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Investigating Light Intensity Through Design of the Solar Bottle Light
Contents:
Introductory Activity: Measuring Light Intensity
Laboratory Protocol: Designing a Solar Bottle Light
Scientific Explanation
Elaborate: Redesign Challenge
Assessment: Evaluate Your Understanding
Photos of Kit Components
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Introductory Activity:
Engage
Consider the following information:
Key Issues in Africa’s Energy Sector
KEY ISSUES IN AFRICA’S ENERGY SECTOR
•
Low access and insufficient capacity - Some 24 percent of the population of sub-Saharan
Africa has access to electricity versus 40 percent in other low income countries. Excluding South
Africa, the entire installed generation capacity of sub-Saharan Africa is only 28 Gigawatts,
equivalent to that of Argentina.
•
Poor reliability - African manufacturing enterprises experience power outages on average 56
days per year. As a result, firms lose 6 percent of sales revenues in the informal sector. Where
back-up generation is limited, losses can be as high as 20 percent.
•
High costs - Power tariffs in most parts of the developing world fall in the range of US$0.04 to
US$0.08 per kilowatt-hour. However, in Sub-Saharan Africa, the average tariff is US$0.13 per
kilowatt-hour. In countries dependent on diesel-based systems, tariffs are higher still. Given poor
reliability, many firms operate their own diesel generators at two to three times the cost with
attendant environmental costs.
How can people in developing areas, such as sub-Saharan Africa, use the sun to light their
homes?
Here is one way:
https://www.youtube.com/watch?v=jBz9ps88pQQ
http://dornob.com/solar-bottle-lamps-water-bleach-10000-liters-of-light/#axzz3E4gyMVKX
Today we will explore whether this really works!
Explore
Before moving on, we need to understand how we can measure the effectiveness of the design
using an instrument that measures light intensity. We should also be aware of how much light
is needed to do certain activities.
Encourage students to develop a plan to use
foot-candle meters to measure the intensity
of light around the classroom and school.
Compare your results with the amount of light
needed for various tasks, shown in the table
to the right.
3 Explain
Foot-Candle Meter
Units:
• Candela (cd): fundamental unit of all photometry that corresponds to the amount of
light--quantity of photons--produced by a standard light source. Originally, the standard
source was a real candle. Today, it's a theoretical construct, like most measurement
standards.
• Lumen (lm): unit of luminous flux, or amount of light radiating out from a light source
through a specific solid angle or cone of space. The efficiency of light sources is
assessed by comparing the energy input in watts with the luminous output in lumens
(lm/W).
• Foot-Candle (fc): a hybrid unit, which uses a metric measurement for luminous flux, in
combination with the square-foot for the unit of area. There are about 10.76 square feet
to the square meter. The foot-candle meter is an instrument for measuring light levels in
terms of foot-candles.
What is a foot-candle meter?
How many lux is one foot-candle?
Using date from the table above calculate the luminous flux in lux recommended for reading or
studying.
Based on your investigation provide a scientific argument to answer the question: Where is
the best place for reading in the school? Be sure to answer the question (claim), provide
data to support your answer (evidence), and connect this to the scientific principles and
measurements we learned about today (reasoning).
4 Laboratory Protocol: Designing a Solar Bottle Light
Provide students with materials to see how much light can be created with a soda bottle and
bleach.
Students will be given the following materials:
• Cardboard boxes of various sizes
• Duct tape
• String
• Bleach
• 1-Liter bottle, 2-Liter bottle
• Deionized water
• Sunny day!
Build a solar bottle light. Consider the variables you want to test. These variables might include:
• the size of the bottle
• the amount of bleach
• deionized vs. tap water
• amount of light
• size of the “room”
Use the foot-candle meter to measure the amount of light generated with and without the solar
bottle light. Record your data.
How does this work?
Scientific Explanation
5 To begin with, cutting a hole in your roof will let some sunlight in. Even a small hole in a
darkened room can make a big difference. There are, however, some problems with a simple
hole.
Besides letting in the rain and insects, you only get as much
light as falls directly through the hole. You can do much better
than that with some grassroots engineering. Putting a waterfilled container in the hole collects more light due to two
fundamental optical effects: Snell's Law and total internal
reflection.
When light moving through the air runs into a denser material
like water, it changes direction. That is, the light beam kinks
where it encounters the water surface so that it's traveling
more directly down into water. That's very helpful if you're
trying to get more light to go down through the hole.
Despite the help that Snell provides, some of the light will still
be on a path to the opposite side of the cylinder. A portion of
the light is trapped in the cylinder because of simple reflection,
which can happen anytime light passes from one transparent
material to another. However, during much of the day, the
light will strike the cylinder walls in such a way that lots of
light will be reflected down toward the hole. This is called total
internal reflection. Light reflected this way will bounce back
and forth as it travels down through the water column, much
like signals passing through an optical cable.
A laser beam trapped in a plastic
illustrates the effect well. So, without a solar bottle, this is all the sunlight you can expect to collect.
When the solar bottle light gets added to your roof, this is how much light you'll gather.
6 Once the light is in the room, the bottle offers another major benefit. If you only had a hole, or
even a small glass window, then sunlight passes straight through, and illuminates a spot on the
floor or wall, like this . . .
Because of multiple reflections and different paths caused by Snell's Law refraction and internal
reflection, light spreads out as it passes through the water column. That's what made the
bottles appear to glow in the video of Solar Demi doing his stuff.
In the first of these two sketches, sunlight passes directly through hole. In the second sketch,
I've added a few of the paths the light can follow as a result of the water column. There are
actually an enormous number of possible paths the light can take, which results in an apparent
overall glow, much like a light bulb.
All in all, it's a simple, clever, cheap, and environmentally sound lighting solution!
From Physics Buzz, http://physicsbuzz.physicscentral.com/2011/09/solar-bottle-superhero.html
7 Elaborate: Redesign Challenge
Using the steps of the engineering
design process consider how you
might improve on this design?
Assessment: Evaluate Your Understanding
Explain why the solar bottle light works better than a window or skylight?
Using what you learned about Snell’s Law and total internal reflection draw a picture to
compare what happens when sunlight strikes the solar bottle light and when sunlight strikes a
hole in the roof.
Test your redesign of this invention. How does it compare with the original?
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