In this experiment, the students determine the wavelength λ of the laser with the help of two-beam interference using Fresnel's double mirror experiment. This experiment is one of the methods developed by Augustin Jean Fresnel, along with the biprism experiment, to generate 2 coherent light beams and bring them to interference. Even though the two experimental setups differ slightly from each other, the basic geometric relationships are the same in both experiments. Because of this, the further tasks of the two experiments are chosen differently. During the experiment with the Fresnel biprism, the refractive angle is determined. Following the measurements with the double mirror, the topic of coherence is addressed.