When a beam of light of any wavelength travels, it bends towards or away from its normal and this phenomena is known as refraction. The refractive index, n, is given:. While travelling from less to a more optically dense medium, the beam of light slows down and bends towards the normal and while travelling from a dense to a less optically dense medium, the beam of light speeds up and bends away from the normal. Strawberry Jelly is an optically denser medium compared to air and hence, when an incident beam of light travels from air into the Jelly, it slows down and bends towards the normal.
In this investigation, the refractive index of a translucent cube of strawberry Jelly is investigated. Regardless of the ingredients, Jelly is formed by crystallization and the rigidity of the structure of the crystals is dependent on its temperature. Thus, the temperature can alter the optical density of Jelly as a result of which, the refractive index of the translucent cube of Strawberry Jelly changes with temperature. Hence, the research question that this experiment aims to answer is “What is the relationship between the temperature and the refractive index of Jelly?
” To form the Jelly, a sachet of ready-to-make strawberry Jelly was used and all instructions on the packet were followed to obtain a well-made Jelly. An electronic water bath was used to keep the jellies at different temperatures. As the Jelly is heated, the water in it would expand and thus, it was hypothesized that higher the temperature, lower would be the optical density of the Jelly and hence, smaller the angle of refraction and the refractive index. VARIABLES: Independent Variable: Variable Method of varying
Temperature of Jelly The Jellies were kept at five different temperatures, each 10oC apart, in the interval of 10oC to 50oC. An alcohol thermometer was used to measure the temperature inside the Jelly. The thermometers were inserted into approximately the same location in each of the experiment. Dependent Variable: Variable Method of measuring Refractive Index of Jelly, n A protractor was used to measure both the angle of incidence (i) and the angle of refraction (r). Controlled Variables: Variable Controlled Purpose of Controlling
Method of Controlling Thickness of Jelly Increase in thickness will lead to an increase in the lateral displacement, as a result of which, the angle of refraction (r) will change. A cuboid of Jelly of the same dimensions was cut out using a sharp brimmed rectangular bowl. Each cuboid was also wrapped up in a single layer of LDPE cling film. Angle of incidence (i) Larger the angle of incidence, larger is the lateral displacement and hence, the angle of refraction. The angle of incidence was kept constant using a protractor (45o).
Moreover, the angle of emergence (e) was confirmed to be 45o before recording r. Type of Jelly Different Jellies have different optical density hence, different refractive indexes. The same Jelly (brand and flavor) was taken for all the experiments. A large portion was initially made from which slabs were cut later on. APPARATUS AND CHEMICALS: Serial No. Name Uncertainty (if any) Capacity OR Quantity Purpose 1. Jelly – Excess To investigate its refractive index. 2. Sharp brimmed rectangular bowl – 1 To cut equal dimensions of Jelly cuboid for each experiment.
3. Low walled rectangular bowl – 1 To hold the cuboid of Jelly while light is being passed through it. 4. White paper board – 1 To function as the plane of refraction. 5. Board pins – 6 To mark the positons of the incident and refracted rays. 6. Protractor ±0. 5o 1 To measure the angles of incidence and refraction. 7. Pencil and Ruler – 1 To trace the path of the beam of light. 8. Electronic water bath – 1 To change the temperature of the Jelly and stably maintain it. 9. Thermometer ±0. 25oC 1 To check the temperature of the Jelly.
Excess of the same kind of Jelly was prepared by following the manufacturer’s instruction at the back of the packaging. 2. The white paper board was laid out flat on the work table and the low walled rectangular bowl was placed on top of it. 3. Using a pencil and a ruler, the low walled rectangular bowl was traced out on the white paper board. 4. With a pencil, ruler and the protractor, an incident ray was drawn on one side of the traced rectangle. 5. One the incident ray, three 6. Using a sharp brimmed rectangular bowl, a cuboid of Jelly was cut out and placed on the low walled rectangular bowl.
7. The piece of Jelly was completely wrapped with one layer of LDPE cling wrap and any air pockets in it were released to ensure a tight wrap. 8. The thermometer was inserted into the top right corner of the Jelly. 9. The entire setup of Jelly in the low walled rectangular bowl was cooled to 10oC in the electronic water bath and the temperature was maintained until it was to be used again in step 12. 10. Using a pencil, ruler and the protractor, an incident ray of light at 45o was drawn entering the rectangle on the white paper board.
11. On the incident ray, three tack board pins were stuck so that they were all collinear. 12. The Jelly was placed on the white paper board inside the rectangle drawn in step 3. 13. From the opposite side of the Jelly, the three pins were viewed in a way that they all seemed to be collinear. 14. Making sure no parallax error was caused, the fourth pin was put down on the opposite side of the Jelly making sure that it too lined up perfectly with the three pins on the other side. The same was repeated with the fifth and the sixth pin. 15.
The Jelly was removed and using a pencil and a ruler, a straight line was drawn through the three locations at which the fourth, fifth and the sixth pin were planted. 16. Procedure 13 and 14 was repeated four more times to obtain five sets of readings. 17. The entire procedure was repeated for all other temperatures: 20oC, 30oC, 40oC and 50oC. Safety Precautions: 1. Ensure a safe distance from the electronic water bath and use gloves while handling the hot water tray. 2. Ensure the necessary dimensions of the low-walled rectangular bowl and the sharp brimmed rectangular bowl are the same.
3. Ensure pins are not too thick and they are not bent. 4. While tracing the path of the pins, adjust height of viewing to ensure no parallax errors. Preferably, close one eye to isolate unnecessary lights. 5. Ensure thickness of cling film is negligible as it can have its own refractive properties. 6. Ensure straight lines for the rays and make sure the normal line drawn are parallel to the other. 7. Make sure that pins are appropriately spaced apart so that a more accurate determination of angle is allowed.