11d sci teachingwaves lenses designlab 15shelleyo

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Shelley Ohashi 11D

10/09/2012

The Effect of the thickness of the convex lens to the focal length

INTRODUCTION

BACKGROUND INFORMATION

A convex lens is a type of lens that has a curve facing outwards. [1] It lets the ray of light toconcentrate on one spot. This one spot is called a focal point and the distance between the

lens and this point is called a focal length. This experiment was investigating on how

thickness of the lens changes any characteristics of lenses.

RESEARCH QUESTION

How does the thickness of the convex lens affect the focal length?

HYPOTHESIS

The prediction for this investigation is that the focal length will change depending on the

thickness of the lens. As the lens gets thicker, the focal point appears clearer and the focal

length increases.

VARIABLES

Independent Variable: thickness of the convex lens (5 types of lens)

Dependent Variable: the image distance and the focal length

Fixed Variable: light source (ray box with three slits) (4V), object distance (distance between

light and lens (8cm))

MATERIALS AND METHOD

MATERIALS

Five different thickness of double sided convex lenses (lens 1, 2, 3, 4, 5)

one white piece of paper to put it under the lens and on the wall

One ray box

One power

Ruler

Pencil

White board to let the focal point appear

Equation to calculate focal length (thin lens formula) [2]

f = focal point

do = distance of object (distance between the ray box and the lens)

di = distance of image (distance between the lens an the image appearing on the white

paper)



Shelley Ohashi 11D

10/09/2012

DIAGRAMS / PHOTOGRAPHS

↑Figure 1.1: Lens 1 view from front ↑Figure 1.2: Lens 1 view from top (minimum)

↑Figure 2.1: Lens 2 view from front ↑Figure 2.2: Lens 2 view from top


Convex lensRay box

Pa er

light direction

A white board

Move to

define the

focal point

rulerObject distance = 8cm



Shelley Ohashi 11D

10/09/2012


↑Figure 5.1: Lens 5 view from front ↑Figure 5.2: Lens 5 view from top (maximum)

PROCEDURE

1. spread a sheet of paper on the table

2. plug in the power source to the outlet. Plug in the ray box to the power

source.

3. Place the ray box on one side of the white paper

4. Use the three slits card for the ray box

5. Place the thinnest convex lens (lens 1) 8cm far apart from the ray box

6. Light up the ray box by turning up the power to 12V

7. Observe where the focal point is made with the lights on the white paper

8. Draw a line of the middle of the lens for the starting point with pencil on

paper

9. Mark the focal point with a pencil on the paper

10. Measure the focal length from the marked lines to the point with a ruler

11. Record the length in the experiment table

12. Repeat step 5 to 11 with lens 2, 3, 4, 5

13. Repeat step 5 to 12 five times each for 5 trials

14. Create a table for the experiment result and calculate the average. The

average should be calculated by dividing the total of 5 test results by 5.



Shelley Ohashi 11D

10/09/2012

RESULTS

RAW DATA

Table 1: Defining the image distance with the focal point from the ray of light

Type of Convex Lens Trial 1 Trial 2 Trial 3 Trial 4 Trial 5

Lens 1 19.60 21.70 21.00 20.30 19.60

Lens 2 7.50 8.70 18.30 6.00 6.20

Lens 3 9.50 9.50 7.50 8.10 5.10

Lens 4 4.20 6.60 9.20 7.00 4.90

Lens 5 6.20 6.80 7.50 7.60 7.70

Observations

The light from the ray box was not clearly appearing on the white paper through the thin

lenses. The image that was showing on the white paper seemed to be appearing directlywithout refracting in the lens. The shape of the light image was a thin, long rectangle which

was the slit of the ray box. The unclear shape was shown when lens 1, 2, 3, and 4 was used.

However the light through lens 5 appeared clearly on the white paper. The shape of the

light image was a small circle that was brighter than the actual light coming out from the ray

box because the light was concentrated on one place.

