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There are three reasons.
First reason is that light always reflects in mirrors obeying the reflection laws. The particular law that is important here is the equality of the incidence and of the reflection angles. To find these angles you need to know the normal at the mirror surface at the point of incidence. For a plane mirror the normal is just the perpendicular to the surface of the mirror. For a spherical mirror the normal is the radius of the circle drawn from the circle center to the incidence point. For this you must draw the outline of the mirror.
Second reason is that, as said above there are two type of mirrors (plane and spherical). When drawing the light rays into these mirrors you need to differentiate between their shape because their focal points are different. For a plane mirror the focal point is at infinity (`f =+oo` ) whereas for a spherical mirror the focal point is at a distance from the mirror equal to half of circle radius (`f =R/2` ).
Third reason (that can be derived from the second reason above) is that the equations relating the distances of the object and image to the focal point are different depending on the type of mirror that reflects the light.
`x_1+ x_2 =0` for plane mirror
`1/x_1 +1/x_2 =1/f` for spherical mirror
where ` ``x_1, x_2 > 0` to the left of the mirror and `f>0` for concave mirrors.
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