ECAT — Engineering College Admission Test (UET)

ECAT Physics — Physical Optics and Thermodynamics (Set 3)

Take the ECAT Physics — Physical Optics and Thermodynamics (Set 3): 30 multiple-choice questions (40 minutes) with instant scoring and a full answer…

What this test covers

30 ECAT Physics

Sample questions with answers

The first 5 questions of this test, with the correct answer and explanation.

Q1. In a Young’s double-slit experiment, light of wavelength 600 nm falls on slits 0.3 mm apart and the screen is 1.5 m away. The fringe spacing is
- A. 0.3 mm
- B. 1.5 mm
- C. 6 mm
- D. 3 mm ✓
Answer: D. 3 mm
Explanation: Δy = λD/d = 600×10⁻⁹ × 1.5/(0.3×10⁻³) = 3 mm.
Q2. If the whole Young’s double-slit apparatus is immersed in water of refractive index 4/3, the fringe spacing
- A. increases to 4/3 of its value
- B. decreases to 3/4 of its value ✓
- C. becomes zero
- D. is unchanged
Answer: B. decreases to 3/4 of its value
Explanation: λ in water = λ/n, so Δy = λD/(nd) falls by the factor 3/4.
Q3. Which of the following pairs of sources can produce a sustained interference pattern?
- A. Two separate sodium lamps
- B. Two coherent sources with a constant phase difference ✓
- C. Two bulbs driven by separate supplies
- D. Two torches of the same colour
Answer: B. Two coherent sources with a constant phase difference
Explanation: A stable pattern needs a constant phase relation; independent sources have randomly changing phases.
Q4. At a point on the screen the path difference between two interfering beams of wavelength 500 nm is 1250 nm. The point is
- A. half-bright
- B. a bright fringe
- C. a dark fringe ✓
- D. the central maximum
Answer: C. a dark fringe
Explanation: Path difference/λ = 2.5, a half-integer multiple, so the waves cancel.
Q5. A thin film of refractive index 1.5 on glass of higher index is to be non-reflecting for light of wavelength 600 nm in air at normal incidence. The minimum thickness is
- A. 100 nm ✓
- B. 400 nm
- C. 200 nm
- D. 150 nm
Answer: A. 100 nm
Explanation: With phase changes at both surfaces, the condition is 2nt = λ/2, so t = λ/4n = 600/6 = 100 nm.

