Physical Optics (202) - Lecture Notes PDF

## Physical Optics (202) ### Nature of light: The simple fundamental facts about light which were known and well established can be summarized as follows: 1. Light travels in straight lines. 2. Light travels through empty space. 3. Light travels with a velocity of $3 \times 10^{10}$ cm/sec., and...

## Physical Optics (202) ### Nature of light: The simple fundamental facts about light which were known and well established can be summarized as follows: 1. Light travels in straight lines. 2. Light travels through empty space. 3. Light travels with a velocity of $3 \times 10^{10}$ cm/sec., and its velocity is independent of color and of the temperature of the source. 4. Light is a form of energy. This is shown by the fact that, if a beam of light is spread out into a spectrum by a prism, the various colors especially on the red side, a rise in temperature notice in a sensitive temperature. 5. White light consists of different colors whose refractive indices in a given medium (e.g. glass) vary continuously, to each color there corresponds a refractive index. This are all facts about the nature of light. ### Huygens and Newton gave two well known theories: #### Huygens wave theory: (Huygens Principle) The theory states that, in a medium traversed by waves, each point on a wave front at any instant may be treated as the source of secondary wavelets. The subsequent position of the wave front is obtained by constructing the envelope of the secondary wavelets due to point source distributed over the initial position of the wavefront. #### Newton's Corpuscular Theory: Newton assumed that light is a form of energy. A ray of light is made of a large number of small material corpuscles, which travel in a straight line with a limited velocity $ (3 \times 10^{10})$ cm/sec. The sum of the kinetic energy $\frac{1}{2} mv^2$ gives the mean energy. The emission of these corpuscles causes a loss in the mass of their source, but the loss may be too small to be detected. Newton assumed also that the corpuscles are perfectly elastic, and obey his law of motion. Accordingly the energy contained in a beam is the kinetic energy of the corpuscles. Rectilinear propagation can be easily explained if the effect of gravity is neglected on account of the smallness of each particle. Reflection and Refraction could be accounted for some how. But the theory is completely unable to account for interference and polarization. ### Conditions for interference: In order to have a well-defined observable interference pattern, the intensity at a region corresponding to destructive interference must remain zero, and that at a region corresponding to constructive interference must remain maximum. To realize such a state, it is necessary that the following conditions be fulfilled: - The two interfering sources must originate from the same source, i.e. coherent sources. This ensures that the phase difference between the two vibrating sources must remain constant so that the difference in phase between the two waves at any point is not changing with time. As a result condition sustained maximum and minimum are obtained. - The light must be of a single wavelength or very nearly monochromatic. This condition is necessary in order to avoid mashing of the pattern due to different wavelengths given out by the light sources. - The two interfering sources must be traveling in the same direction or make a very small angle with each other.