06 Feb, 2013
In the water
PRESENTATION: A mechanical wave is the propagation of a perturbation through a medium that transports energy and that, when it encounters an obstacle or other waves, produces phenomena of superposition characterised by an increase or decrease in the space of the amplitude of the resulting wave.
- Wavetank in a glass, Dave Van Domelen, Phys. Teach. 50, 56 (2012)
- Modeling the 2004Indian Ocean Tsunami for Introductory Physics Students, Gregory A. DiLisi and Richard A. Rarick, Phys. Teach. 44, 585 (2006)
INTRODUCTION: In wave mechanics, interference is the result of the superposition of two or more waves with identical or similar frequencies. The principle of wave superposition establishes that the magnitude of wave perturbation at any point in the medium is equal to the sum of the displacements at that point of all the waves present. If the original waves oppose each other, then the resultant wave is smaller and the interference is said to be destructive. If the resultant wave is greater than the original waves because they have reinforced each other, then the interference is constructive.
In Physics, diffraction is a characteristic phenomenon of waves that is based on the curving and scattering of waves when they encounter an obstacle or pass through a crack. Diffraction occurs due to the way in which the waves propagate themselves; it is described by the Hygens-Fresnel Principle. There are several analytical models that can be used to determine the shape of a diffraction pattern: Fraunhoffer and Fresnel are two such. Both have provided the study of waves with great discoveries.
MATERIALS: transparent plastic box, plastic ruler, wooden block, comb, clay, powerful light source.
SETUP: Fill the plastic box with cold water. Place the wooden block with the ruler on it in such a way that the end of the ruler reaches the box. Mould same-sized balls of clay and mount them onto the comb at equal distances. Place the comb at the end of the ruler and make sure that the balls are in contact with the surface of the water but not submerged.
EXPLANATION: Tapping on the free end of the ruler with a finger causes small amplitude oscillations. Their frequency can be altered by varying the length of ruler hanging over the edge of the wooden block. This change of wavelength in turn affects the angles of maximum and minimum. These maximums and minimums appear as a consequence of the interference of the waves created at each source point, that is, each clay ball. In order to see the diffraction phenomenon, a barrier is placed as an obstacle for the propagation. In this way, spatial variations in the intensity of the diffracted wave can be seen.
CONCEPTS: waves, diffraction, interference, wavelength, maximums and minimums, frequency, wave intensity.
- Giancoli, D.C. Física para ciencias e ingeniería con Física moderna, Prentice-Hall, 2009.
- Sears F.W., Zemansky M.W., Young H.D. y Freedman R.A. Física universitaria. Pearson, 2010.
- R. Ehrlich, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, Princeton University Press, 1997.
STUDENTS 2010-2011: Sandra Martínez, Diana Martínez, Elena Maceiras.
LINK pdf STUDENTS: