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

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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 α is 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. If the illumination on both surfaces is the same and the distance from the source is the same we can derive from the Law of Lambert that: I2 = I1 (r22 / r12).

In order to perform an experiment with a photometer, both light sources are switched on and the distance of the second one is changed. When the source is placed at a location where the illumination of the two surfaces is equal, the distances r1 and r2 and the light intensity of one of the sources I1 can be used to find the intensity of the second source I2.

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Variables

Considering the fact that we have 3 tasks, the quantities that we measure change their nature. For the 1st and for the 2nd task:

Independent variable – distance r1  

Dependent variable – distance r2

Controlled variables – voltage V, light intensity I

For the 3rd task:

Independent variable – voltage V

Dependent variable – distance r2

Controlled variables – distance r1, light intensity I

 

Method

  1. With the help of the photometer we determine the distances r1 and r2 between the two lamps and the light source.
  2. With experiments we determine the light intensity of an unknown light ...

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