Intensity in Physics: Definition & Measurement

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  • 0:02 What Is Intensity?
  • 0:40 Intensity of Sound Waves
  • 1:37 Intensity of…
  • 3:46 Lesson Summary
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Lesson Transcript
Instructor: Sergey Segal

Sergey has a Masters in Biomedical Engineering and has taught science and mathematics courses at the University of Wisconsin-Madison.

Intensity in physics is a fundamental concept that scientists and engineers often use. Learn what the definition of intensity is and how to measure it in this lesson.

What Is Intensity?

Imagine you are standing on the sidewalk and start hearing a faint ambulance siren that is speeding your way from the right. The siren's sound intensity is low while the ambulance is far away; it gets stronger as the ambulance approaches you, reaching a maximum when it is closest to you and decreasing again as the ambulance speeds away past you.

Intensity describes how much there is of physical quantities, such as sound and light output from a source. It refers to the magnitude, or strength, of a given physical quantity at a given location in space. Depending on the type of physical quantity, intensity can be measured in different ways. Let's look at some examples.

Intensity of Sound Waves

Referring back to the ambulance siren, sound intensity, perceived as loudness by the human ear, is measured as the sound power per unit area at a given location. Since it is easier to talk about how loud something is when we have a reference point, scientists use the term sound intensity level, denoted LI and measured in decibels (dB), to describe the loudness of sounds from any source.

Here's a chart of different sound intensity levels, with data taken from the U.S. Occupational Safety & Health Administration website:

Sound Chart

The sound intensity level is calculated as the ratio of the sound intensity, I, with respect to our threshold of hearing, Io. Note that I and Io are measured in Watts per square meter. Here is the equation that is used to calculate L I:

eq3_sound_level_intensity

Remember that since the logarithmic scale is used, the intensity of sound waves of a conversation at 60 dB is orders of magnitude higher than that of a whisper at 35 dB.

Intensity of Electromagnetic Waves

Now let's describe how a celestial body's brightness can be measured and calculated. Note that although we will only mention stars in our discussion, it is equally applicable to any object in outer space that emits or reflects light, including planets. That is, these equations can be applied to any celestial body that emits or reflects light.

Imagine looking up at the night sky and watching the stars. You'll notice that some stars appear to be brighter than others. Astronomers calculate this apparent brightness by comparing a given star's light intensity with respect to a reference star.

We can quantify the light output intensity of a given star by dividing its luminosity, or electromagnetic energy emitted per unit of time, by the product of 4*pi and the Earth-to-star distance squared:

Luminosity Intensity

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