JNTU
JNTU B.Tech I Year ENGINEERING PHYSICS I Previous Question Paper (October/November - 2020 - R16)
Answer any five questions. All questions carry equal marks.
Main
Explain the formation of colors in thin films and show that films which appear light in reflected light appear dark in transmitted light.
Answer
When white light is incident on a thin film, the light reflected from the top and bottom surfaces undergoes interference. The path difference between these rays depends on the film thickness, refractive index, and the angle of incidence. Since the path difference varies with wavelength, specific colors undergo constructive interference while others undergo destructive interference, resulting in the observed colors.
For a film of thickness t and refractive index μ, the path difference for reflected light at near-normal incidence is 2μt cos r. Due to the phase change of π upon reflection at the denser medium (top surface), the condition for destructive interference (dark) is 2μt cos r = nλ. In transmitted light, the light undergoes two reflections at the rarer medium interface, resulting in no net phase change, so the condition for constructive interference (bright) is 2μt cos r = nλ. Thus, a condition resulting in a dark band in reflection corresponds to a bright band in transmission, making them complementary.
Explain why the center of Newton’s rings is dark in the reflected system.
Answer
The center of Newton's rings is dark because the air film thickness at the point of contact between the plano-convex lens and the glass plate is effectively zero. However, the ray reflected from the bottom surface of the air film undergoes a phase change of π radians (equivalent to a path difference of λ/2) because it reflects from a denser medium (glass). The ray reflected from the top surface of the air film does not undergo this phase change. Consequently, the two rays interfere destructively at the point of contact, resulting in a dark spot.
A soap film 5 × 10-5 cm thick is viewed at an angle of 350 to the normal. Find the wavelength of light in the visible spectrum which will be absent from the reflected light [µ = 1.33].
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What is the difference between interference and diffraction?
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Briefly explain the N-slits of diffraction pattern.
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Examine if two spectral lines of wavelengths 5890 Å and 5893 Å can be clearly resolved in the (i) first order and (ii) second order by a diffraction grating 2 cm wide and having 425 lines/cm.
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State and explain Malus’s law of polarization.
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What is double refraction? Explain the construction of Nicol prism.
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What are the quarter and half wave plates and explain the principle.
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Why is population inversion necessary to achieve lasing action?
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Describe the construction and working of Helium-Neon laser.
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Calculate the relative population in the laser transition levels in a ruby laser in thermal equilibrium (without pumping of atoms). The wavelength of the ruby laser light is 6943 Å at 300 K.
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Derive an expression for the numerical aperture of an optical fiber.
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Describe the structure of different types of optical fibers with ray paths.
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Explain the transmission of a signal through the step index fiber.
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Explain briefly the basic principle of optical fiber.
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Explain the transmission of a signal through the graded index fiber.
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Discuss the various factors contributing to attenuation in optical fiber.
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What is Bravais lattice? What are the different space lattices in the cubic system?
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Find the packing fraction for the BCC and FCC.
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What are Miller indices? How do you obtain for a given plane in a crystal?
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Derive Bragg’s law.
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Explain the significance of Burger’s vector.
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Define the terms: i) Stacking faults ii) Tilt boundaries iii) Grain boundaries.
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