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a line emanating from a center, as a more or less distinct portion of radiant energy (light or heat), proceeding in a specific direction.
α-r's high-speed helium nuclei ejected from radioactive substances; they have less penetrating power than beta rays. See also alpha particles.
actinic r's light rays that produce chemical action, especially those beyond the violet end of the spectrum.
alpha r's α-rays.
β-r's (beta r's) electrons ejected from radioactive substances with velocities as high as 98 per cent of the velocity of light; they have more penetrating power than alpha rays but less than gamma rays. See also beta particles.
cosmic r's very penetrating radiations that apparently move through interplanetary space in every direction.
digital ray a digit of the hand or foot and corresponding metacarpal or metatarsal bone, regarded as a continuous unit.
γ-r's (gamma r's) a type of electromagnetic radiation emitted by an atomic nucleus during a nuclear reaction; see also gamma rays.
grenz r's very soft electromagnetic radiation of wavelengths of about 2 angstroms.
infrared r's radiations just beyond the red end of the visible spectrum, having wavelengths of 0.75–1000 μm; see also infrared.
medullary ray a cortical extension of a bundle of tubules from a renal pyramid.
roentgen r's x-rays.
ultraviolet r's radiant energy beyond the violet end of the visible spectrum, of wavelengths 0.39 to 0.18 μm; see also ultraviolet rays.
x-r's see x-rays.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


1. A beam of light, heat, or other form of radiation. The rays from radium and other radioactive substances are produced by a spontaneous disintegration of the atom; they are electrically charged particles or electromagnetic waves of extremely short wavelength.
2. A part or branch that extends radially from a structure.
[L. radius]
Farlex Partner Medical Dictionary © Farlex 2012
A small photon intensity element used to subdivide an intensity-modulated beam for intensity distribution optimisation or dose calculations treatment planning


A gene on chromosome 12q24.31 that encodes a member of the Rab subfamily of Ras-related small GTPases, which is involved in endocytosis and is the essential rate-limiting regulator of a fast recycling pathway back to the plasma membrane. During cytokinesis, RAB35 is needed to ensure intercellular bridge stability and abscission.
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.


1. A beam of light, heat, or other form of radiation. The rays from radium and other radioactive substances are produced by a spontaneous disintegration of the atom; they are electrically charged particles or electromagnetic waves of extremely short wavelength.
2. A part or branch that extends radially from a structure.
3. One of the grooves of the embryonic hand and foot indicating where the digital rays (e.g., hand rays) will develop.
[L. radius]
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


  2. a cartilagenous fish of the genus Raia, closely allied to the skate.
  3. a bony rod supporting the fin of a fish.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005


In geometrical optics, a straight line representing the direction of propagation of light.
axial ray A ray that is coincident with the axis of an optical system.
chief ray A ray joining an object point to the centre of the entrance pupil of an optical system (Fig. R2). See pencil of light.
emergent ray A ray of light in image space either after reflection (reflected ray) or after refraction (refracted ray).
extraordinary ray See birefringence.
incident ray A ray of light in object space that strikes a reflecting or refracting surface.
marginal ray A ray joining the axial point of an object to the edge or margin of an aperture or pupil (Fig. R2).
ordinary ray See birefringence.
paraxial ray A light ray that forms an angle of incidence so small that its value in radians is almost equal to its sine or its tangent. (i.e. sin θ = θ or tan θ = θ. These are approximate expressions referred to as the paraxial approximation (or the gaussian approximation). See paraxial optics; paraxial region; gaussian theory.
principal ray A ray joining the extreme off-axis object point to the centre of the entrance pupil or aperture (Fig. R2).
ray tracing Technique used in optical computation consisting of tracing the paths of light rays through an optical system by graphical methods or by using formulae. Nowadays, computer methods are used. See sign convention.
Fig. R2 Rays of light incident to the eye (E, centre of the entrance pupil of the eye)enlarge picture
Fig. R2 Rays of light incident to the eye (E, centre of the entrance pupil of the eye)

