Investigation 4 - Measuring refractive index of water and observing refraction PDF

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Measuring refractive index of water and observing refraction ...

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Investigation 4- Refractive Index of Water and Observing Refraction Aim: To observe Coins in order to measure the refractive index of water and to also observe the refraction of light in water using Snell’s law and ray tracing diagrams. Risk Assessment Tasks

Hazards

Associated Risks

Existing Risk Controls

Risk ratings with existing controls C L R 2 D M

Pouring water into the container

Spilling water

Slipping or falling due to spilled water

Handling glassware

Dropping glass jug

Cutting yourself on broken glassware

Pour water carefully into the container and if spills do occur wash immediately Handle 2 glassware with caution and keeping glassware at the centre of the table

C

M

Part 1: Measuring nwater Results hwater(cm)

hcoin(cm)

Uncertainty in hwater(cm) 0.05 0.05 0.05 0.05 0.05

4 8 12 16 20

0.8 4.7 7.3 9.6 12.2

Height of Water vs Height of Coin 25

Height of Water (cm)

20

15

10

5

0 0

2

4

6

8

Height of Coin (cm)

Analysis

10

12

14

Part 2: Observing Refraction Vertical Pencil: The observed angle is the same as the angle measured. 15 degrees: The observed angle seems to exceed 15 degrees as the pencil clearly seems to bend underwater.

30 degrees: The observed angle exceeds the real angle of 30 degrees. However the pencil seems to bend even more

45 degrees The observed angle once again exceeds the actual angle and the bending of the pencil is even more prominent than the last two angles.

60 degrees: The observed angle again exceeds the actual angle and the bending of the pencil is more prominent than the previous angles.

Questions 1) Explain (using equations and diagrams) why the gradient of the graph you plotted in part 1 is equal to the refractive index of water (nwater). For this question do the derivation algebraically, do not refer to the value you have measured. Refraction occurs when light passes from one medium to another. Each medium involved in the refraction of light has a different refractive index given by the equation: NMedium = CVaccum / Cmedium Each medium bends light and slows it down. The more dense a medium is the more light will slow down and bend.

Hence gradient of graph is equal to the refractive index of water. 2) Compare the value you have recorded for nwater to the accepted value of nwater = 1.33. Discuss any discrepancies; make sure that you refer to the size of your uncertainty in your answer. The value that I obtained 1.48 in comparison to the value of nwater is relatively small. This different is most likely due to the fact that the procedure carried out to measure the coin from the outside of the jar was fairly unreliable, but since the uncertainty was small the overall index wasn’t largely affected.

3) Suggest a better technique to measure nwater. This technique does not need to be limited by the equipment you have at home, you may use any equipment you want. A better way to measure the refractive index of water more accurately would be by using the air cell method.

The air cell method consists of an air cell in the centre of a water filled container with two glass plates with a narrow gap between them. A monochromatic light source is used to pass a beam of light through the container and is viewed through the slits either side of the container. The cell is then rotated until no light passes (total internal reflection) through the apparatus and a reading of the air cell in this particular orientation is recorded. From here the air cell is rotated until no light passes through again and the angle of rotation from one position to another is found. NL sinA = ng sinr = na sin90 = na and hence the refractive index of water is given by: Nwater = na / sin A = 1/sin A

4) Use ray tracing to clearly explain why the straight object looks the way you observed it to look in water. This answer requires diagrams and words (and possibly equations). Marks will be awarded for clarity. HINT: consider light reflected off the far end of the straight object. How does it get to your eye?

When observing a pencil in water it seems bent because of the difference in densities between water and air. When light rays travels through less dense air into more dense water they refract. The density of different mediums correlate with their refractive index (the more dense an object is the higher its refractive index) given by snell’s law.

Conclusion In conclusion this experiment allowed me to successfully measure the refractive index of water. Although my value of 1.48 was slightly above the theoretical value of 1.33. The second part of this experiment allowed me to observe the refraction of light in water which makes object seem like they are bending due to the refraction of light in different mediums....