Labs from Chicago, Fall 1993 :
Light as a form of Energy.

Dr. Rich Kron, Dr. Heidi Newberg, and Luisa Rebull
Labs written for the CARA Space Explorers, Fall 1993

This is meant to be handed out to the students.

I. Introduction

The fancy word for light is electromagnetic radiation. In fact, light is just one kind of electromagnetic radiation - other kinds include: These types of "light" may seem to be very different, and they have different-sounding names, but they are all the same phenomenon. They differ in one simple property, namely a property called the wavelength (or frequency). The list above is ordered from long wavelength (radio waves are typically meters long) to short wavelength (gamma rays are less than a ten-billionth of a meter long). The range of visible wavelengths - from deep blue to deep red - turns out to be a very small piece of the whole range. To detect invisible types of light, special instrumentation is required.

Not only can light be considered to be a wave of some sort, but the energy of the light is directly related to the wavelength. Specifically, if you know the wavelength of a piece of light (called a photon), you can calculate its energy with the following equation:

E = hc/l , where E is the energy of the photon, h is a constant called Planck's constant, c is the speed of light, and l is the wavelength. We will not use this equation directly, but you are supposed to know that blue light has shorter wavelength than red light, and therefore it has higher energy.

Another key concept is that electromagnetic radiation can carry energy across a vacuum - for example, the heat and light from the Sun travel through empty space to the Earth.

In general, there are three ways to transfer heat: conduction, convection, and radiation. Recall the lab where we heated up a mass of water with a resistor. The hot resistor, since it was in direct contact with the water, imparted (gave) thermal energy to the water. We would say that energy was transferred from the resistor to the water by conduction. The water that was next to the resistor then moved to other parts of the thermos, and water that was not yet heated moved in contact with the resistor. This is convection.

Both conduction and convection require matter to transfer the heat. People try to reduce the heat loss from their homes by putting insulation in the outside walls. Insulation typically has small pockets of air in it; air does not conduct heat very well, and by making the pockets small you keep the air from transferring the heat by convection. This also explains why it is better to wear several layers of clothing in the wintertime. The layers of air between your clothes are good insulation.

The Sun's energy cannot travel to the Earth by conduction or convection since there is hardly any matter between the Earth and the Sun. The light waves (electromagnetic radiation) emitted at the surface of the Sun carry heat to the Earth. Once the energy is on the surface of the Earth or in the Earth's atmosphere, the heat can be transferred by any of the three methods, or can be converted to other forms of energy.

Our atmosphere protects us from some of the electromagnetic radiation that can harm us, for instance, ozone protects us from ultraviolet light. This is why everyone is worried about the "ozone hole" in our atmosphere -- if the ozone is gone, it can't protect us from the ultraviolet light.

II. The Experiment

The purpose of this lab is to make some direct measurements of the amount of radiant light that hits a light-sensitive instrument called a photocell. (This is the same kind of gadget used in light meters in cameras.) We will make measurements by varying the amount of light. This will be done in two ways: we will use light bulbs with different wattages, and then we will measure the same light bulb at different distances.

The equipment is a photocell assembly, a meter that reads the resistance of the photocell, a light, and a meter stick. When you make a measurement, try to make sure that the light from your neighbor's set-up does not interfere.

The meter needs to be set to "omega" (omega), the symbol for electrical resistance (units of ohms). The higher the reading, the lower the intensity of light. (We will use the word intensity to indicate "amount of light at a particular place." As you might expect, intensity is directly related to the amount of energy being radiated or received.) Note also that k = kilo = 10^3, as in kilo-ohms, and M = mega = 10^6, as in mega-ohms.

Part A

Let's first answer this question: if I have four 50-Watt light bulbs and one 200-Watt light bulb, Commonwealth Edison will charge me the same for their use, since the amount of energy consumed is the same. But, do these two set-ups yield the same amount of light? If not, why not?

Procedure

Place a low-wattage bulb in its stand about a foot (30 centimeters) away from your photocell, and take a reading of the resistance. Then, replace the bulb (being careful not to burn your hand!) with one of higher wattage and repeat the experiment, filling in the table below. It is important to make sure that the distance is the same in all cases, that the illumination of the photocell is not shadowed, etc. It pays to take care in the alignment of your set-up!

After you fill in the first two columns, calculate the reciprocal (inverse, 1/R) of the resistance and enter it into the third column.

distance between light bulb filament and photocell _________ (cm)

wattage		  ohms		 1/ohms

_________ 	_________ 	_________ 

_________ 	_________ 	_________ 

_________ 	_________ 	_________ 

_________ 	_________ 	_________ 

_________ 	_________ 	_________ 

Plot your data on a graph, where the x-axis is (1/ohms) and the y-axis is the wattage of the light bulb.

Suppose I have four 50-Watt light bulbs and one 200-Watt light bulb.

(a) How much energy does it take to light the four 50-Watt bulbs for one hour?

____________ kilowatt-hours

(b) How much energy does it take to light the 200-Watt light bulb for one hour?

____________ kilowatt-hours

Do these two set-ups yield the same amount of light? If not, why not?

Part B

In this part, you will choose one of the light bulbs, and vary the distance. The closest distance should be about 5 cm, and the largest should be at least 100 cm. For example, you can try the following distances: 5, 10, 20, 40, 80, and 160 cm. Take the same precautions for aligning the apparatus as before: the photocell should be illuminated similarly by the light bulb at all distances.

Fill in the following table and make another plot.

This time, you do not need to take the inverse of the resistance, but you do need to square the value of the distance. On the x- axis, plot the resistance in ohms, and on the y-axis, plot the distance squared. Based on your graph, answer the following question: "How does the intensity of light decline as I move away from a light bulb?"

wattage of light bulb _________ (W)

distance (cm)	(dist2)		Resistance(ohms)

_________ 		_________ 	_________ 

_________ 		_________ 	_________ 

_________ 		_________ 	_________ 

_________ 		_________ 	_________ 

_________ 		_________ 	_________ 

_________ 		_________ 	_________ 

_________ 		_________ 	_________ 

_________ 		_________ 	_________ 

_________ 		_________ 	_________ 

Now that you plotted your results, you could estimate the resistance in ohms you would get for distances other than the ones you measured. One thing scientists often do is to make a mathematical formula that will give you the same answer as you would get by estimating from the graph. In this case, you should be able to write down a formula that allows you to plug in the resistance measured, and find out how far away the bulb is. (This is one way astronomers find out how far away stars are!!!) What is your formula?

distance =

Example Quiz Questions:

1. There are three ways that heat energy can be transported: conduction, convection, and radiation. Which of these applies to each of the following circumstances? 2. Which takes more energy to run, four 25-Watt light bulbs or one 100-Watt light bulb? Which produces more radiant energy?

3. A rainbow provides a continuous range of color from deep blue to red. What is the relationship between color and wavelength? What is the relationship between color and energy?

4. The planet Jupiter is about 5 times as far from the Sun as is the Earth.

Important Disclaimers and Caveats


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