Measure the Refractive Index of HydrogelLu Yu, Wen Zhou, Yichen Gu, Yunhui Ni, Austin Abel

Instructors: Thomas Brown, Wayne Knox

Introduction Experiment Measurements Discussion

Summary

Conclusion

Project manager: Lu Yu

Photographer: Yunhui Ni

Laboratory Technician: Yichen Gu

Document Control: Austin Abel

Theory Expert: Wen Zhou

This project aims to measure the index of refraction of a hydrogel ball. Since a hydrogel ball is round, it is not possible to use Snell’s Law the index of refraction. Hence, if a concentrated solution of sugar has the same index of refraction with hydrogel, it is possible to determine the index of refraction of Hydrogel. Sugar is used to increase the concentration of the solution, and hence increase its refractive index. A layer of hydrogel is put into a transparent plastic box with certain concentration of sugar. A green laser beam shoots through the layer of hydrogel. It is possible to determine where the beam is traveling straight through the solution. At this point, record the concentration of the sugar solution , and refer to the refractive index database of Sugar, in order to determine the refractive index of Hydrogel. The results show that as the concentration of the sugar water increases from 1.64% to 7.69% the interface between the hydrogel and the sugar water becomes less noticeable. This illustrates that as the index of refraction of sugar water, becomes more similar to index of refraction of hydrogel, the beam travels along a more linear path. The experiment results show that the focal length of hydrogel is 11.62mm and the effective focal length is 11.77mm

-Hydrogel: Hydrogel is a super absorbent polymer that absorbs and releases water. They are highly absorbent (they can contain over 90% water) natural or synthetic polymeric networks. Hydrogels also possess a degree of flexibility very similar to natural tissue, due to their significant water content. In optical engineering, hydrogels are used to make contact lenses.

-Polarization: Polarization is a property of waves that can oscillation within more than one plane. Electromagnetic waves such as light show polarization. In an electromagnetic wave both the electric field and the magnetic field are oscillating but in different direction. Polarization of light refers to the polarization of the electric field.

Apparatus: Transparent plastic container, a bottle of hydrogels, a green laser pointer(wavelength: 532mm), clamp stand, a measuring cylinder, polarizer, a bottle of corn syrup (density: 1380 kg/m3), sugar (density: about 1 g/ml), flashlight (white light).

2.Measure the refractive index of a hydrogela. Put 60 ml water in an open rectangular transparent container, wait until the temperature is same as the room temperature(20℃).b. Put Hydrogels into the container filled with water.c. Shoot a green laser beam(wavelength 532nm) into hydrogels. d. Observe and record the reflected intensities at the interfaces between hydrogel and the solution.e. Add 1g sugar into the water and smoothly mix up until all the sugars are dissolved.d. Repeat step c.Repeat d. ad e. until the interfaces between hydrogel and solution are not distinct.f. Calculate the concentration of that solution and transform it into refractive index with the chart.

3. Do the different polarizers influence the focal length of hydrogela. Repeat the setup from experiment 1.b. Place the right polarizer of a 3D glasses in front of the green laser, and measure any change in the focal length of the hydrogel, with a ruler.c. Repeat part b with the left polarizer of a 3D glasses in front of the green laser.d. Observe any change in the focal length of the hydrogele. Repeat the setup from experiment 2.f. Place the right polarizer of a 3D glasses in front of the green laser.g. Place the left polarizer of a 3D glasses in front of the green laser.h. Observe any change in the optical path after being polarized.

1. Measure the effective focal length of a hydrogel: a. Place one hydrogel on a flat surface.b. Shoot a flashlight(white light) onto the hydrogel from a distance of 76cm and keep the flashlight stationary with a stand and a clamp.c. Carefully move the screen, by either bringing it towards or moving it away from the hydrogel in order to see where the light focuses.d. The point that converge on the screen is the focal point. Then, use a standard ruler to measure the distance between the center of the hydrogel to the screen.

n = Refractive Index of Hydrogel (1.3418)D = diameter of the hydrogel, 12mmOnce the focal length of the hydrogel from procedure one is measured, experimenter then figured out the index of refraction of the hydrogel, which can be apply into the equation above to determine the actual focal length of the hydrogel to see how accurate or precise of the experiment.

