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# Aim of the Laboratory Experiment: Understanding of propagation of light waves phenomena by using a photometer device.

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Introduction

Alexander 11 IB C

11.04.2012

Teacher: Plamen Malinov

Investigating Propagation of Light Waves

Lab Report № 13

Aim of the Laboratory Experiment: Understanding of propagation of light waves phenomena by using a photometer device.

Hypothesis: The luminous intensity of a lamp will increase as the voltage increases. However, when the distance between two lamps decreases, the voltage will also decrease, and as a result of this I predict that the light intensity will also decrease.

General Background: Light consists of electromagnetic waves with wavelength in vacuum from 400 to 700 nm. From a point source light propagates in every direction. The light intensity of a source I is measured by the international unit called candela (cd). The illumination E of a unit surface (1 m2) is measured by unit called lux (lx). E is connected with the light intensity of the source and its distance r by the law of Lambert. The law of Lambert states: E = (I / r2) cos α, where I is the light source intensity, r is the distance to the source, E is the illumination of a surface with area of 1 m2, and αis the angle between the distance to the source and the normal (imaginary line perpendicular to the surface).

A photometer is a device which consists of two surfaces which are illuminated by two different light sources. A photometer is illustrated in this picture:

Both surfaces are at the same angle α in respect to their light source.

Middle

14.8

14.7

14.8

14.8

± 0.1

Residuals r2

0

0.1

0

2

20

22

19

21

20.7

± 1.7

Residuals r2

1.3

1.7

0.3

3

25

25.5

24.0

26.5

25.3

± 1.3

Residuals r2

0.2

1.3

1.2

The error for the distance = one quarter of the cm or 0.25 cm

Table 1: Experimentally checking the photometer

The average (mean) value of the distance r2 can be calculated when the total result from the three measurements is divided by 3.

Table 2: Determining the light intensity of an unknown light source

 No r1, cm First measurement r2, cm ± 0.25 cm Second measurement r2, cm ± 0.25 cm Third measurement r2, cm ± 0.25 cm Average r2, cm Experimental error ∆r2, cm 1 15 13.5 13.7 14.0 13.7 ± 0.3 Residuals r2 0.2 0 0.3 2 20 17.2 19.6 18.2 18.3 ± 1.3 Residuals r2 1.1 1.3 0.1 3 25 19.6 20.7 19.8 20.0 ± 0.7 Residuals r2 0.4 0.7 0.2

The light intensity of the second lamp for the first measurement of the 2nd task is:

I2 = I1 (r22 / r12), where r2 is the average distance.

I2 = 25 x (13.72 / 152) = 20.9 cd

The light intensity of the second lamp for the second measurement is:

I2 = 25 x (18.32 / 202) = 20.9 cd

Conclusion

Evaluation

It was difficult for the team to set the light intensity  I1 andI2 to be exactly the same for the two sources because we did not have more precise device than our eyes, which resulted in a small inaccuracy. The sunlight entering through the windows of the laboratory was creating troubles for the team to see only the luminous light. It is evident on my picture that the photometer used is old and it used to confuse us when we were setting the light intensity I1 andI2 to be equal. An old and a poor calibrated instrument increases the systematic errors, thus increasing the experimental error.

Improvements

The experiment can be improved when it is being held in a darker room, with limited or no presence of sunlight, because in this way human eyes will be more sensitive to the differences in the light intensity and would determine the equal light intensity of the sources more accurately and precisely. The photometer can be replaced with a newer and better one, a digital photometer if possible that would measure the equal light intensity electronically like this photometer:

This student written piece of work is one of many that can be found in our International Baccalaureate Physics section.

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