# Relationship between illuminance and distance

### Inverse-square law - Wikipedia

The inverse-square law, in physics, is any physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that . The intensity (or illuminance or irradiance) of light or other linear waves radiating from a point source (energy per unit of area. ILLUMINATION AND DISTANCE To quantify the amount of light, we will use units called lux. Prepare a graph to analyze the relationship mathematically. Hello, We had a lab in physics about the relationship between illuminance and distance. Our teacher gave use data so we make a graph of.

When you make measurement, hold the null-photometer between the lights, and move it back and forth along an imaginary line between them until both halves of the photometer's window appear the same.

### The Inverse-Square Law

Your partner can check to make sure the photometer really is on a line between the two lights, and then measure the distances Da and Db. Ideally, these distances should be measured from the center of each light-bulb to the nearest side of the photometer, as shown in the diagram.

Once both distances have been recorded, flip the photometer over so the left face is now the right face, and vice versa.

Re-position the photometer between the lights, move it so both halves appear the same, and again measure the distances. Repeat two more times, again flipping the photometer each time.

You should now have four separate measurements of the two distances. Each set of four measurements can be averaged to get a more precise value; you can also look at the range of values for each measurement to get some idea of the accuracy of your work.

Before moving on to the next pair of lights, be sure to record the luminosities La and Lb. When you write up your lab report, first compute averages for your measurements of Da and Db for each pair of lights. If the last two columns of each row are equal, allowing for experimental error, then the inverse-square law passes the test.

Measuring luminosity The same procedure can be used to measure the luminosity of a light-bulb. We will set up a pair of lights and tell you the luminosity of one light; your job is to calculate the luminosity of the other. Follow the same procedure you used when testing the law: Record your measurements for Da and Db, along with the known luminosity La.

When you write up your lab report, compute averages for your measurements of Da and Db just as you did when testing the law. Then plug your averages and the known luminosity La into the equation In astronomy, we sometimes know the distance to a star but not its luminosity.

A measurement like this can be used to find the star's luminosity. Measuring distance A similar procedure can be used to measure an unknown distance, given the luminosities of both light-bulbs.

## Inverse-square law

We will set up one last pair of lights and tell you both luminosities. Once again, repeat another three times, flipping the photometer each time. Record your measurements for Da, along with the given luminosities La and Lb.

When you write up your lab report, compute averages for your measurements of Da, and plug your result into the equation In astronomy, stars come in a range of luminosities, and we can sometimes figure out the luminosity by measuring the star's color. If we know the luminosity, we can then use this technique to measure the star's distance. This report should include, in order, the general idea of the experiments, the equipment you used for this work, a summary of your experimental results, and the conclusions you have reached.

In somewhat more detail, here are several things you should be sure to do in your lab report: In your own terms, explain the difference between luminosity and brightness, and summarize the inverse-square law.

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List individual values for each Da and Db you have measured. Estimate your experimental errors from the range of values you got for each distance, and check for bad data values. It's likely that the In this case, you should exclude the The three remaining values give you an average of Make a table with your results as described in the subsection on Testing the law. Do your results support the inverse-square law?

### Illuminance and distance | Physics Forums

Give reasons for your conclusion. Why should you flip the null-photometer over between measurements?

• The Inverse-Square Law
• Homework Help: Illuminance and distance

Hence, the intensity of radiation passing through any unit area directly facing the point source is inversely proportional to the square of the distance from the point source. Gauss's law is similarly applicable, and can be used with any physical quantity that acts in accordance with the inverse-square relationship. Gravitation[ edit ] Gravitation is the attraction between objects that have mass.

The gravitational attraction force between two point masses is directly proportional to the product of their masses and inversely proportional to the square of their separation distance.

The force is always attractive and acts along the line joining them. If the distribution of matter in each body is spherically symmetric, then the objects can be treated as point masses without approximation, as shown in the shell theorem. Otherwise, if we want to calculate the attraction between massive bodies, we need to add all the point-point attraction forces vectorially and the net attraction might not be exact inverse square.

However, if the separation between the massive bodies is much larger compared to their sizes, then to a good approximation, it is reasonable to treat the masses as a point mass located at the object's center of mass while calculating the gravitational force.

As the law of gravitation, this law was suggested in by Ismael Bullialdus.

Indeed, Bullialdus maintained the sun's force was attractive at aphelion and repulsive at perihelion. Hooke's Gresham lecture explained that gravitation applied to "all celestiall bodys" and added the principles that the gravitating power decreases with distance and that in the absence of any such power bodies move in straight lines.

ByHooke thought gravitation had inverse square dependence and communicated this in a letter to Isaac Newton: