Refraction of Light

Jennifer C. MaglaqueCTP Feb 2010

Objective• To measure the refractive

index of materials using the critical angle method.

Abstract • Refraction is bending of light wave

when it enters a new medium where its speed is different. • The refraction of light when it passes

from a past medium to a slow medium bends the light towards the normal to the boundary between the two media

• The amount of bending depends on the indices of refraction of the two media and is described quantitatively by Snell’s law.• As the speed of light is reduced in

the slower medium, the wavelength is shortened proportionally.

Materials

• Crown glass• Glycerin• Ice• Perspex• Salt solution• Vegetable oil• Water

• Ruler• Beaker• Glass container• Flashlight• Powder

Description of Methods used• Put the water in the beaker, measure

its height and put some powder on surface of the water. Place the source of light at the bottom of the beaker and measure the diameter of the light on the surface of the water. Spread small amount of powder on the surface of water to see the diameter of light clearly.

• Compute for the index of refraction of water by getting the radius of the light (d/2). Get the tangent of angle x using (r/y) after getting the tangent (x) you can get now the value n of the water using 1/sin (x) and compare the measured n by the actual n.

How to get n=1/sinx

• Liquid/Glass

• Water• Y= 4.3 cm • ‘d = 9 cm• ‘r = 9/2• Tan Ɵ = r/y = 4.5/4.3 = 46.30• ‘n = 1/sinƟ = 1.38

y

r

• Do the same procedure for all the liquid materials and compare measured n to the actual n.

• For the crown glass and Perspex glass measure the thickness of the glass and put some powder on top of it and place the source of light at the bottom of the glass and measure the diameter of the light. Do the same computation same as in liquids.

• Compute for the percentage of accuracy by subtracting actual value from the measured value and divided by measured value multiply by 100 percent.

Experimental Set-up

Discussion of the results

MaterialsLength (y)(cm)

Diameter(cm)

‘r =d/2(r/y)

tanƟ ‘n= 1/sinƟ

Crown glass 2.2 3.9 1.95/2.2 41.55 1.50

Glycerin 2.8 5.6 2.8/2.8 45 1.41

Ice 3.0 6.5 3.25/3 47.29 1.36

Perspex glass

2.23 4.5 2.25/2.23 45.26 1.41

Salt solution 4.8 9.0 4.5/4.8 43.15 1.46

Vegetable oil

2.5 5.0 2.5/2.5 45 1.41

Water 4.3 9.0 4.5/4.3 46.30 1.38

Results of the studyMaterials Measured n Actual n Percentage of error

Crown glass 1.50 1.52 1.33

Glycerin 1.41 1.47 4.2

Ice 1.36 1.31 3.6

Perspex glass 1.41 1.49 5.6

Salt solution 1.46 1.54 5.4

Vegetable oil 1.41 1.47 4.2

Water 1.38 1.33 3.6

Conclusions and Recommendations• The results I obtained from the

experiment were quite closer to the actual values of the indices of refraction of the materials used. The small difference between the experimental value and actual value was maybe because of the source of light I used, distance of source of light to the surface of the materials and errors from me using the ruler to measure the diameter of light.

• I suggest for further validating of the results to use source of light which the intensity is sharp and make sure that from the source of light itself the materials surface is attach to it and use thin glass with a wide opening for convenient measurement.

References

• Optics third edition by Eugene Hecht, Addison Wesley Longman pg 94

• Physics GCE O’ level

Acknowledgements• Thank you for the following

Temasek foundationNIE-NTU CoordinatorsProf. Augustine Tan Tuck LeeAteneo de Manila UniversityOur Principal and Head Teacher

Almighty God