What is the ruling on completing a mixed-gender physics degree in universities and working in its mixed-gender field? Is the earning from mixed-gender education or teaching unlawful? And is it better to switch to studying medicine to avoid mixed-gender environments and reduce interactions with females?

Mixing of genders in schools and universities is a reprehensible act, and it is not permissible to study in them unless it is the only way to acquire knowledge and there is no other option. In such a case, it is permissible, provided the student adheres to the etiquette of Islamic law, such as lowering the gaze and avoiding seclusion. It cannot be said that studying in them is absolutely forbidden due to the hardship involved, and because acquiring worldly knowledge is a communal obligation (fard kifayah). Earning from teaching in such institutions is not forbidden because it is in exchange for teaching knowledge, not for gender mixing. If you are able to study in a non-mixed university, then do so. Otherwise, it is permissible for you to study in a mixed one, with the obligation of lowering your gaze, avoiding seclusion, and keeping away from intermingling with women. It is not obligatory for you to change your specialization if you would lose three years of study.