ST_GeometryN: Returns the 1-based Nth geometry if the geometry is a GEOMETRYCOLLECTION, MULTIPOINT, MULTILINESTRING, MULTICURVE or MULTIPOLYGON. Return NULL if the geometry is not a polygon. ST_ExteriorRing: Returns a line string representing the exterior ring of the POLYGON geometry. ST_AsSVG: Returns a Geometry in SVG path data given a geometry or geography object. ST_AsKML: Returns the geometry as a KML element. ST_AsGML: Returns the geometry as a GML version 2 or 3 element. ST_AsGeoJSON: Returns the geometry as a GeoJSON element. ST_AsEWKT: Returns the Well-Known Text (WKT) representation of the geometry with SRID meta data. ST_AsEWKB: Returns the Well-Known Binary (WKB) representation of the geometry with SRID meta data. ST_EndPoint: Returns the last point of a LINESTRING geometry as a POINT. ST_AsBinary: Returns the Well-Known Binary (WKB) representation of the geometry/geography without SRID meta data. ST_AsText: Returns the Well-Known Text (WKT) representation of the geometry/geography without SRID metadata. For “geography” area is in square meters. For “geometry” type area is in SRID units. ST_Area: Returns the area of the surface if it is a polygon or multi-polygon. The following SQL query shows an example of WKB representation (the call to encode() is required to convert the binary output into an ASCII form for printing): Note that in addition to a text parameter with a geometry representation, we also have a numeric parameter providing the SRID of the geometry. The most common use of a constructor is to turn a text representation of a geometry into an internal representation: ST_GeomFromText(text, srid) returns geometry Fortunately, PostGIS supports emitting and consuming geometries in a large number of formats: In order for external programs to insert and retrieve useful geometries, they need to be converted into a format that other applications can understand. Within the database, geometries are stored on disk in a format only used by the PostGIS program. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License. We recommend using aĪuthors: Paul Peter Urone, Roger Hinrichs

Use the information below to generate a citation. Then you must include on every digital page view the following attribution: If you are redistributing all or part of this book in a digital format, Then you must include on every physical page the following attribution: If you are redistributing all or part of this book in a print format, Want to cite, share, or modify this book? This book uses the To obtain constructive interference for a double slit, the path length difference must be an integral multiple of the wavelength, or We illustrate the double slit experiment with monochromatic (single λ λ size 12 is the distance between the slits. Young used sunlight, where each wavelength forms its own pattern, making the effect more difficult to see. Why did Young then pass the light through a double slit? The answer to this question is that two slits provide two coherent light sources that then interfere constructively or destructively. Incoherent means the waves have random phase relationships. By coherent, we mean waves are in phase or have a definite phase relationship. Furthermore, Young first passed light from a single source (the Sun) through a single slit to make the light somewhat coherent. Why do we not ordinarily observe wave behavior for light, such as observed in Young’s double slit experiment? First, light must interact with something small, such as the closely spaced slits used by Young, to show pronounced wave effects. Without diffraction and interference, the light would simply make two lines on the screen. Here pure-wavelength light sent through a pair of vertical slits is diffracted into a pattern on the screen of numerous vertical lines spread out horizontally. Figure 27.10 Young’s double slit experiment.

Double slit geometry x coordinates