Table R1 Differences between the sine and the tangent values of various angles (in degrees and radians). The error is calculated between the sine value and the value in radians and between the value in radians and the tangent value
angle (deg)angle (rad)sine
tangent valueerror (%) sine errorerror (%) tangent error
0.50.008 7270.008 7270.008 7270.000.00
10.017 4530.017 4520.017 4550.010.01
20.034 9070.034 8990.034 9210.020.04
30.052 3600.052 3360.052 4080.050.09
40.069 8130.069 7560.069 9270.080.16
50.087 2660.087 1560.087 4890.130.25
60.104 7200.104 5280.105 1040.180.37
70.122 1730.121 8690.122 7850.250.50
80.139 6260.139 1730.140 5410.330.65
100.174 5330.173 6480.176 3270.511.03
150.261 7990.258 8190.267 9491.152.35
5200.349 0660.342 0200.363 9702.064.27
300.523 5990.500 0000.577 3504.7210.27
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann


1. Beam of light, heat, or other form of radiation.
2. A part or branch that extends radially from a structure.
[L. radius]
Medical Dictionary for the Dental Professions © Farlex 2012

Patient discussion about ray

Q. Is an X- Ray dangerous to my fetus? I fell down while I am pregnant and was sent to the ER. I was given an x- ray there, is the radiation dangerous to my fetus?

A. As far as I know one x-ray cannot harm your fetus since there is not enough radiation there to harm it. If you are worried consult a Doctor.

Q. What does radiation do for cancer patients? We found out today that my grandmother has cancer and my mother said that the oncologist is planning on using radiation to ease her pain. My question is, what does radiation do? I know, eases pain, but how?

A. hello;radiation therapy/an anticancer drugs are used to suppress or arrest the rate of cell division in any tumor cells, the rad also kills good cells also.

Q. Is it proven that cellular radiation can damage health?

A. it was proven that people that talk a lot with cellular phones tend to develop problems in their salivary gland (the Parotid gland, right under the ear)that is on the side they speak the most.

could be that in 15 years from now the amount of brain cancer will increase and they will know for sure it's from cellular phones. , why take a chance- use as less as possible, use an earphone and don't give a cellular phone to your children until they are 17.
you can never know what will they find next...

More discussions about ray
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References in classic literature ?
Ray Pearson lost his nerve and this is really the end of the story of what happened to him.
"I'll try to obey mother ALWAYS," wrote Sara Ray, with a tremendous sigh, as if she fully realized the difficulty of keeping such a resolution.
"If you had to read seven chapters of it every time you were naughty I don't believe you would like it either," retorted Sara Ray with a flash of spirit.
These blue and red shifts broaden the spectrum of X rays emitted from the iron atoms in the accretion disk.
Devil Rays catcher Josh Paul, pitching coach Mike Butcher, third base coach Tom Foley and special advisor Don Zimmer participated in the DAV Celebrity Entertainment Program to visit recovering service members at the Polytrauma Center at the James A.
As described in last month's column, the brightness of rays is due to two separate causes that have been only recently understood.
His surface isn't completely smooth, so when light rays hit him, they get reflected at all different angles--a process called diffuse reflection.
On September 22, 2004, the direct rays of the sun were at the equator, which is at 0 degrees latitude.
If you're planning to spend long days in the sun, protective clothing is the surest way to ward off harmful ultraviolet (UV) rays, which are invisible but nevertheless pose a serious health threat.
In the United States, three types of ionizing radiation have been approved for irradiating food: gamma rays, high energy electrons, which are sometimes referred to as electron beams (or e-beams), and X-rays.
Although also based on the variable absorption of x rays by different tissues, computed tomography (CT) imaging, also known as "CAT scanning" Computerized Axial Tomography), provides a different form of imaging known as cross-sectional imaging.
A sunscreen with a Sun Protection Factor (SPF) of at least 15 is recommended for protection against skin cancer and premature aging of the skin, even on cloudy days when 80 percent of the sun's rays penetrate the clouds.