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Which of the following points always lie(s) on the principal axis of a spherical mirror? i) Principal Focus ii) Point of incidence iii) Pole
Both (i) and (iii)
Only (i)
Only (iii)
Both (ii) and (iii)
Both (i) and (iii)
The principal axis of a spherical mirror is defined as the straight line passing through the pole (P) and the center of curvature (C) of the mirror. Both the pole (the geometric center of the mirror surface) and the principal focus (the point where parallel rays converge or appear to diverge) by definition lie on this axis.
The principal axis of a spherical mirror is defined as the straight line passing through the pole (P) and the center of curvature (C) of the mirror. Both the pole (the geometric center of the mirror surface) and the principal focus (the point where parallel rays converge or appear to diverge) by definition lie on this axis.
Think of the principal axis like the 'spine' of the mirror; just as a human spine supports the head and torso in a straight line, the principal axis connects all the key landmarks (Pole, Focus, Center of Curvature) in a perfectly straight line.
Remember 'PAC': Pole, Axis, Center of Curvature (all lie on the same line).
f=2RтАЛ тАФ where f is the focal length and R is the radius of curvature.
In optics, the principal axis acts as the axis of symmetry for the spherical mirror. By geometric convention, the pole is the vertex of the mirror, and the line connecting the vertex to the center of curvature is chosen as the reference line for all incident and reflected light path calculations.
The Pole (P) is the center point of the mirror's surface.
The Principal Focus (F) lies halfway between the Pole and the Center of Curvature.
The Point of Incidence is any random point on the mirror where a ray strikes, not necessarily on the principal axis.
Used for image formation calculations in concave and convex mirrors.
Used to define ray diagram geometry in ray optics.
The point of incidence (ii) is only on the principal axis if the light ray strikes the pole of the mirror directly.
Therefore, (ii) is not 'always' on the principal axis, making it an incorrect option.
A is correct тАФ Both the pole and the principal focus are fixed points that reside on the principal axis of any spherical mirror.
In competitive exams, always look for the keyword 'always'тАФit excludes points like 'point of incidence' which depend on the path of the specific light ray.
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