These kinds of crystals have one or two optical axes.

A point R at the intersection of the optical axis and the image plane.

This axis is known as the optical axis of the material.

This ray crosses the optical axis at the locations of the pupils.

The vertex of the lens surface is located on the local optical axis.

Consider a photon which is polarised parallel to the optical axis of P,.

Each cylinder lines up parallel to the optical axis into which electrons enter.

For light traveling perpendicularly to the optical axis the beams will have the same direction.

The cardinal points lie on the optical axis of the optical system.

Note that the aperture must be centered on the optical axis for this to work as indicated.

This is more appropriate than the paraxial model in systems that lack symmetry about the optical axis.

The optical axis is the centre of a lens where light travels through and is not bent.

The optical axis is an imaginary line that defines the path along which light propagates through the system.

Some materials can have multiple such optical axes.

If the pupil is centered on optical axis, it causes positive chromostereopsis.

This ray is useful, because it crosses the optical axis again at the locations where an image will be formed.

Sometimes glasses are given with the optical axis shifted away from the visual axis.

Such rays do not cross the optical axis anywhere, and are not parallel to it.

The distance between the chief ray and the optical axis at an image location defines the size of the image.

The surface vertices are the points where each surface crosses the optical axis.

However, even an expensive star diagonal will deliver poor performance if it is not in alignment with the optical axis of the telescope.

Discrete frames are formed as each successive face of the mirror passes through the optical axis.

Such a ray will be reflected from the mirror as a ray parallel to the optical axis.

Exceptions arise when the light propagates either along or orthogonal to the optical axis.

Parallel orientation of the optical axis of minerals like nepheline.