position and if you are an eligible Candidate, ISRO certainly considers you for that position. You should keep yourself updated with opportunities available at their official website.