In telecommunication, the term optical axis has the following meanings:
1. Of a refractive or reflective optical element, the straight line that is coincident with the axis of symmetry of the surfaces.
Note: The optical axis of a system is often coincident with its mechanical axis, but it need not be, e.g., in the case of an off-axis parabolic reflector used to transmit signals to, or receive signals from, a geosynchronous satellite. Contrast withoff-axis optical system.
2. In a lens element, the straight line which passes through the centers of curvature of the lens surfaces.
3. In an optical system, the line formed by the coinciding principal axes of the series of optical elements.
The wavelength is the linear distance parallel to the axis between two points in the same phase, or occupying equivalent positions on the wave, for example, the distance from maximum to maximum in the case of a sine function representation.
Unless light is traveling parallel to an axis of symmetry with respect to the structure of the crystals (the opticaxis of the crystal), it is separated into two parts that travel with different velocities.
The component with the electric vector vibrating in a plane containing the opticaxis is the ordinary ray; its velocity is the same in all directions through the crystal, and Snell’s law of refraction holds.
The opticaxis passes through the centre of a spherical lens or mirror and through its centre of curvature.
If a narrow beam of rays travelling along the opticaxis is incident on the spherical surface of a mirror or a thin lens, the rays are reflected or refracted so that they intersect or appear to intersect at a point on the opticaxis.
A narrow beam of rays travelling along the opticaxis and incident on a concave mirror is reflected so that it intersects the radius at the focal point, or principal focus, halfway between the pole, or centre, of the mirror's surface and the mirror's centre of curvature.
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