PHY260 LAB REPORT: REFRACTIVE INDEX OF A PRISM PDF

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Lab report about refractive index of a prism...

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UNIVERSITI TEKNOLOGI MARA, SABAH KOTA KINABALU CAMPUS

GEOMETRICAL OPTICS (PHY260) LABORATORY REPORT EXPERIMENT NO: 1 TITLE: REFRACTIVE INDEX OF A PRISM

Format Method Data 0.5

0.5

1.5

Lab report (8)

Analysis Discussions Conclusion Questions Marks 2

2

Skills

0.5

Attitude & team work

(6)

Marks EXPERIMENT 1

(6)

1

8

Total (20)

Title: Determining the refractive index of a glass prism using a spectrometer Objective: To determine the refractive index of a glass prism using the Minimum Deviation Method Apparatus: 1. Spectrometer 2. Lamp holder 3. Sodium lamp 4. Glass prism Theory: Refraction through a prism; Consider a ray HM incident on a prism of refracting angle A figure 1 below. If i1, r1, i2 and r2 are the angles of incidence and refraction at M and N as shown and n is the refractive index of the prism then, by using the law of refraction, Sin i1=nSin r1 Sin i2=nSin r2

From the geometry of figure, we can write: A = r1 + r2 as the angle NST is the exterior angle of the triangle MSN. The deviation angle, D, of the light (angle BOK) is given by: D = (ii - r1) + (i2 – r2)

Minimum deviation;

Experiment shows that as the angle of incidence i is increased from zero, the deviation D begins to decrease continuously to some value Dm and then increases to maximum as i is increased further to 90o. Experiment and theory shows that the minimum deviation, Dm, of the light occurs when i1 = i and r1 = r2 = r. Then we can write equation as: sin i sin r

n=

A = 2r Dm = 2i – 2r =2i – A i=

Dm + A 2

Substituting equation gets the refractive index of the prism: Dm + A 2 A sin 2

sin n=

Procedure: Refracting angle, A 1. The prism was placed on the table of the spectrometer with the angle to be measured directed toward the lens of the collimator. The two surfaces containing the refracting angle A with parallel light were illuminated by the incident light. 2. The telescope was turned till got an image formed by reflection on the surface the prism. 3. The position of the telescope(A1) was determined by reading the scale. 4. The telescope was turned in the opposite direction to get another image of the slit reflected on the second face of the prism, the position(A2) was determined in this case. 5. The angle of rotation of the telescope from T1 to T2 was equal to A1 + A2 = 2A, and, hence the angle of the prism, A was obtained. 6. Step 2 and 5 was repeated and another reading for 2A was obtained. The average A was calculated

Angle of minimum deviation, Dm 1. The glass prism was placed with its refracting angle A pointed away from collimator. 2. The telescope was turned until an image of the slit was obtained on the cross-wires at the position, T1. 3. The table slowly rotated so that angle of incidence decreased and the image of the slit is kept on the cross-wires by moved the telescope at the same time. 4. The telescope was turned till got spectral and the minimum deviation angle (D m) for each spectral line was measured, and the results was recorded. 5. Step 1 to 4 was repeated to get another value for Dm. The average Dm was calculated.

RESULT/DATA: Refracting angle, A; Attempt

1st telescope

2nd telescope

1 2

reading 356 350

reading 340 339

2A

A

16 11 Average

8 5.5 6.75

Minimum deviation, Dm; Attempt 1 2

1st telescope reading 8 5

Dm + A 2 A sin 2

sin The index of refraction, n =

3+6.75 2 6.75 sin 2

sin =

=1.44

2nd telescope reading 5 8 Average

Dm 3 3 3

DISCUSSION: The objective of this experiment was to determine the refractive index of a glass prism using the Minimum Deviation Method. According to the theory of refractive index of a glass prism, the refractive index of a glass prism is 1.5. Based on this experiment, after the data was calculated, the refractive index of the glass prism is 1.44 which a little bit lower from the theory. The percentage error that has been calculated was 4%. Even though the experiment was conducted carefully to minimize errors, there were still errors which may have affected the results and giving 4% as the percentage error. The possible errors that may have done was the two surfaces of the glass prism which contain the refracting angle was not parallel with the incident light that affect the scale of the telescope’s position. This is because, this error could affect the average of the refracting angle, A and next affect the refractive index of the prism’s result. Other than that, the blurry image of slit is might also the errors in this experiment that could affect the value of refractive index of the prism where the blurry and clear image of the slit has a little bit different in scale of the telescope’s position. The eyes might be not parallel with the reading scale of the telescope’s position that could give an error even small error in the results. Some of precautions need to be taken to get a better result. One of the precautions that have taken in this experiment was clean the glass prism and make it sure the glass prism was free from air bubble. Other than that, place the prism on the table of the spectrometer with the two surfaces containing the refracting angle A parallel with the incident light as could as can to minimize the error in taking the scale reading of the telescope’s position. Last but not least, make sure the eyes is parallel with the reading scale to minimize the errors.

CONCLUSION: In conclusion, the objective of this experiment was achieved where the refractive index of a glass prism was determined using the Minimum Deviation Method. Based on this experiment, the refractive index of a glass prism was 1.44.

PRE-LAB DISCUSSION: 1. Explain the meaning of the phrase ‘the refractive index of water is 1.33’. ‘The refractive index of water is 1.33” means that the ratio of the light’s velocity in vacuum to the light’s velocity in water is 1.33 2. What happens to the velocity of light as it moves from air into a medium of higher optical density. The velocity of light as it moves from air into a medium of higher optical density will refracted or bent towards the normal. The light travels more slowly in a denser medium make the bending occurs since a higher optical density indicates how much slower the light travels through that material.

REFERENCES: I.

Goyal, S. (2016, February 25). Refraction of light through a glass prism. Retrieved March 10, 2020, from https://www.jagranjosh.com/general-knowledge/refraction-oflight-through-a-glass-prism-1456391639-1

II.

(n.d.). Retrieved March 10, 2020, from https://amrita.olabs.edu.in/? sub=1&brch=6&sim=246&cnt=4...