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powerClinical research The probability of detecting a treatment effect of a given magnitude when a treatment effect of at least that magnitude truly exists Vox populi Energy. See Crypto-nuclear power, Durable power of attorney, Guild power, Intellectual processing power, Mass stopping power, Null hypothesis, Police power, Pyramid power, Scanning power, Social power.
powerrate of doing work. In mechanics, over any period of constant applied force, power = force multiplied by displacement in the direction of that force, divided by the time for which it has operated, i.e. = force×velocity. In electricity, power = current×voltage.
aligning power See vernier visual acuity.
approximate power See nominal power.
back vertex power (BVP) The reciprocal of the back vertex focal length. It is equal to
dioptric power See refractive power.
dispersive power See dispersion.
effective power The power of a lens or surface measured in a plane other than the principal plane and usually remote from the lens or surface. If a thin lens or surface of power F is illuminated by parallel incident light, the effective power Fx of another lens placed at a distance d from the original lens and forming an image in the same position, is given by the equation
equivalent power The refractive power of a lens or an optical system expressed with reference to the principal points. It corresponds to the refractive power of a thin lens placed in the second principal plane which would form an image of a distant object of the same size as that produced by the system that it replaces. It is equal to
equivalent viewing power (EVP) A term used to describe the magnifying effect of a lens (or lens system). It is equal to the power resulting from the combination of a magnifier Fm assumed to be a thin lens and a near addition (or the accommodation exerted) Fa, i.e. EVP = Fm + Fa − zFm Fa where z is the distance (in metres) between the magnifier and the eye (or spectacle plane). EVP may also be expressed as EVP = M ✕ Fa, where M is the enlargement ratio (lateral magnification). If the magnifier is placed against the spectacle lens (z = 0), EVP = Fm + Fa. If the magnifier is placed against the object being viewed (z = fa), EVP = Fa. If the magnifier is held at its focal length from the eye (z = fm), EVP = Fm: EVP is then equal to the power of the magnifier, irrespective of any near addition (or accommodation). Example: A patient uses a lens +16 D placed 12 cm from the eye and wears a near addition of +4.00 D. EVP = 16 + 4 − 0.12 ✕ 16 ✕ 4 = +12.3 D, the enlargement ratio (M = EVP/Fa) is 12.3 / 4 = 3.1✕, and at a distance of 4 cm from the eye the enlargement is equal to 4.4✕, whereas the conventional magnification remains Fm/4 = 4✕.The reciprocal of EVP is called the equivalent viewing distance (EVD). It represents the focal length of the equivalent magnifying system where the target must be placed to be seen clearly. See lateral magnification.
power factor See spectacle magnification.
focal power See fundamental paraxial equation; refractive power.
front vertex power (FVP) The reciprocal of the front vertex focal length. It is equal to
magnification power See spectacle magnification.
magnifying power See apparent magnification.
nominal power An estimate of the power of a lens, calculated as the sum of the front and back surface powers, i.e.
prism power The amount of deviation of a ray of light transmitted through a prism or lens (outside its optical centre). It is usually expressed in prism dioptres (D) and given by the following approximate formula for small angle prisms (in air)
prismatic power See prism power.
refractive power The ability of a lens or an optical system to change the direction of a pencil of rays. It is equal to
resolving power See limit of resolution.
surface power The dioptric power of a single refracting or reflecting surface. It is equal to
true power See equivalent power.
vergence power See fundamental paraxial equation; refractive power; vergence.
vertex power See back vertex power; front vertex power.
|Table P5 Power (in dioptres) of the surfaces and structures of an average adult Caucasian eye*|
|anterior surface of the cornea||48.21|
|posterior surface of the cornea||−5.97|
|complete corneal system||42.34|
|anterior surface of the lens||7.92|
|posterior surface of the lens||13.54|
|complete lens system||21.19|
|refraction of the eye||+0.50|
|*see constants of the eye.|
|Table P6 Powers of the surfaces of contact lenses of thickness d = 0.20 mm, in which the radius of curvature of the back optic zone is constant and that of the front optic zone varies to produce various back vertex powers and equivalent powers. The index of refraction of these lenses is assumed to be 1.49|
|radius of back optic zone (mm)||back surface power (D) F2||radius of front optic zone (mm)||front surface power (D) F1||power of lens (D) considered thin F1 1 F2||equivalent power (D) Fe||back vertex power (D) F′v|
|Table P7 Contact lens power (or ocular refraction) corresponding to a spectacle lens situated at two vertex distances|
|spectacle lens power (D)||contact lens |
|10 mm||14 mm|
|Table P8 Surface power of the anterior surface of the cornea (in dioptres) corresponding to various radii of curvature (in mm). Calculations were made using 1.376 as the index of refraction of the cornea|
Patient discussion about power
Q. My myopic son is wearing power glasses. Are there any other nutritional supplements to support eye sight? My myopic son is wearing power glasses from the age of 2 years. His power is not very high yet but the rate of his eye power is doubling every year. Doctor had given him some medicines and had told him to have lots of carrots. We are giving him carrot juice every day. But soon he stopped taking it for some months. But he is having juice now but I wish to know are there any other nutritional supplements to support eye sight?
here is a "snopes" about it-