At what angle does the beam of light fall into the mirror if the beam is reflected makes an angle of 40 ° with that mirror​

The angle of incidence is the angle formed by the incident ray and the perpendicular to the surface. Together, the angle of incidence and the angle the incident ray makes with the surface form a right angle (90 degrees). The angle of incidence is therefore given by the following expression:

Angle of incidence = 90 degrees - angle ray makes with the surface

In your example

Angle of incidence = 90 degrees - 40 degrees = 50 degrees

The law of reflection states that the angle of incidence equals the angle of reflection, which is also measured with respect to the perpendicular to the surface, so

Angle of reflection = Angle of incidence = 50 degrees.

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