Using the table at background information and change the sugar concentration. With 4g of sugar and 60g of water. We get the most fine straight line. % concentration of Sugar Water=4g/(60g+4g)=6.25%≈6% Which relates to a refractive index of 1.3418. From experiment 2, the image distance(l’) is 11mm, the object distance is 760mm, then use the lens-maker equation:

EFL=nD/(4(n-1)) =1.3418*12mm/(4(1.3418-1))≈11.77mm

The theoretical effective focal length of 11.77mm is similar to the Actual focal length measured in the experiment 11mm.%error=(11.77mm-11.162mm)/(11.77mm)*100=5.166% (acceptable because under 20%)

A minor error in this experiment is precisely measuring the concentration of sugar water. A major error in this experiment is qualitatively measuring the straightness of the laer as the concentration of the sugar increases. A major error in this experiment is qualitatively measuring the reflected intensity of the green laser. A systematic error is the exposure from the camera in order to measure the reflected intensity. A random error is measuring the focal length of the hydrogel with the ruler. This experiment used two different types of sugar, Corn Syrup and granulated sugar. However for future experiment, it is important to be more consistent with the type of sugar used during the experiment, because different types of sugar have different concentration As the hydrogel is soaked inside the sugar concentration, it becomes bigger. This makes it difficult for the experimenter to measure, and cannot ensure a laser beam is traveling through the center points of the hydrogels. .

The results from this experiment illustrate that as the concentration of the sugar water increases from 0% to 6% the green laser travels along a more linear path. Diagram B illustrates that at a sugar concentrations of 1.64 %, the interface between the Hydrogel and the solution is well defined. However, as the concentration of the sugar water increases from 1.64% to 7.69% the interface between the hydrogel and the sugar water becomes less noticeable. Diagram E shows that at high sugar concentration of 7.69% the interface is not noticeable, and little blemishes are present. This illustrates that as the index of refraction of sugar water becomes more similar to index of refraction of hydrogel, the beam travels along a more linear path.

The polarization does not have detectable effects in our system. The focal length of the hydrogel is miniscule to measure an observable change. Before polarizing our system, the focal length of hydrogel is 11.16 mm. After placing the polarizer glasses (left lens and right lens) there is no change in the focal length. There is no measurable change in the focal length and the only difference is the light passes through the system become dimmer. Then a pair of polarization glass is placed in front of the laser pointer. The glass are designed to be left and right polarized so it is expected to shift the optical path after the polarization. However, again there is no observable change in optical path and the light is only going dimmer. It is not necessary to setup this experiment to observable a change in the intensity of polarized light.

Works Cited Edmund Optics. N.p., 2 Feb. 2001. Web. 11 Dec. 2014. <http://www.edmundoptics.com/technical-resources-center/optics/understanding-ball-lenses>.

Nation Stem Center. N.p.: n.p., n.d. Nation Stem Center. Web. 11 Dec. 2014. <http://www.nationalstemcentre.org.uk/dl/46855b8e0c682346f07acc473b728065867182c4/8567-catalyst_18_1_335.pdf>.

Refractive Index. N.p.: n.p., n.d. Refractive Index. Web. 11 Dec. 2014. <http://faculty.weber.edu/ewalker/Chem2990/Chem%202990%20Refractive%20Index%20Readings.pdf>.

Polarization Principles. N.p., 3 Mar. 1999. Web. 11 Dec. 2014. <http://background.uchicago.edu/~whu/polar/webversion/polar.html>.

To confirm this , use the equation in data analysis.

We are grateful for advice and assistance from Professor Thomas Brown, Professor Wayne Knox, and Maggie Han