1. What is meant by interference of light? How does it occur in Newton's rings?
Answer: Interference of light is the phenomenon where two or more light waves superpose to produce a resultant wave of greater or lesser amplitude. In Newton's rings, interference occurs between light waves reflected from the top surface of the air film (between the lens and glass plate) and the bottom surface. The phase difference due to the path length variation in the air film leads to constructive (bright rings) and destructive (dark rings) interference.2. State the principle of superposition of waves
Answer: The principle of superposition of waves states that when two or more waves overlap in space, the resultant displacement at any point is the algebraic sum of the displacements of the individual waves at that point.3. Why is a sodium lamp used as the light source in this experiment?
Answer: A sodium lamp is used because it emits monochromatic light (primarily at 589 nm), which produces a clear and distinct interference pattern. Polychromatic light (e.g., white light) would cause overlapping patterns of different wavelengths, making the rings indistinct and harder to measure.4. What is the significance of using a plano-convex lens with a large radius of curvature?
Answer: A large radius of curvature ensures that the air film between the lens and glass plate changes thickness gradually, producing well-spaced and observable rings. A smaller radius would result in closely spaced rings, making measurements difficult.5. What are some necessary conditions for sustained interference?
Answer: For sustained interference, waves must come from coherent sources with the same frequency, and comparable amplitudes, while maintaining a small path difference to ensure a stable interference pattern.6. How does the air film between the lens and glass plate contribute to the formation of rings?
Answer: The air film's thickness varies radially from the point of contact, causing a path difference between the reflected waves. This path difference, combined with a phase change upon reflection, leads to interference, forming alternate bright and dark concentric rings.7. What is the difference between bright and dark fringes in Newton's rings?
Answer: Bright fringes occur due to constructive interference when the path difference is an integral multiple of the wavelength (2t = m?). Dark fringes occur due to destructive interference when the path difference is an odd multiple of half the wavelength (2t = (2m+1)?/2), with an additional phase shift of ? from reflection at the denser medium.8. What is a coherent source of light?
Answer: A coherent source of light emits waves that have a constant phase difference and the same frequency.9. Why do the rings get closer together as the ring order (n) increases?
Answer: The diameter of the nth ring is proportional to \( \sqrt{n} \) (from \(D^2_n = 4n\lambda R \) As n increases, the increase in D<sub>n<\sub> becomes smaller relative to the previous ring, causing the rings to appear closer together.10. What would happen if a white light source were used instead of a sodium lamp?
Answer: A white light source contains multiple wavelengths, producing overlapping interference patterns for each wavelength. This results in colored rings near the center (due to varying constructive interference conditions) and a less distinct pattern farther out, making precise measurements difficult.11. How does the radius of curvature (R) affect the size of the rings?
Answer: The diameter of the rings is proportional to \( \sqrt{R} \) (from \(D^2_n = 4n\lambda R \). A larger R increases the diameter of the rings, making them more spread out and easier to measure, while a smaller R reduces the ring size and spacing.12. Explain why the center of Newton's rings is dark in reflected light.
Answer: At the center, the air film thickness is zero, and the path difference is zero. However, light reflected from the glass plate (denser medium) undergoes a ? phase shift, while the reflection from the lens does not. This results in destructive interference, producing a dark spot at the center.13. How can this method be modified to measure the refractive index of a liquid?
Answer: Introduce a liquid (e.g., water or oil) between the lens and glass plate. The wavelength of light in the liquid becomes \( \lambda /n \), where n is the refractive index of the liquid. Measure the ring diameters with and without the liquid, and use the modified formula \( D^2_n = 4n \lambda /n R \) to calculate n by comparing the two sets of results.14. Why are Newton's rings circular?
Answer: The air gap between the two surfaces varies in thickness in a circularly symmetric manner, creating concentric bright and dark rings where constructive and destructive interference occur, respectively. The circular symmetry of the lens causes the rings to be circular.