Light is a form of a wave that vibrates back and forth

# Light is a form of a wave that vibrates back and forth -...

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ANALYSIS Light is a form of a wave that vibrates back and forth (oscillates) as it moves. Individual light waves each have their own wavelength, as well as direction of vibration. Thus, they can be characterized as particles that are radiating. By definition, anything that radiates, such as sound waves, gravity, etc. follows the Inverse Square Law including light. 1 This law states that a specified physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity. 2 The light from a point light source spreads out uniformly in all directions. The intensity at a given distance r from the light will be equal to the power output of the light divided by the surface area of the sphere through which the light has spread. Since the area of the sphere varies as the square of its radius, r, the intensity will theoretically vary inversely as the distance squared, or 1/r 2 . For that, we can say, I 1r2 Thus, for a comparison of two light, = Ir2 k = [ ] I1r12 I2r22 1 Remarkably in the first part of this experiment, there are two ways of decreasing the light intensity observed in the photometer having a fixed distance. These are by: Introducing a neutral density filter, that is capable of decreasing the transmittance of the light Moving the light source to a farther distance from the photometer ___________________________________________ 1 J. Peatros, Physics f Light and Optics, Hurley Publishing House, 2 nd Edition, 2006

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2 B. Saleh, and M. Teich, Fundamentals of Photonics , Wiley-Interscience, 5 th Edition, 2001 In this experiment, the said methods are done simultaneously such that the light seen at two sides of the eyepiece of the photometer are comparatively of the same luminosity.
1 The image presented above shows the set-up for the first part of the experiment. It follows that the filter is mounted on the side where light source 1 is directed. Adjustments on the distance of the light source 2 are done afterwards, so that the light seen at the apparatus are comparatively the same. Table 1 shows the results for the first part of the experiment. It follows that as the transmittance 2 is decreased, the required distance between the photometer and the light source needed for an equal amount of light seen in the photometer is increased. It is due to not all the light have passed through the medium , thus the distance, in accordance to inverse square law, must be increased at a distance equal to the square root of the fraction of light passed through and the distance of the other light source at 100% permeability. Table 1. Inverse Square Law Transmittance Experimental r 2 = ( ) r2 I1 r1 2I2 % Error r 1 =30cm 100% (I 2 =I 1 ) 30 cm 30 cm 0.00 % 75% (0.75I 2 =I 1 ) 26.1 cm 25.98 cm 0.46 % 50% (0.50I 2 =I 1 ) 21.4 cm 21.21 cm 0.88 % 25% (0.25I 2 =I 1 ) 15.3 cm 15.00 cm 2.00 % ___________________________________________ 1 Y. B. Band, Light and Matter , Wiley and Sons (2006), pp. 34-40 2 Transmittance

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Light is a form of a wave that vibrates back and forth -...

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