DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

light bridge Observed in white light, a
bright tongue or streak penetrating or crossing a
sunspot umbra. The appearance of a light bridge
is frequently a sign of impending region division
or dissolution.
light cone The 3-dimensional hypersurface
in spacetime which is generated by light rays
(i.e., light-like geodesics) originating at a single
event p (future light cone) and meeting at
this event (past light cone); the event p is referred
to as the vertex of the light cone. In the
flat spacetime of special relativity such hypersurfaces
with vertices at different events of the
flat spacetime are isometric (i.e., are identical
copies of each other). In general spacetimes,
the hypersurfaces may be curved and may end
at folds, or self-intersections. In curved spacetimes,
light cones with different vertices are in
general not isometric.
The notion of a light cone is important in observationalastronomy:
theimageoftheskyseen
by an observer at the momentt=t0 is a projection
of the observer’s past light cone at t=t0
onto the celestial sphere. All the events in our
actual spacetime that could have been observed
by a given observer up to the moment t =t0
on his clock lie on or inside the past light cone
of the event t=t0 of the observer’s history, a
restatement of the fact that an eventq can be observed
by an observerO only by the time when
the light-ray emitted at q reachesO.
light-curve A plot of light received from a
star vs. time. A variable star is one that has a
light-curve with peaks and troughs. Often these
features are periodic (in the case of eclipsing
binaries) or nearly (or quasi-) periodic in the
case of variations in an isolated star, such as
sunspots (starspots) or stellar oscillations.
For planetary bodies, the variation in brightness
due to an object’s rotation. A light-curve
can be due to shape effects (seeing different
cross-sectional areas of an object as it rotates),
albedo changes on the surface (seeing different
bright and dark patches as the object rotates), or
some combination of the two.
light deflection The curving of light rays
by the gravitational field. In the absence of all
gravitational fields, i.e., in the flat spacetime of
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light pollution
special relativity, light rays are straight. However,
in curved spacetime containing a massive
body, a light ray passing by the massive body
curves toward the body. If a body of mass M
is spherical, and the closest distance between
the ray and the center of the body is r, then
the angle of deflection φ is given by the approximate
Einstein formulaφ = 4GM/(c 2 r),
where G is the gravitational constant and c is
the velocity of light. The formula applies only
whenr≫ 2GM/c 2 (i.e., when the angleφ is
small). For a ray grazing the surface of the sun,
φ = 1.75 sec of arc. This effect was measured
by Eddington in 1919, and more recently confirmed
with greater precision for radio-waves
from radio-sources; deflection of light by the
sun has been verified to parts in 1000 by observations
of the direction to extragalactic radio
sources, as the sun passes near their position
in the sky. An observer placed far enough behind
the deflecting mass can see light rays that
passed on opposite sides of the deflector. For
such an observer, the deflector would be a gravitational
lens. In the case of distant sources,
deflection by intervening galaxies or clusters of
galaxies causes lensing, leading to the appearance
of multiple images, and of rings or arcs
of distorted images. Light rays approaching the
very strong gravitational field of a black hole
can describe very complicated orbits.
light-harvesting complex A pigment-protein
complex containing chlorophyll that supplies
additional energy in photosynthesis.
lightlike current (cosmic string) See electric
regime (cosmic string).
lightning Electric discharge in the atmosphere
that produces a lightning flash. Two fundamental
types can be distinguished: ground
flashes where the discharge occurs between the
ground and the cloud with the flash either propagating
upward or downward, and cloud flashes
or intracloud flashes, where the discharge occurs
between clouds and the flash does not strike the
ground.
light pollution Deleterious effects of outdoor
lighting on astronomical observations (which require
dark sky and which are severely affected
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