PROBLEM : To determine the intensity of light sources.

MATERIALS:

SAFETY FIRST, Be sure to take every possible precaution to prevent INJURY and FIRE. SAFETY GOGGLES, when you strike a match, sometimes a piece of match will fly off in any direction. Fire is a possibility, be sure to have means to put a fire out, best thing is to have on hand, is a fire Extinguisher and a source of water! PVC plastic pipe can catch on fire, so keep flame well away from Plastic pipe. CAUTION: It is also important to wrap the 2 and 1/2 inch of PVC pipe, the one to which you will attach the candle to, with aluminum foil to prevent the PVC plastic from igniting. CAUTION: If a person has long hair, do not work over the candle area. A length of 38 inches of 1/2 inch PVC pipe, Attach a metric stip to the pipe, or easier; measure with a tape and then convert to metric, and record the Du and Sc, Supports ( feet ) from SBB, a screen made from ( plastic gallon bottle ) miniature 4 watt clear bulb, miniature white bulb, a small 15 watt regular bulb.

PROCEDURE AND OBSERVATION:

1. Construct the following set up shown in below.

2. Obtain a square piece of white paper to fit the plastic frame, use a drop of oil, for the grease spot.

3. With the candle at one end and bulb at the other end of the meter stick; (our 38 inch PVC pipe) Dim the lights, then move the screen back and forth until the grease spot practically disappears. Be sure to check from both sides!

4. Now,record the distance between the candle and the screen, you should be able to read the number under the plastic handle or using your tape measure and convert to metric.

5. Using the following formula, find the intensity of the bulb.

Iu - the unknown intensity of the bulb.

Cs - the intensity of the candle power of the standard.

Du - the distance between the screen and the bulb.

Ds - the distance between the screen and the standard.

	( Du ) 2
Iu   =   Cs   X	----------
	( Ds ) 2

BUNSEN PHOTOMETER

	Distance between bulb & candle	Du	Ds	Candle power of bulb Iu	Distance beween bulb candle	Du	Ds	Candle power of bulb Iu	Average candle power of bulb
Minature clear bulb	100 cm				80 cm
Minatuer white bulb	100 cm				80 cm
Small regular bulb	100 cm				80 cm

QUESTIONS:

1. In the formula below, explain what does each letter represent?

	I
E   =	----------
	R 2

2. What are the two factors that the illumination of a light on a surface depend?

3. Explain, how an umbra and a penumbra are produced.

4. What is the light intensity of a lamp if the grease spot on the screen disappears when it is 15 centimeters from a standard 10 candle power lamp and 30 centimeters from the unknown lamp?

5. What is the light intensity of a lamp if the screen in a photometer is 25 centimeters from a standard lamp of 25 candle power and 50 centimeters from the unknown lamp?

6. How far from a newspaper should a 200 candle source of light be placed to provide 10 lumens per square foot?

7. How many lumens of light fall on a surface of a map 10 feet from a lamp having an intensity of 225 candles?

8. Explain the difference between luminous and illumination?

Talking about light, in years past when Eskimos used to build their houses from compacted snow, to make an igloo, and a window was an important necessity. They just got a block of ice and made an opening and inserted into the wall.

In the early history of this country, pioneers would build log cabins, for their houses. To have light in the log cabin, and there were no corner hardware stores, to buy glass for the windows, they would improvise. Well, the pioneers used to being very resourceful, used paper for their windows, not just any paper that is used today, because one rain storm and you would have to replace the paper. Paper that was used in time past, was made from animal skins to make parchment or vellum, used for writing. I am sure that you have heard of the 'Dead Sea Scrolls' well, these were written on vellum. As a matter of fact, the whole Bible was written by the scribes on vellum. Even today, you will see on Jewish holidays Rabbi's carrying large Scrolls, that contain the Torah. So when books are referred to in the Bible, it is understood that they are rolls or scrolls. Even the founding Fathers of this country used vellum for all their documents, like the U.S. Constitution and the Bill of Rights. Today, you will find vellum used mostly in documents that have to last. If and when you get a degree, it will be most likely printed on vellum, and quite often referred to as a sheep skin. The sheep skin is durable, but to make it more translucent from mostly opaque, so that more light can pass through you have to apply oil to the entire vellum. You now have a durable window, where it will last many rain storms. You know what happens from weathering, the vellum loose some of the oil and therefore becomes less translucent, allowing less light to pass through the window. Every so often you have to keep reapplying the oil to maintain maximum translucency. .

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