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Thursday, January 20, 2011

Physics Practical Report: How can the distance of an object from a specific convex lens affect its image formed on the other end of the lens?


Problem to be investigated:
How can the distance of an object from a specific convex lens affect its image formed on the other end of the lens?
Back ground on the topic to be investigated:
Convex lenses are of different thickness, density and curvature however all the convex lenses have few common properties:
Any lens always a fixed focal point, thickness, density and a centre of curvature.

When an object is placed in front of a lens, the lights rays travelling from the object  fall on the lens and get refracted. The refracted converging rays produce an image at a point where all the rays meet or intersect.  For demonstrating this as a diagram, two rays are taken and constructed for showing the point of intersection. In general objects at different distances from the lens would create different sized images.
Hypothesis – An object is placed on one side of a convex lens can give an image of different magnification or size with change in the placement of object. So, if the object is placed farther away from a lens the image formed would be smaller and nearer to the lens on the other side. The image formed will also be inverted.
Variables
1)      Independent – Distance between the object and the lens (u).
i)        Dependent –
a) Distance between the lens and the image (v)
b) Size of the image

2)      Constants –
i)        Convex Lens, Its:
a) density
 b) thickness
c) refractive index
ii)      Height of the lens and the object
iii)    The unit of measurement

Manipulation of Variables

The distance of object from lens can be altered by hand movement. As the light passes the lens it gets refracted to a different position and the so the distance of the image changes as well as the size of the image.
The convex lens used remains constant so its density, thickness and refractive index stay constant. The lens is placed on a stand and a candle of some height is placed in front of it so their height also remains constant. Finally the unit measured in should be kept constant or there might be mistake.



Material required
ü     Wax Candle
ü     Matchstick (for lighting the candle)
ü    Wooden block(if candle height is too small)
ü    Lens Holder
ü    Convex lens
ü    Screen
ü    Meter Rule

Method
1)    Firstly all the apparatus are collected with a proper lens and a proper thickness (not too thick nor too thin)
2)    Now the experiment is set up as per figure 1.1. The lens is in the lens holder, candle on the wooden block and a screen to capture the image.
3)    After, all three are in place; the candle is lighted and placed at a measured distance from the lens. The different (u) value length taken will be:
i)                   25
ii)                 30
iii)               35
iv)               40
v)                 45
vi)               50
vii)             55
viii)           60
4)    Now the image formed is observed. The screen is shifted up and down to get the clearest image of the candle light.
5)    Then we measure this distance ‘v’ and note it.
6)     Now the steps 1-5 are repeated for the same ‘u’ values two more times.
7)    Next all the data is recorded in a table which looks like as follows:







Calculated data with the focal length and magnification of the image
Focal Length = (uv)/ (u+v)

Magnification = v/u

Safety Measure:
1.    The ruler used should not be bent or there can be mistake in the reading.
2.     The Lens should be handled carefully or it might break.
3.    The flame used should be controlled and small and playing with the match sticks or lighters shouldn’t be implemented.



Data Processing
(all units to be measured are in cm except magnification)

Raw Data:


Try 1
Try 2
Try 3

Sl. No.
object distance (u)
image distance (v)
image distance (v)
image distance (v)
Average
1
25
64.3
64.3
64.3
64.30
2
30
46.5
46.5
46.4
46.47
3
35
38.5
38.6
38.5
38.53
4
40
33.7
33.8
33.5
33.67
5
45
31.4
31.5
31.5
31.47
6
50
29.7
29.6
29.9
29.73
7
55
28.5
28.5
28.6
28.53
8
60
27.7
27.5
27.6
27.60













Thus the analysed table is as follows:

Sl. No.
object distance (u)
image distance (v)
Magnification = v/u
Focal Length = (uv)/ u+v
1
25
64.30
2.572
18.00111982
2
30
46.47
1.548888889
18.23016565
3
35
38.53
1.100952381
18.34088849
4
40
33.67
0.841666667
18.28054299
5
45
31.47
0.699259259
18.51787271
6
50
29.73
0.594666667
18.64548495
7
55
28.53
0.518787879
18.78691141
8
60
27.60
0.46
18.90410959

A graph can help illustrate the data collected more effectively as follows:
A graph on Object distance vs. Image dista


 


Please use a spreadsheet, cant upload graphs
 
A graph on Object distance vs. Magnification:

Please use a spreadsheet, cant upload graphs
 



Analysis

These two graphs and tables demonstrate that with an increase in the object distance from the lens, the size of the image decreases as well as the distance of the image formed from the lens clearly. Both the lines are progressing in a negative slope signifying that the further away the object is from lens the smaller and nearer the object will form.




Conclusion:
The investigation on how can the distance of an object from a specific convex lens affect its image formed on the other end of the lens had fruitful results. The hypothesis stated if the object is placed farther away from a lens the image formed would be smaller and nearer to the lens on the other side. According the planning and manipulation of variables, the experiment had successful results after all. The hypothesis has been proved and illustrated even on graphs. It was also observed that when the candle grew shorter and the object level dropped, its image level increases and also the image formed were always inverted.

Evaluation
The investigation conducted has been a success. It has proved my hypothesis correctly. The variables were correctly controlled however the height of the wax candle had decreased since it was burning for long time. The readings were much or less correct but weren’t precise. This could have been the problem because of small parallax errors or maybe because the wooden ruler wasn’t properly straight. Another reason why the readings weren’t correct was because it is not possible to place the screen exactly at the perfect meeting position of the rays. So the next time I can keep these small things in mind and get a better result.

Suggestions
Few suggestions to the methods could have been:
1)      Candle flame could have been replaced with a torchlight which would remain a stationary beam of light at point along with a stronger light beam.
2)      The ruler used should have been maintained as a straight rigid bar. (Not bent). 




P.S. : Please leave a comment or tell whether it was useful, interesting or not,TY

5 comments:

  1. VERY useful! thanks :)

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  2. this was rubbish and i didnt understand anythign you culd of madke itr mere simples for mo coz e is simplse

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  3. i found this very useful but i did not understand the method

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  4.        0 its a train................... i like trains

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