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Which of the following statements is/are NOT true about the principal focus of a spherical mirror? i) It always lies on the principal axis. ii) An incident ray passing through the focus after reflection becomes parallel to the principal axis. iii) Principal focus is a point where all the incident rays after reflection from the mirror meet.
Only (i)
Only (iii)
Only (ii) and (iii)
Only (i) and (ii)
Only (iii)
The principal focus of a spherical mirror is a point on the principal axis where light rays parallel to the axis either converge (in concave mirrors) or appear to diverge from (in convex mirrors) after reflection. Statement (i) is true as it must lie on the axis. Statement (ii) is the principle of reversibility of light, which is true. Statement (iii) is NOT true because it describes the focus as the meeting point for 'all' incident rays, but in reality, this only applies to rays traveling parallel to the principal axis.
The principal focus of a spherical mirror is a point on the principal axis where light rays parallel to the axis either converge (in concave mirrors) or appear to diverge from (in convex mirrors) after reflection. Statement (i) is true as it must lie on the axis. Statement (ii) is the principle of reversibility of light, which is true. Statement (iii) is NOT true because it describes the focus as the meeting point for 'all' incident rays, but in reality, this only applies to rays traveling parallel to the principal axis.
Think of the focus like a 'target point' for parallel lanes on a highway; only the cars staying in their designated parallel lanes are directed to that specific exit, while cars arriving from other random angles go in different directions.
f=2RтАЛ тАФ The focal length (f) is half the radius of curvature (R).
For a spherical mirror, light rays traveling parallel to the principal axis strike the reflecting surface and follow the law of reflection (anglei=angler). Due to the curvature of the mirror, these specific parallel rays converge at or appear to originate from a single point on the optical axis known as the principal focus (F). Rays that are not parallel do not necessarily pass through this focus.
Principal focus lies strictly on the principal axis.
Only rays parallel to the principal axis converge at/diverge from the focus.
The statement 'all incident rays meet at the focus' is scientifically incorrect because rays incident at arbitrary angles reflect to different points.
Concave mirrors have a real focus, while convex mirrors have a virtual focus.
Headlights of vehicles
Solar furnaces
Statement (i) is True. The focus is a point on the axis.
Statement (ii) is True. It is the definition of the path of a ray passing through the focus.
Statement (iii) is False. Rays coming from different directions (non-parallel) do not meet at the focus.
B is correct тАФ Statement (iii) is the only false statement among the three provided, as the principal focus is specific to incident rays that are parallel to the principal axis.
Always remember that ray optics definitions in exam questions are highly specific; words like 'all' are usually indicators of a trap statement.
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