in this video we're going to talk about the law of reflection so what exactly is that what is its state the law of reflection states that the angle of incidence is equal to the angle of reflection so as an example let's say if we have a flat surface and let's say this is the normal line which is perpendicular to the surface and let's say if we have a light ray that strikes the surface at an angle of incidence of 30 degrees the angle of incidence is between the light ray and the normal line according to the law of reflection the ray is going to bounce back at the same angle so the angle of reflection which is also between the normal line and the light ray that's going to be 30 degrees and so that's the law of reflection now keep in mind these two angles are complementary the normal line always forms a right angle with the surface so this angle is 60 and this angle is 60. now let's work on this problem a light ray strikes the mirror at an angle of 30 degrees from the horizontal what is the angle x where the ray leaves the second mirror so here this is the first mirror and this is the second mirror so this is the incident ray and let's start by calculating the angle of incidence so we know that the normal line makes a 90 degree angle with the surface so these two angles add up to 90. so 90 minus 30 is 60 and so that's the angle of incidence and based on the law of reflection the angle of incidence is equal to the angle of reflection so the angle of reflection for the first mirror is also 60 which means this is 30. now the three angles of a triangle must add to 180 so what is this missing angle which we'll call y well 30 plus 130 plus y is equal to 180 now 30 plus 130 is 160 and so now we need to subtract both sides by 160 so 180 minus 160 is 20 and so that's the missing angle here so the angle of incidence for the second mirror is going to be 90 minus 20 which is 70. and the angle of reflection for the second row is 70 as well which means x is 90 minus 70 so x is 20 degrees so that's the angle that the reflected ray leaves the second mirror so the angle of reflection for the second mirror is 70 but the angle at which it leaves the mirror is 20 which is what we're looking for you