light -reflection & refraction
A paraxial ray is a ray which makes a small angle (θ) to the optical axis of the system, and lies close to the axis throughout the system.
The marginal ray in an optical system is the meridional ray that starts at the point where the object crosses the optical axis, and touches the edge of the aperture top of the system
The visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc. It also is called the object's angular size.
virtual object/Virtual image?convex mirror always forms a virtual image whenever object is not at infinity.
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