The angle of incidence, in
geometric optics
Geometry (; ) is a branch of mathematics concerned with properties of space such as the distance, shape, size, and relative position of figures. Geometry is, along with arithmetic, one of the oldest branches of mathematics. A mathematician ...
, is the angle between a
ray incident on a surface and the line perpendicular (at 90 degree angle) to the surface at the point of incidence, called the
normal. The ray can be formed by any waves, such as
optical,
acoustic,
microwave
Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
, and
X-ray
An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
. In the figure below, the line representing a ray makes an angle θ with the normal (dotted line). The angle of incidence at which light is first totally internally reflected is known as the
critical angle. The
angle of reflection and
angle of refraction are other angles related to beams.
In
computer graphics
Computer graphics deals with generating images and art with the aid of computers. Computer graphics is a core technology in digital photography, film, video games, digital art, cell phone and computer displays, and many specialized applications. ...
and
geography
Geography (from Ancient Greek ; combining 'Earth' and 'write', literally 'Earth writing') is the study of the lands, features, inhabitants, and phenomena of Earth. Geography is an all-encompassing discipline that seeks an understanding o ...
, the angle of incidence is also known as the illumination angle of a surface with a light source, such as the
Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
's surface and the
Sun. It can also be equivalently described as the angle between the
tangent plane
In geometry, the tangent line (or simply tangent) to a plane curve at a given point is, intuitively, the straight line that "just touches" the curve at that point. Leibniz defined it as the line through a pair of infinitely close points o ...
of the surface and another plane at right angles to the light rays.
This means that the illumination angle of a certain point on Earth's surface is
0° if the Sun is
precisely overhead and that it is
90° at
sunset
Sunset (or sundown) is the disappearance of the Sun at the end of the Sun path, below the horizon of the Earth (or any other astronomical object in the Solar System) due to its Earth's rotation, rotation. As viewed from everywhere on Earth, it ...
or
sunrise.
Determining the angle of reflection with respect to a planar surface is trivial, but the computation for almost any other surface is significantly more difficult.
Grazing angle or glancing angle
When dealing with a beam that is nearly parallel to a surface, it is sometimes more useful to refer to the angle between the beam and the
surface tangent, rather than that between the beam and the
surface normal
In geometry, a normal is an object (e.g. a line, ray, or vector) that is perpendicular to a given object. For example, the normal line to a plane curve at a given point is the infinite straight line perpendicular to the tangent line to the ...
. The 90-degree
complement to the angle of incidence is called the grazing angle or glancing angle. Incidence at small grazing angles is called "grazing incidence."
Grazing incidence diffraction is used in
X-ray spectroscopy
X-ray spectroscopy is a general term for several Spectroscopy, spectroscopic techniques for characterization of materials by using x-ray radiation.
Characteristic X-ray spectroscopy
When an electron from the inner shell of an atom is excited b ...
and
atom optics, where significant reflection can be achieved only at small values of the grazing angle.
Ridged mirrors are designed to reflect atoms coming at a small grazing angle. This angle is usually measured in
milliradian
A milliradian (International System of Units, SI-symbol mrad, sometimes also abbreviated mil) is an SI derived unit for angular measurement which is defined as a thousandth of a radian (0.001 radian). Milliradians are used in adjustment of ...
s. In optics, there is
Lloyd's mirror.
See also
*
Effect of Sun angle on climate
*
Illumination angle
*
Phase angle (astronomy)
*
Plane of incidence
*
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. The ...
*
Refraction
In physics, refraction is the redirection of a wave as it passes from one transmission medium, medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commo ...
*
Scattering vector
*
Total internal reflection
References
External links
*
geometry: rebound on the strip billiards Flash animation
{{DEFAULTSORT:Angle Of Incidence
Geometrical optics
Angle