We did preliminary tests to ensure that all our equipment was in good condition and all working.
Method
After setting up our equipment we set the bulb at the height of 30cm. We then placed the glue stick (13.5 cm tall) 20cm away from the bulb and took the measurement. We then moved the glue stick another 5cm so that it was 25cm away from the light source. We then took the measurement. We carried on moving it 5 more centimetres away until we reached 70cm away from the bulb. After that we turned off the bulb, turned it back on and repeated the experiment another two times. This meant that we got 3 sets of results, which we calculated into an average length for each distance, this ensured more accurate results.
Fair test
To ensure a fair test we kept certain factors the same, these were:
- The bulb – we kept the bulb we used throughout the experiment was 150W we kept it the same because a more or less powerful bulb would alter our results.
- Same object – we used a glue stick for each of our results because changing it would create a different shape and sized shadow.
- Same position – we carried out the experiment in the same lesson so we didn’t move the experiment at all because that would change the amount of natural light shining in.
- Same time of day – if it was carried out at two or more different times of day the position of the sun would be different which means a different amount of natural light would be shining through which would affect our results.
- Same height – we kept the height at 30cm for each reading taken because a higher light source would change the length of the shadow.
- Same equipment – this must be kept the same because this may affect the way in which it is carried out which would affect the results.
Range of measurements
We took three readings of a length of 20cm, three of 25cm, three of 30cm and so on until 70cm away. We took three for each distance because it ensured the results were more accurate by adding each result up and divided the by three because it produced an average.
Risk Assessment
To make sure that our experiment was as safe as possible we:
- Ensured that the bulb had completely cooled down before removing it.
- We made sure that the retort stand was on a flat surface and not too near the edge of the table, so it wouldn’t fall off and be a danger to someone.
- We ensured that no unnecessary equipment or belongings could affect our experiment and were moved out of the way.
Obtaining Results (O)
I am putting my results in a table so that it is easier to spot patterns and rules.
Analysis (A)
The first impression from looking at the graph is that it is almost perfect/very strong positive correlation almost perfectly fitting the line of best fit. The points that don’t fit exactly are on the distances 20cm away, 30cm, 35cm, and 55cm away; but at most they were only 0.5cm longer or shorter than the line says which shows that there is almost a perfect proportional relationship between the distance of a light source and length of its shadow. I know this because each time the distance from the light source increases by five the average length of its shadow increases by either 2 or 3 cm (to the nearest whole number).
For example the difference between 20 and 25cm away from the light source the shadow is 3cm longer and the same difference between when the distance is 25 and 30cm.
Looking at the Average length results column there seems to be a pattern emerging in the numbers after the decimal point.
10.16
13.16 from distance 20cm – 40cm away
- all are averaged to [n .16]
From 50cm – 65cm away there is a pattern in the numbers after the decimal point again.
E.g. 26.16
28.83 Rounded to [n .16]
31.16
- Rounded to [n .83]
Conclusion
From my graph I can see that there is an almost directly proportional relationship between the distance from the light source and the length of the shadow.
I can see that the further away an object is from a light source the longer it’s shadow will be.
I know this because the line of best fit is going upwards. This means that my original prediction was correct and this was what I was expecting to see.
This diagram shows why when the object is further away, the shadow is longer.
Evaluation (E)
Suitability of the procedure
We encountered a few problems during our experiment but we discovered the majority of them during our preliminary tests. The main problem we discovered was that we performed the experiment during the day, which meant there was a lot of natural light coming in through the windows.
We did not overcome this problem completely because it was impossible to come back to school in the evening or find a location completely dark so we had to shut the blinds and cover up all the places where outside light was seeping in.
The method was a suitable way of obtaining results at the level we are at as GCSE students but at a higher level or for more in depth investigation it would be better to do the procedure at night or in a room with no windows to ensure the clarity of the shadows which would give a more accurate results.
Accuracy of measurements
Our measurements were reasonable accurate but because the light coming through the window meant that the outline of the window was not totally clear but if it was totally dark they would be much more defined which means the results would be much more accurate. They evened out to be more accurate though because we repeated our readings 3 times. Our repeated readings were similar. The biggest difference between repeat readings was 1.5 cm, which was when the object was 35cm away. This may have been because the shadow was not clearly defined which made it difficult to measure.
Our results were accurate because there was no anomalous results which means there were nor results that didn’t fit the pattern.
Improvements
We could improve our practical procedure by using more accurate measuring devices than a metre stick and therefore be able to have each result measured to 2 decimal places. We could also have improved our procedure by doing it in a different location where it was darker meaning the results would again be more accurate.
Further work
We could extend our investigation by investigating whether the height of the light source as well as the distance of the light source affects the length of the shadow.
We also could investigate the width of the shadow and how that is affected.
Bibliography
Modular science for GCSE textbook
Encarta CD ROM
GCSE science revision guide 2002
