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In optics
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...
, a conjugate plane or conjugate focal plane of a given plane ''P'', is the plane ''P′'' such that points on ''P'' are imaged on ''P′''. If an object is moved to the point occupied by its image, then the moved object's new image will appear at the point where the object originated. In other words, the object and its image are interchangeable. This comes from the principle of reversibility which states light rays will travel along the originating path if the light's direction is reversed. The points that span conjugate planes are called conjugate points.''Fundamentals of Optics'', Fourth Ed. (1976) Francis A. Jenkins, Harvey E. White p. 48
It comes from the mirror formula 1/v+1/u=1/f where if u and v are interchanged then the equation remains same.
In a telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
, the subject focal plane is at infinity and the conjugate image plane, at which the image sensor
An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of c ...
is placed, is said to be an infinite conjugate. In microscopy and macro photography, the subject is close to the lens, so the plane at which the image sensor is placed is said to be a finite conjugate. Within a system with relay lenses or eyepiece
An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks through the device. The ...
s, there may be planes that are conjugate to the aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.
An ...
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