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 in transmitted light?
Answer
When white light falls 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. Constructive or destructive interference occurs for specific wavelengths, causing certain colors to be reinforced and others to be suppressed, resulting in the observed colors.
For a film of thickness t and refractive index μ, the condition for constructive interference in reflected light is 2μt cos r = (n + 1/2)λ, while for destructive interference it is 2μt cos r = nλ. In transmitted light, the phase change of π occurs only if the light reflects internally at the bottom surface, which is not the case for transmitted rays. Consequently, the conditions for interference are reversed. If a film appears bright in reflected light due to constructive interference, the same wavelength undergoes destructive interference in the transmitted light, making it appear dark, and vice versa. Thus, reflected and transmitted systems are complementary.
Explain why the center of Newton’s rings dark in the reflected system.
Answer
In the Newton's rings experiment, the air film thickness at the point of contact between the plano-convex lens and the glass plate is zero. However, the ray reflected from the bottom surface of the air film (the top surface of the glass plate) undergoes a phase change of π radians because it reflects from a denser medium. The ray reflected from the top surface of the air film (the bottom surface of the lens) does not undergo this phase change. Due to this additional path difference of λ/2, the two rays interfere destructively at the point of contact, resulting in a dark center.
A soap film 5 × 10-5 cm thick is viewed at an angle of 35° 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 the population inversion is 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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