- Simple Result Table & Simple diagram of general theory:
î D R
0°
10°
20°
30°
89°
Theory:
When the light ray travels from the fast medium (air) to the slow medium (glass block) its speed has decreased, wave frequency stays the same and the wavelength decreases. So parts of wave reaches boundary first and slows down which causes the change in direction. See diagram above.
Preliminary work:
Use a ray box with single slit to make the light ray through a rectangular glass block by changing the angle of incidence and measure the angle of relative refractions. (See the relative graph on the left and simple diagram on the right). My preliminary work also helped me to plan an accurate procedure: ①Avoid too many lines on the paper to prevent lines overlap each other. ②Avoid the range between each reading is too small to prevent the change of refraction angle is too small.③Minimized the error by using long ruler to draw the lines.
From the graph, I can see the general pattern that as the angle of incidence increases, the angle of refraction increases but slowly at the large value of i. This enables me to make the following prediction.
Prediction:
As the angle of incidence increases, the angle of refraction increases, but more slowly. From the formula (d= i – r), i rises, r rises slowly, so d rises rapidly at large i.
Analysis:
After I collected all the evidence, I organized and sorted them out then showed them on a result table and a graph.
I arranged the results as the table shown below.
From graph 1 shown on the right, it is showing angle of deviation against angle of incidence for refraction in a glass block. The general pattern shows us as i increases, d increases non-uniformly. This means when I is increasing, d increases but at a greater rate, making graph line steeper at large values of i.
Explanation of the trend:
When the light ray travels from the fast medium (air) to the slow medium (glass block) its speed has decreased, wave frequency stays the same and the wavelength decreases. So parts of wave reaches boundary first and slows down which causes the change in direction. See the wave diagram on the left.
As the diagram shows the incident ray was refracted and bends toward the normal when the light ray from air crosses to glass.
Angle of incidence is the vertically opposite angles with angle r and d. so the formula will be (i= r + d), we can transfer this formula to (d= i– r).
Although angle of incidence and refraction are both increasing, but I increases much more than r at large i. (See diagram X). From the formula (d= i– r), we can work out that d increases with I and at a greater rate at large value of i. From the previous points, I found out the graph should bend upwards.
This prediction showed a pattern of as the angle of incidence increases, angle of refraction increases, but more slowly which cause d increases rapidly at large value of i. Compare with my first graph and the conclusion I made, they are slightly the same. So my results support my prediction.
Evaluation
On the whole, the procedure I used is suitable and can be controlled easily. However there is one point I need to improve which is I should keep the light box close to the glass block to ensure a narrow beam. Due to this experiment having been done properly, the results I collected are sufficient and reliable evidences. From d vs i graph in the obtain section, I found it was a nice and smooth graph which support my evidences which are reliable.
I checked all the evidences to ensure their quality. During this I found there was an anomalous group of result (see table on the right). This group of result is different from the others, because the value of angle of deviation didn’t have too much change compare last group of results. Except this problem, the rest of results are reliable and repeats are close to each other which mean they are accurate. I also checked the consistency in table of result, all the values of measurement are approximated to integers and the average values are approximated to 1 decimal place.
Changes to improve the accuracy of results from the experiment:
- To use sharper pencil
- To use thinner single slit which can make narrower beam
- To use long transparent ruler to draw lines
- To make sure the line of vision is vertical to ruler which can help me to decrease the parallax.
Suitability of procedure:
I think my procedure is suitable, because we could make it fair test by keeping other variables constant. The results are enough and reliable.
Improvements:
- To do the experiment more times and gain more repeats to decrease the error
- To use narrower slit that makes the beam narrower to go through the block (incorporating focusing lens)
- To use larger glass block giving more accuracy on where light emerges, so the measurement error of angle deviation falls.
Accounting for anomaly:
The causes of anomaly in my results might be affected by this factor. ①Parallax error is caused especially wider beams at large angle of incidence. ②Possible chips on block can affect the track of refracted ray.
Even through my results have one anomaly, doesn’t affect the general conclusion. Because it still agreed with my prediction and the line on the graph is good enough. So the evidence is reliable enough to firmly definitely support my conclusion that as i increases d increases at an even greater rate.
Further Works:
We can investigate the same problem by using different ways.
- Investigate by using single colour (Get d vs I graph for different colours of spectrum). Simple apparatus seen below
See how angle of deviation varies with angle of incidence each time.
- Investigate deviation going from slow medium to fast medium (Simple apparatus below).
- Use different material of block (E.g. Perspex)