PROCESSED DATA AND ANALYSIS

Table 2: Defining the image distance with the focal point from the ray of light

Type of Convex LensImage distance (cm)

Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Average

Lens 1 19.60 21.70 21.00 20.30 19.60 20.44

Lens 2 7.50 8.70 18.30 6.00 6.20 9.34

Lens 3 9.50 9.50 7.50 8.10 5.10 7.94

Lens 4 4.20 6.60 9.20 7.00 4.90 6.38

Lens 5 6.20 6.80 7.50 7.60 7.70 7.16

Calculation



Shelley Ohashi 11D

10/09/2012

Graph 1: Defining the image distance with the focal point from the ray of light

Table 3: The Effect of the thickness of the convex lens to the focal length

Type of Convex Lens Image distance(cm) Focal Length

Lens 1 20.44 0.173923679

Lens 2 9.34 0.232066381

Lens 3 7.94 0.250944584

Lens 4 6.38 0.281739812

Lens 5 7.16 0.264664804

Calculation

f = focal length

do = distance of object (distance between the ray box and the lens)

di = distance of image (distance between the lens an the image appearing on the white

paper)

0.00

5.00

10.00

15.00

20.00

25.00

Lens 1 Lens 2 Lens 3 Lens 4 Lens 5

I m a g e d i s t a n c e ( c m )

Type of Convex Lens



Shelley Ohashi 11D

10/09/2012

Graph 2: The Effect of the thickness of the convex lens to the focal point

The graph above is categorized in 5 different types of lenses. The result of this graph was

calculated by the average of 5 tests that was done in the experiment. As it is shown, lens 1

had the furthest focal length which also means, there was a certain distance for the ray of

light to appear clear on the board. Other lenses had similar results for the focal length.

However lens 4 is an anomaly that is off the trend. The focal length was shorter than lens 5

which was supposed to be longer. Table 3 and Graph 2 were processed with the data

calculated with the formula to define the focal length. However lens 4 remains as an

anomaly after the calculation.

DISCUSSION

CONCLUSION

The hypothesis for this investigation was proved in the experiment. The lens thickness

changed the focal length however it did not make much difference. This is proved in the

Tables and Graphs made above. Lens 1 is the minimum thickness of the lens and as the

number increases, lens 5 is the maximum thickness of the lens. However the diagram of the

lens is not in order. Lens 4 has a larger area and diagram than lens 5 however it is thinner

than lens 5. According to Table 1, Table 2, Table 3, Graph 1, and Graph 2, the image distance

and focal length changes as the thickness of the convex lens get thicker.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Lens 1 Lens 2 Lens 3 Lens 4 Lens 5

F

o c a l L e n g t h ( c m )

Types of Convex Lens



Shelley Ohashi 11D

10/09/2012

EVALUATION

Procedure

The method for the experiment was well planned with a clear set of variables. The

experiment was successfully done for 5 tests in two lessons including the processing the

data. However the materials for this experiment, especially the types of convex lenses did

not have the same diagram and certain pattern of thickness to measure the test accurately.

Validity

Other lights in the room that could be one of the distractions of defining the focal point will

affect the image distance. The image distance was measured by human which is inaccurate

for example, the angle where it was viewed was different each time or the focal point was

shown in wide distance.

Issues and Improvements

Each trail did not necessary have the same range of results therefore; there is no certainanswer for each lenses. The final processed data was constructed only from the trial

averages. The reason was the general types of convex lenses. This investigation was focused

on the thickness however there was not a certain pattern. Also the diagram of the lens was

not decided therefore some lenses were thinner but had larger area than the thicker,

smaller lenses. Additionally, depending on the diagram of the lens, the ray box was elevated

to let the ray of light go through the centre of the lens. To improve this issue, either the

diagram or the thickness should be included in the controlled variable. The thickness and

diagram should be measured as well to be included in the data such as, 2mm, 5mm, 8mm,

11mm, etc.

This experiment was investigating on different types of lenses which the graph wascategorized by a bar graph. This had a difficulty of displaying a trend line. If it was a

comparison experiment and a scatter graph was constructed, it is more obvious to analyse

and explain a trend.

There was some issue on planning the experiment with the limited resources. The

experiment plan was changed a few times according to the formula that was going to be

used. The test that was planning in the beginning was about changing the object distance

and defining the focal length however it did not change anything. To prevent this issue in

any other experiments, the plan should be tested first before the actual experiment starts.

FURTHER WORKTo expand this investigation, the object distance could be the set as an independent variable

to find the relationship of the objectives in the thin lens formula. Another experiment could

be done by comparison of convex lens and concave lens.

BIBLIOGRAPHY

[1] Bortner, Larry. "Background." Lens Equation. N.p., n.d. Web. 19 Sept. 2012.

<http://www.physics.uc.edu/~bortner/labs/Physics%203%20experiments/

Lens%20Equation/Lens%20Equation%20htm.htm>.

[2] "Concave Vs. Convex." Where You Came From. N.p., n.d. Web. 21 Sept. 2012.

<http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/

glossary/term-full.php?t=concave_vs_convex>.

11d sci teachingwaves lenses designlab 15shelleyo

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