Can a person taking their undergraduate degree in humanities, apply for a masters in scientific fields?
Check the stated requirements of the program you're interested in. There's no easy answer.

Example: if you're interested in a Masters in Biology @ Harvard Extension School, then you're in luck because their website says:


  Eligibility: Before enrolling in any degree-applicable courses, you need to possess a four-year, US regionally accredited bachelor's degree or its foreign equivalent. You cannot already have or be in the process of earning a master's degree in biology or related field.


So you should be in the clear. However if you want to do Aeronautics and Astronautics @ MIT, then:


  You must have a strong background in mathematics and physics, and, with rare exception, familiarity with an engineering discipline. In some cases, unfulfilled entrance requirements may be satisfied during your first year of graduate study. If you're radically changing fields, advanced study before applying is recommended. You should also address your reasons for changing you field when you are writing your Statement of Objectives, a very important part of our graduate application.


It's up to you to assess if you have a strong background in mathematics &amp; physics from your humanities degree, and if so, to convince the admissions people.