In this study it is investigated to what extent the use of the Thought Experiment (TE) "Einstein's elevator" in the teaching of the "principle of equivalence" can help students of the upper secondary school to approach this principle and some of its consequences. This work is a learning process study to investigate students learning processes during the teaching of "principle of equivalence" by the help of the TE as it is presented by Einstein himself in books that popularize physics theories. It is concluded that students of the upper secondary school, starting from their experiences and being helped by the TE, can be led to the formulation of the "principle of equivalence", are able to explain physical phenomena based on this principle and reach the conclusion that "the deflection of light in the gravitational fields" is a consequence of this principle.