Principal Axis: Imaginary straight line passing through the two centers of curvature.

Principal Foci ($F_1, F_2$): Two focal points for lenses.

Focal Length (f): Distance from O to F.

Aperture: Effective diameter of the circular outline of a spherical lens.

Ray parallel to principal axis:
- Convex: Passes through $F_2$ after refraction.
- Concave: Appears to diverge from $F_1$ after refraction.
Ray passing through $F_1$ (convex) or directed towards $F_2$ (concave): Emerges parallel to the principal axis after refraction.
Ray passing through optical center (O): Emerges without any deviation.

Object Position	Image Position	Image Size	Image Nature
At infinity	At $F_2$	Highly diminished, point-sized	Real and inverted
Beyond $2F_1$	Between $F_2$ and $2F_2$	Diminished	Real and inverted
At $2F_1$	At $2F_2$	Same size	Real and inverted
Between $F_1$ and $2F_1$	Beyond $2F_2$	Enlarged	Real and inverted
At $F_1$	At infinity	Infinitely large or highly enlarged	Real and inverted
Between $F_1$ and O	On the same side of the lens as object	Enlarged	Virtual and erect

Object Position	Image Position	Image Size	Image Nature
At infinity	At $F_1$	Highly diminished, point-sized	Virtual and erect
Between infinity and O	Between $F_1$ and O	Diminished	Virtual and erect

Lens Formula: $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
$u$: object distance, $v$: image distance, $f$: focal length.
Magnification (m): $m = \frac{h'}{h} = \frac{v}{u}$
Negative $m$ indicates real and inverted image. Positive $m$ indicates virtual and erect image.

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