# Joly Photometer: A Hands-On Activity for Learning About Light Intensity

Here’s a quick (15 min), inexpensive ($2/group) activity for students to learn about the marvels of light intensity. Students can build this in just a few minutes and use it to compare or even measure light intensities.

The Joly photometer it is a simple instrument invented by Irish physicist, John Joly (1857-1933), and is used for measuring the relative intensity of light. This elegant, simple method allows students to make quick, concrete comparisons of the intensity or irradiance of light sources.

## Understanding the Joly Photometer

The Joly photometer is composed of two pieces of paraffin wax and a piece of reflective material such as aluminum foil. The reflective surface is sandwiched between the two pieces of wax, and prevents the light from travelling to the wax piece on the other side. Each side of the Joly photometer is illuminated by only one light source, allowing us to compare the amount of light falling on each side. When both sides of the Joly photometer appear to be the same shade of grey, the light intensity on both sides is equal. If one side is brighter, the light on that side is more intense.

## Making a Joly Photometer

It takes just a few minutes to construct a Joly photometer. The slideshow below shows the steps.

## Using the Joly Photometer

Once students have their own Joly photometer they can do all sorts of hands on activities to learn about light intensity. In the video below, we will use a Joly photometer to compare the light intensity of two different cellphone flashlights, then we will show students how to calculate a ratio of the light outputs.

The basic method is:

Place the Joly photometer in-between two unequal light sources.

Adjust the position until both sides appear to be the same shade of grey.

Measure the distance from the center of the Joly photometer to each of the light sources.

Use the equation at right to calculate the relative power output of (in this case) the left bub, compared to the right bulb. Depending on the students’ comfort with math, they can either derive the equation (as shown in the video) or use the equation given to them.

Here are some activities students can do with the Joly Photometer

## Compare the light output from cellphone flashlights.

Have students use cellphones with flashlights, or give them small flashlights, a meter stick and their Joly photometer. Then have them position the phones or flashlights at either end of the meterstick and move the the Joly photometer in between until both sides are equally illuminated. You can then have them calculate the light output ratio to determine which light is brighter, either by having them derive the equation or just giving them the end equation from the video (equation on above). If two students have the same kind of phone, have one of them change the brightness setting of the flashlight and determine how much the brightness settings reduce the brightness.

## Compare the color of different light sources.

Although they may appear to be “white” light, many light sources are different color. When viewed with a Joly photometer, it is easy to compare the colors of two light sources. Incandescent bulbs often have a sickly yellow glow, where are many modern LED lights have a color more similar to the sun during mid-day hours.

## Measure the light output of the sun.

Bring a powerful light outside. Have students position a Joly Photometer between the light and the sun so that both sides are equally illuminated. Measure the distance from the Joly photometer to the center of the bulb. Use the known distance from the earth to sun. Calculate the output of the sun relative to the bulb.

Although these activities are effective and fun, there are more in-depth activities that can be done with the Joly photometer. Also, each of these is time consuming to set up. For quicker access to student-centered interactive learning, here are some Pivot Interactives activities that students can use to discover and apply the concepts that underlie the irradiance of light.

Activity 1: ** Discover** the inverse-square law for intensity vs distance for a point source. In this activity students will make measurements using a Joly photometer and plot data that reveals the inverse-square relationship between light intensity and distance.

Activity 2: ** Derive** an equation using the inverse-square relationship to compare the efficiencies and light output from different light bulbs This activity is intended for students in AP, IB, or college level physics courses.

Activity 3: ** Apply** the inverse-square relationship to compare the light power output from of four different types of bulbs: standard incandescent, halogen incandescent, CLF, and LED. Students can use data and an equation described in the activity to compare the efficiencies of each bulb. They compile class data to make comparisons and looks for trends, and practice arguing from evidence. (There is an optional part of this activity that guides students through each step of the equation derivation process for the efficiency equation, or they can simply apply the given equation.)

Activity 4: ** Apply** the inverse-square relationship to measure the output of the sun. The video in this activity allows students to measure the location where the light intensity coming from an electric light is equal to the light from the sun. Using this measured distance and the known distance to the sun, students can calculate the light output from the sun relative to the light bulb.