Magnum
Jiraboondilok and
Jake Fantozzi: 2006
Research Questions
If sunlight is passed through a biconvex
lens, an image of the sun is created on the colored paper,
how does the size of the image produced by the lens affect the
heat absorbed by the colored paper, and how does the color of
the paper affect the temperature rise?
Hypothesis
As the image of the light source decreases in size, the amount
of heat produced will increase. The amount of heat absorbed by
the object will change depending upon the color of object. If
the object is black it will have the most absorption, while a
white object will have the least amount of absorption.
Introduction
When light passes through a convex lens, the light rays converge at the focal point. The light bends because when light passes through the glass lens, the light is passing into a medium with a higher refractive index, causing the light to bend towards the normal. The light that passes through the front surface exits the lens entering the air again causing the light to bend away from the normal since it is passing into a medium with a lower refractive index.
As an screen is moved towards the focal point the image decreases in size, becoming more concentrated into a smaller area.
Apparatus
A Vernier Surface Temperature Sensor which is a thermocouple was secured behind the colored paper to measure the temperature increase of the paper due to the exposure to light.
Materials and Procedure
For this experiment the sun was used as the light source. A biconvex glass lens was used to produce the image of the Sun. Paper of three different colors, white, green, and black, was used as objects to absorb the heat. The lens and the paper were held with clamp stands. To ensure that heat losses were kept to a minimum and were consistent a cardboard box was placed surrounding the apparatus to block the wind. A surface temperature thermometer was placed under the paper to measure the temperature of the paper.
The white piece of paper was placed at a distance for which the image formed by the lens had a diameter of 7.5 cm, and subsequently, 7.0 cm, 6.3 cm, 5.5 cm, 5.0 cm, 4.5 cm, and 3.5 cm. Each time the paper was left for three minutes of exposure to the light. The final maximum temperature of the three minute period was recorded. The same process was repeated for the green and black paper.
Data
Temperature versus
image diameter graph of collected data.
Analysis
Temperature versus light intensity graph displaying relation between paper colors.
Discussion
As the image of the sun is reduced in
size the intensity (Watts per square cm) increases and the temperature
reached by the papers after three minute exposures rises accordingly.
The black paper absorbed the most heat, then came green and least
was the white paper. The expectations outlined above have been
shown to be correct, black paper increases in temperature more
that white but there is one significant finding that was not expected.
The green and white papers reflected light in a very similar way.
Green plants remain relatively cool in direct sunlight. They reflect the most intense part of the spectrum (in the green) and absorb for the purpose of photosynthesis in the blue and the red, depending on the exact mixture of pigments, which varies from species to species.
If plants were black, absorption would quickly raise the leaf temperature above optimal levels.
Evaluation
The Vernier Surface Temperature Sensor (thermocouple) used here had a very small thermal capacity and consequently a fast response time. It was held in place, tightly against the back of the heated paper, by slipping it into a pocket made by stapling two pieces of paper together. The temperature measurement was made in one fixed position close to the center of the image and was not an average over the whole area. In future the infrared remote sensing thermometer which records surface temperature over a 2 cm circle could be used to eliminate these problems.
The method of varying the intensity is only if the intensity of the sunlight is constant and the same on each day. Bangkok is a tropical location with the sun overhead at mid day. The measurements were made within one hour of 12.00 o'clock on days that appeared equally bright and clear. In future measurements a high wattage quartz halogen lamp could be used instead of sunlight to eliminate variations in the primary light source.
The measurements were made on calm days and the apparatus was placed in a large box but there was a little unv certainty introduced because of wind. In future, using a lamp inside would elimate the effect of wind.
In spite of these three considerations the data in this present work is accurate enough to draw definite conclusions, to show the similarity between green and white paper and the very different absorption of black paper.