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Grazing incidence optics (sometimes abbreviated GIO) is optics that reflects EMR such that a ray's direction-change is small; the angle between the incoming EMR's path and the reflecting surface (grazing angle) is small, e.g., less than 2°. Gracing incident optics is commonly used in X-ray telescopes: X-rays generally penetrate material rather than reflect off of it, but they are reflected if such small grazing angles are used. Wolter telescopes, of which there are a few kinds, are designed to use only grazing incident optics. A consequence of this design-restriction is that the telescopes are relatively long: both Chandra X-ray Observatory and XMM-Newton have telescopes on the order of 10 meters in length.