Is it possible to determine public key if you know private key? Is it possible to determine public key if we know private key?

Employees has RFID card and RFID is a private key for employee.

I want to encrypt some user's (employee's) data with employee's public key, but I don't want to anybody read encrypted data without employee's RFID card.

My idea is that If we can use some function which will create public key based on RFID as a private key, than we can store that public key in rest of data of employee so encrypted data can be read only if we have RFID private key.

Is this possible and which function are you suggest?
Since you tagged the question with "rsa": Yes, with rsa it would be very easy to determine the public key (n) from the private key (p &amp; q), because n = p * q.