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Measurements of light refer to the intensity. These measurements are obtained by dividing either a power or a luminous flux by a solid angle, a planar area or a combination of the two. Intensity is the power transferred per unit area. When intensity is measuring waves the average power transfer over one period of the wave is used.

Intensity is found by taking the energy density at an arbitrary point in space and multiplying it by the velocity at which the energy is moving. The resulting vector has the units of power divided by area. In the SI system, the unit is watts per meter squared.

For a point source of radiating energy in three dimensions where there is no energy lost to the medium, the intensity decreases in proportion to the distance from the object squared. Applying the law of conservation of energy we obtain the following equation

P = ∫I * dA


P is the net power radiated

I is the intensity as a function of position

In photometry and radiometry, intensity has different meanings. It is the luminous or radiant power per unit solid angle. This can cause confusion in optics where intensity can mean any of radiant intensity, luminous intensity or irradiant. This depends on the background of the person using the term. 

Categories within Intensity

Inverse Square Law

Postings: 6

The inverse-square law is any physical law stating that a specified physical quantity is inversely proportional to the square of the distance from the source.

X-ray Intensity

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Environmental Factors in Everyday Life

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Write a summary of the issues presented in this article. The case is attached Wireless Radiation Frying Your Brains? Radio waves, microwaves, and infrared all belong to the Electromagnetic radiation spectrum. These terms reference ranges of radiation frequencies we use every day in our wireless networking environments. However, the very word radiation strikes fear in many people. Cell towers have sprouted from fields all along highways. Tall rooftops harbor many more cell stations in cities. Millions of cell phone users place microwave transmitters/receivers next to their heads each time they make a call. Computer network wireless access points have become ubiquitous. Even McDonald's customers can use their machines to browse the Internet as they eat burgers. With all this radiation zapping about, should we be concerned? The electromagnetic spectrum ranges from ultra-low frequencies to radio waves, microwaves, infrared, visible light, ultraviolet, x-ray, and up to gamma ray radiation. Is radiation dangerous? The threat appears to come from two different directions, the frequency and the intensity. A preponderance of research has demonstrated the dangers of radiation at frequencies just higher than those of visible light, even including the ultraviolet light used in tanning beds, x-rays, and gamma rays. These frequencies are high (the wavelengths are small enough) to penetrate and disrupt molecules and even atoms. The results range from burns to damaged DNA that might lead to cancer or birth defects. However, radiation's lower frequencies ranging from visible light (the rainbow colors you can see), infrared, microwave, and radio waves have long waves unable to penetrate molecules. Indeed, microwave wave lengths are so long that microwave ovens employ a simple viewing screen that can block these long waves and yet allow visible light through. As a result, we can watch our popcorn pop without feeling any heat. Keep in mind that visible light consists of radiation frequencies closer to the danger end of the spectrum than microwave light. Lower radiation frequencies can cause damage only if the intensity is strong enough, and that damage is limited to common burns. Microwave ovens cook food by drawing 800 or more watts and converting them into a very intense (bright) microwave light. Cellular telephones, by comparison, draw a very tiny amount of current from the phone's battery and use the resulting microwaves to transmit a signal. In fact, the heat you feel from the cell phone is not from the microwaves but rather from its discharging battery. It is extremely unlikely that either device can give the user cancer, though a microwave oven could cause serious burns if the operator disables its safety features.

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Sound Intensity Levels

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