1. Which method is used to measure the speed of light accurately?[BP 2013]
- Fizeau's method
- Foucault's method
- Michelson's method
- None of these
(c) Michelson's rotating octagonal mirror method (1926) achieved the most accurate measurement of c before modern laser techniques. 2. Velocity of light is equal to[BP 2009]
- 1/√(μ₀ε₀)
- √(μ₀/ε₀)
- 1/(μ₀ε₀)
- μ₀ε₀
(a) From Maxwell's equations: c = 1/√(μ₀ε₀) where μ₀ is permeability and ε₀ is permittivity of free space. 3. Time taken by light to cross a glass slab of thickness 5 cm and refractive index 1.5 will be[MOE 2013]
- 2 × 10⁻¹⁰ s
- 2.5 × 10⁻¹⁰ s
- 3 × 10⁻¹⁰ s
- 3.5 × 10⁻¹⁰ s
(b) t = (n × d)/c = (1.5 × 0.05)/(3×10⁸) = 2.5 × 10⁻¹⁰ s 4. For which color the velocity of light is maximum in diamond?[MOE 2011]
(a) Red light has the longest wavelength ⇒ least dispersion ⇒ highest velocity (v = c/n, where n is smallest for red). 5. The refractive index of diamond is 2.4, velocity of light in diamond is[MOE 2010]
- 1.25 × 10⁸ m/s
- 2.5 × 10⁸ m/s
- 1.5 × 10⁸ m/s
- 2.0 × 10⁸ m/s
(a) v = c/n = (3×10⁸)/2.4 = 1.25 × 10⁸ m/s 6. The correct sequence of electromagnetic waves in decreasing order of frequency:
- X-ray, visible ray, microwave
- Infra-red, visible ray, UV-ray
- X-ray, visible ray, UV-ray
- Microwave, UV-ray, X-ray
(a) EM spectrum order: γ > X > UV > visible > IR > microwave > radio 7. The velocity of light in glass (n=1.5) is 2×10⁸ m/s. The refractive index of liquid is[IOM 2014]
(b) Given as glass with n=1.5 (though velocity should be 2×10⁸ m/s for n=1.5 matches c/n). 8. Which correctly shows regions of the electromagnetic spectrum in order of increasing wavelength?[KU 2014]
- gamma → infra-red → visible → radio
- Microwave → ultraviolet → Visible → infra-red
- Radio → microwave → infra-red → visible
- Ultraviolet → visible → infra-red → microwave
(d) Correct increasing λ order: γ < X < UV < visible < IR < microwave < radio 9. Which part of the EM spectrum has frequency > infra-red and wavelength < X-ray?[KU 2014]
- Gamma rays
- micro-waves
- Ultraviolet
- Radio waves
(c) UV fits between X-ray (higher f, lower λ) and visible (lower f, higher λ) in the spectrum. 10. A light wave has a frequency of 100 Hz. The wavelength of the wave is:[IE 2011]
- 2 × 10⁶ m
- 3 × 10⁶ m
- 2 × 10⁸ m
- 3 × 10⁸ m
(b) λ = c/f = (3×10⁸)/100 = 3×10⁶ m 11. Which has lower energy than infrared waves?[IE 2013]
- Visible
- Microwaves
- X-rays
- None
(b) EM energy order: γ > X > UV > visible > IR > microwave > radio 12. The velocity of light in vacuum depends upon[IE 2012]
- Frequency
- Wavelength
- Amplitude
- None
(d) c is a fundamental constant independent of light's properties. 13. In Fizeau's experiment (720 teeth, d=9.633 km, 750 rpm), speed of light is
- 2.10×10⁸ m/s
- 3.10×10⁸ m/s
- 4.10×10⁸ m/s
- 2.9×10⁸ m/s
(b) c = 2D × N × n/t = 2×9633×(750/60)×720 ≈ 3×10⁸ m/s (n = teeth, N = rotations) 14. In Foucault's experiment (r=20m, 20 rps), angle of rotation is
- 3×10⁻⁴ rad
- 5.33×10⁻⁴ rad
- 6.22×10⁻⁴ rad
- 3.33×10⁻⁴ rad
(d) θ = ω × (2r/c) = (20×2π) × (40/3×10⁸) ≈ 3.33×10⁻⁴ rad 15. In Michelson's experiment, if distance between mirrors is doubled, speed of octagonal mirror should be
(c) v ∝ 1/d ⇒ halving speed compensates for doubled path length. 