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11TH CLASSES PHYSICS SUBJECT: Single Slit Diffraction AIM: To understand the diffraction of light waves DURATION: 120 min. REAL LIFE: PRESENTATION: DAILY PLAN DATE: parallel light beams emerging from the grating and they are brought to a focus at point P (Figx). A clear image of the illuminated slit normal to the grating is formed on the screen (Fig11a, b). The kind of the interference depends on the path difference of two parallel rays Diffraction: The spreading of light into a region emerging from two adjacent slits of the grating. If behind an obstruction in path of the light is called the optical path difference between the two rays is diffraction. A slit opening, a fine wire, a sharp edged integer object or a pinhole may serve as a suitable obstruction interference takes place, and in the path of a beam of light from a point source. formed. If the path difference is odd multiplies of half of Diffraction Gratings the wavelength destructive interference takes place, and multiples of the wavelength constructive a bright fringe is We have seen that the wavelength of light could a dark fringe is formed. The wavelength of the light be measured by making use of Young’s double slit used can be calculated as it is calculated in Young’s experiment. However since the fringes formed in this double slit experiment experiment are wide and their intensities are weak, in this method the wavelength cannot be measured accurately. By increasing the number of slits to a very large number narrow fringes (lines) of higher intensities can be produced (Fig11). Moreover the fringes are formed far apart from one another. For this purpose Fraunhofer made the first grating. An ideal diffraction grating consists of a large number of fine equidistant parallel grooves ruled on a plate of glass. The region between two grooves is transparent and allows the light to pass through, while the grooves themselves do not allow the light to pass through. The grooves are usually ruled on the glass plate by a sharp diamond point. The regions between any two grooves behave like equally spaced parallel slits. Standard gratings may have 1000-10000 grooves or slits per centimeter. The distance between the centers of two adjacent slits of a grating is called the grating constant. The grating constant of a grating containing 6000 lines per centimeter is equal to 1 d cm/lines . 6000 Measurement of the Wavelength by Diffraction Gratings A grating of width 2-5cm (Fig10) is placed on d.sinθ the order of the diffraction, is the angle of deviation (diffraction angle) in Nth order, and d is the grating constant. When the angle and the order N are known the wavelength, can be easily calculated since the grating constant, d is known for all diffraction gratings. When a light and is incident perpendicularly on the grating. The light propagates through the grating and the wavelets from spectroscope can observe this coincide. The fringes of the shorter wavelength will be closer to the central bright fringe. When white light is used the central bright fringe will be white, and a continuous spectrum of the rainbow colors is formed on both sides of the central bright fringe. However this spectrum is different from that in Young’s double slit experiment where the image is wide and the colors are mixed. As a matter of fact a diffraction grating analysis the light to its component colors very well (better than a prism). HOMEWORK: MULTIMEDIA: DEMONSTRATION: EXPERIMENT: interference pattern. The lens of the spectroscope collects the TEACHER: different fringe where the central fringes of the two waves pattern of diffraction fringes is formed. The telescope the two each wavelength in both sides of the central bright different slits interfere. As a result an interference of including wavelengths is used, a set of fringes is formed for spectroscope (see Ch9 Fig11). The collimator of the wavelength, . Parallel light exists from the collimator Eq8.9 Where is the wavelength of the light used, N is the spectroscope table instead of the prism of the spectroscope is illuminated with monochromatic light of N DIRECTOR: