Lab Report

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THIN LENSES 10

LabReport

THINLENSES

Names

Thinlenses

SECTIONA: LIGHT RAYS

A1:Testing the Law of Reflection

Schematicsof set-ups

Thediagram below represents the schematic used in the measuring of theangle of incidence as well as reflection

Measurements

Reading

Angle of incidence (1)

Angle of reflection (2)

1

48

50

2

50

49

3

24

25

Logicalevidence-based explanation

Whenlight meets a boundary i.e. a mirror, the light rays are reflected.For example, the moon is seen because it reflects the light from thesun. The angle of reflection is always equivalent to the angle ofincidence (Knight, 2013).

Conclusion

Fromthe experiment, the angles are observed to be nearly equal. However,they may not be equal due to some errors during the measurement andexperimental set-up.

A2:Testing Snell’s Law

Schematicsof set-ups

Measurements

Reading

Angle of incidence (1)

Angle of refraction (2)

2

1

40

26

1.466

2

40

26

1.466

3

38

25

1.457

4

63

38

1.447

5

38

24

1.514

Average

1.47

LogicalEvidence Based Explanation

Lightnot only changes direction when it meets a reflective surface butalso changes direction when it passes across a boundary between twodifferent media of propagation. For instance, a border between glassand water (Beyerer et al., 2016). The change in direction is referredto as refraction.

Accordingto Snell’s law,

isthe refractive index of air and isof the second media

Conclusion

Therefractive index of the plastic block was found by averaging therefractive index calculated for every reading. A material with arefractive index of 1.47 is Pyrex (University of Utah et al., 2016).Therefore, we concluded that the material of the block is Pyrex.

SECTIONB: FORMATION OF IMAGE

B1:Object-Image Relationships

Predictions

  1. No image would form upon removal of the lens

  2. The image is likely to get smaller and sharper. However, when it is moved away from the lens, a larger but blurry image is likely to be observed.

  3. If object is made to slide closer or towards the lens by a few centimeters, there is a higher likelihood of the image enlarging. If the object is moved away from the thin lens, it is likely that a smaller image will be observed.

Observations

  1. On removing the lens, no image is observed.

  2. On moving the screen closer or away from the lens by some few centimeters, two comments are made. The image gets tinier as the display is taken far from the lens. The second observation is that the picture gets bigger when the screen is taken far from the lens.

  3. Through observation, the picture gets tinier as the object moves far from the lens. If the object is transferred towards the lens, a larger image was seen.

Measurements

The following measurements were obtained

Position of object (±0.0005) m

Position of lens (±0.0005) m

Location of image (±0.0005) m

0.1

0.75

1.08

015

1.09

0.2

1.105

0.25

1.12

0.3

1.15

0.35

1.195

0.4

1.27

Explanation

Thelens acts as a medium that refracts the rays of light and convergesor diverges the rays to form an image. In the eye, the part where animage is formed is known as the retina. Lights rays from the objectsreach the eye and get refracted by the cornea and the lens. The raysconverge at the retina to provide an image of the object. On takingthe object closer to the lens, distance of image from lens alsoincreases (Knight, 2013). The angle of refraction of the light raysfrom the object increases, thereby converging and longer distancesfrom the lens.

Raydiagram

B2:Partial Lens

Prediction

Theimage of the object is unlikely to be observed

Observation

Thescreen was found to be dark. No picture of the object was seen

Explanation

Manyrays leave a point. However, these rays trace the same path from theobject to the image. When an opaque object is placed in the path ofthe rays, they get blocked before converging (University of Utah etal., 2016). An image cannot be formed unless the refracted raysconverge at a point. In the above case, no object could have beenobserved because the rays were blocked immediately after refraction.

B3:Eyeballing the image

Prediction

Theeye should be placed in region A i.e. the region between the lens andwhere the screen was.

Observation

Theobject can be seen in area A

Explanation

Forthe image to be formed, rays from the object must be intercepted bythe eye. Region C has no rays because they already converged inregion B. The eye must be moved towards the lens to get a sharpimage. Two lenses are involved. These are the lens in the eye and theone used in the experiment. The image observed is upright because theeye intercepts the rays from the object.

SECTIONC: FOCAL LENGTH

C1:Eyeballing the Image as a means of estimating the focal length

Prediction

Atexactly one focal length, an image will not be formed

Observation

Whenthe object distance is reduced by sliding the object along theoptical axis, the resulting image becomes larger. A point is reachedwhen the image observed is upright.

Explanation

Thedistance at which the light rays converges after passing through thelens is affected by the distance between the object and the lens. Ifthe object and the lens are nearer, the rays converge further fromthe lens (Branca,2013).If the object is a long distance from the lens, the rays willconverge at a distance nearer the lens.

Fromthe experiment, the measured focal length was 25 cm

C2:Parallel Rays

Prediction

Therays would converge at the focal length.

Observation

Abright white spot is observed. The object is located away from thelens with its center in line with the center of the lens.

Explanation

Theparallel rays coming from the object are refracted by the lens andconverges at a point forming a bright spot. The focal point is wherethe rays converge while the distance between the lens to the point ofconvergence of the rays is called the focal length. The differencewith the other images observed before is that this is a spot ratherthan a reflection of the object. The ray diagram is shown below.

Theestimated focal length was 26 cm

C3:Graphical Analysis

  1. Graph #1

  1. Graph #2

Explanation

Thetwo figures indicate inverse proportionality between the imagedistance (ID) and object distance (OD). On increasing the OD, the IDreduces. The second graph shows that reciprocal of the ID and that ofthe OD (Beyerer et al., 2016). From the graphical analysis, the focallength can be determined by knowing the y-intercept of the secondgraph. The y-intercept will show the reciprocal of the focal lengthsince from the analysis of straight line equation with relationship y= mx + b

Usingthe statistical tool in excel, the y-intercept was determined to be4.35. Therefore, the focal length is the reciprocal of 4.35 which is0.23 m. In centimeters, the focal length is 23 cm.

Theslope of the graph is negative this shows the inverseproportionality between image and object distance.

C4:Method Comparison

Thefocal length obtained in #C3 is likely to be a better estimatecompared to the one achieved in #C1 and C2. In #C3, the errorsresulting from the estimation of image distance is neutralized byfinding a straight line that best fits the graph. The errors arereduced by balancing the values. By adjusting a set of values,accuracy can be determined. Therefore, the best way to determine thefocal length is the graphical analysis (Conrady, 2013). Also, for #C1to #C2, errors are likely to be high since it depends on the power ofthe eye of the person doing the experiment. The result might,therefore, be misleading.

C5:Two Focal Lengths

Prediction

Thereare two positions where an image can be seen.

Observation

Aftersliding the lens along the ruler, two locations were found where animage is formed. One was when the lens was very close to the object,and the other was formed when the lens and the object were furtherapart.

Explanation

Ifthe length between the lens and object is longer than the FL (focallength), the rays converges at a point, and an image is formed. Theimage formed is real for this case. An image is virtual if thedistance from the object and lens is less than the focal length. Inthis case, the refracted rays do not pass through the pictureposition. The object, as well as the image, are located on one sidethis demonstrates that the lens has two focal lengths. Every side ofthe lens has a focal length.

SECTIOND: DIVERGING LENS

Prediction

Asimilar technique is used in the determination of the focal distanceas seen above cannot be utilized in the determination of the focallength of the diverging lens this makes it difficult to detect animage. It is because diverging lens do not produce real images(Beyerer et al., 2016). Instead, it requires a converging lens andthe diverging lens whose focal length is to be measured.

Explanation

Thearrangement for image formation by the converging lens is maintained.The DL (diverging lens) utilizes the picture formed by the CL(converging lens) as its object. The DL is positioned between thepoint where the image for the converging lens is formed and the lensthis is to make sure that the lens picks the rays just before theyconverge to create a picture (Branca,2013).The arrangement allows us to get the real image formed and thusmeasurement of distances can be done. By the use ofdistance betweenthe object and the lens and that of image and lens, the focal lengthcan be calculated. The relationship between the two distances and thefocal length is used in the determination of any one of thedistances.

Thefigure below shows the ray diagram used

Pand q are used to determine the focal length

References

Branca,M. (2013). Converging or Diverging Lens?. ThePhysics Teacher,51(2),86-86.

Beyerer,J., Puente, L. F., &amp Frese, C. (2016). Machinevision: Automated visual inspection: theory, practice, andapplications.

Conrady,A. E. (2013). AppliedOptics and Optical Design, Part One.Newburyport: Dover Publications.

Knight,R. D. (2013). Physicsfor scientists and engineers: A strategic approach.Boston: Pearson.

Universityof Utah., United States., &amp United States. (2016).Chromatic-aberration-correcteddiffractive lenses for ultra-broadband focusing.Washington, D.C: United States. Dept. of Energy. Office of EnergyEfficiency and Renewable Energy.

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