Final answer:
Q1 must be a positive charge and Q2 must be a negative charge because the electric field points from Q2 to Q1, following the convention that field lines originate from positive charges and end at negative charges.
Step-by-step explanation:
The question concerns two charges, Q1 and Q2, which are equal in magnitude, and the electric field is said to be pointing from Q2 to Q1 when measured at the midpoint between them. This scenario suggests that Q2 must be negatively charged and Q1 must be positively charged. The reason is that electric field lines originate from positive charges and terminate at negative charges. Given that the direction of the electric field mid way is from Q2 to Q1, it implies that the electric field lines start from Q1 and end at Q2, reinforcing the idea of Q1 being positive and Q2 being negative.