Principal Focus Definition for Concave Mirrors
The principal focus of a concave mirror is defined by the behavior of light rays after reflection. Specifically, it is the point on the principal axis where rays parallel to this axis converge after reflecting off the mirror's surface.
Ray Convergence in Concave Mirrors
When light rays strike a concave mirror parallel to the principal axis:
- These parallel rays reflect off the curved surface.
- After reflection, they all meet, or converge, at a specific point.
- This point of convergence is known as the principal focus (F).
This convergence is a key characteristic distinguishing concave mirrors and their ability to form real images.
Why Other Options Are Incorrect
- Option 1: Rays passing through the center of curvature (C) reflect back along their original path. They do not necessarily converge at F.
- Option 2: The intersection of the mirror surface and the principal axis is the pole (P) of the mirror.
- Option 3: This describes the behavior of parallel rays reflecting from a convex mirror, which appear to diverge from a virtual focus. Concave mirrors cause convergence.
Option 4 correctly states that parallel rays converge at the principal focus after reflection for a concave mirror.
Important Questions from Mirrors and Images
- A concave mirror forms a real and inverted image of a distant object at a distance of $15 \text{ cm}$ from the mirror.
If an object is placed $20 \text{ cm}$ in front of this mirror, what will be the nature and magnification of the image formed? The number of images observable between two parallel mirror is
Which of the following pair is correct?
I. Mirror formula : (1/v) – (1/u) = (1/f)
II. Lens formula : (1/v) + (1/u) = (1/f)
- A beam of parallel light, originating from a distant source, is first incident on a convex lens with focal length $f_2$.
Subsequently, the light passes through the lens and then reflects from a concave mirror having a focal length $f_1$.
The concave mirror is placed at a distance $d$ from the convex lens.
For the light rays to retrace their original path and ultimately emerge from the lens as a parallel beam heading back towards the distant source, the separation distance $d$ between the lens and the mirror must be: A ray of light is made incident on a concave mirror, parallel to its principal axis. After reflection it will -