The other 25 questions

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Q6. In a Michelson interferometer, a mirror is moved by 0.1 mm and 400 fringes cross the field of view. The wavelength of the light is
- A. 500 nm
- B. 400 nm
- C. 250 nm
- D. 1000 nm
Q7. A diffraction grating has 5000 lines per cm. Light of wavelength 500 nm falls normally on it. The angle of the first-order maximum satisfies
- A. sinθ = 0.5
- B. sinθ = 0.1
- C. sinθ = 0.025
- D. sinθ = 0.25
Q8. A grating has d = 2 μm and is illuminated by light of wavelength 600 nm. The highest order of maximum observed is
- A. 5
- B. 4
- C. 2
- D. 3
Q9. For single-slit diffraction of width 0.2 mm with light of wavelength 500 nm, the first minimum occurs where sinθ equals
- A. 2.5 × 10⁻⁴
- B. 2.5 × 10⁻³
- C. 5 × 10⁻³
- D. 1.25 × 10⁻³
Q10. When light passes through a polaroid and then a second polaroid whose axis is at 60° to the first, the fraction of the (polarized) intensity transmitted by the second is
- A. 1/4
- B. 3/4
- C. 1/8
- D. 1/2
Q11. Polarization of light confirms that light waves are
- A. stationary
- B. particles only
- C. longitudinal
- D. transverse
Q12. In X-ray diffraction, X-rays of wavelength 0.2 nm reflect from planes 0.2 nm apart. The first-order Bragg angle is
- A. 60°
- B. 45°
- C. 90°
- D. 30°
Q13. Two moles of an ideal gas expand isothermally at 300 K to twice their volume. The heat absorbed is approximately (R = 8.31 J/mol K)
- A. 6000 J
- B. 4990 J
- C. 1730 J
- D. 3460 J
Q14. A gas at constant pressure 2 × 10⁵ Pa expands from 0.01 m³ to 0.03 m³. The work done by the gas is
- A. 400 J
- B. 4000 J
- C. 6000 J
- D. 2000 J
Q15. A gas is compressed so that 300 J of work is done on it, while 120 J of heat flows out to the surroundings. The change in its internal energy is
- A. +420 J
- B. −180 J
- C. −420 J
- D. +180 J
Q16. In an adiabatic compression of an ideal gas, the temperature of the gas
- A. stays constant, because Q = 0 throughout
- B. rises, because work done on the gas increases its internal energy
- C. falls, because no heat enters the gas
- D. rises, because heat flows in from the surroundings
Q17. Ideal gas at 300 K and 1.2 × 10⁵ Pa is heated at constant volume to 450 K. Its new pressure is
- A. 1.2 × 10⁵ Pa
- B. 2.7 × 10⁵ Pa
- C. 0.8 × 10⁵ Pa
- D. 1.8 × 10⁵ Pa
Q18. The root-mean-square speed of gas molecules is doubled when the absolute temperature is
- A. increased by a factor of 2√2
- B. doubled
- C. halved
- D. increased by a factor of 4
Q19. The average translational kinetic energy per molecule of an ideal gas at 300 K is (k = 1.38 × 10⁻²³ J/K)
- A. 1.2 × 10⁻²⁰ J
- B. 6.2 × 10⁻²¹ J
- C. 3.1 × 10⁻²¹ J
- D. 4.1 × 10⁻²¹ J
Q20. For a monatomic ideal gas, the ratio of the molar specific heats C_p/C_v is
- A. 3/2
- B. 5/3
- C. 4/3
- D. 7/5
Q21. Two moles of a monatomic ideal gas are heated at constant volume, raising the temperature by 20 K. The heat required is (R = 8.3 J/mol K)
- A. 498 J
- B. 332 J
- C. 1245 J
- D. 830 J
Q22. A Carnot engine operates between 600 K and 300 K. If it absorbs 1200 J per cycle, the work output per cycle is
- A. 300 J
- B. 900 J
- C. 600 J
- D. 1200 J
Q23. A Carnot engine has an efficiency of 40% with its cold reservoir at 300 K. The hot reservoir temperature is
- A. 700 K
- B. 420 K
- C. 500 K
- D. 750 K
Q24. A heat engine takes 800 J from a hot source and rejects 600 J to a cold sink. Its efficiency is
- A. 12.5%
- B. 75%
- C. 33%
- D. 25%
Q25. When 1000 J of heat flows reversibly into a body at a constant temperature of 250 K, its entropy change is
- A. 4 J/K
- B. 250 J/K
- C. 0.25 J/K
- D. −4 J/K
Q26. Which statement about a refrigerator is consistent with the second law of thermodynamics?
- A. Its coefficient of performance can be infinite
- B. It converts all absorbed heat into work
- C. It needs work input to move heat from a cold to a hot region
- D. It moves heat from cold to hot with no work input
Q27. Ice melting into water at 0 °C is accompanied by
- A. no change in entropy
- B. an increase in entropy
- C. a decrease in entropy
- D. a fall in temperature of the system
Q28. Huygens’ principle states that every point on a wavefront
- A. acts as a source of secondary wavelets that spread forward with the wave speed
- B. moves at twice the speed of the wavefront
- C. emits only light of the same colour backward
- D. absorbs the wavelets from neighbouring points
Q29. In Newton’s rings viewed in reflected light, the centre of the pattern is
- A. bright
- B. coloured
- C. dark
- D. blurred
Q30. A reversible process in an ideal gas has an isothermal and an adiabatic curve through the same point on a P–V graph. The adiabatic curve is steeper because
- A. the work done is always smaller in an adiabatic process
- B. no heat flows in an isothermal process, so its pressure stays low
- C. adiabatic curves always have a slope of −1 on a P–V graph
- D. a falling volume also raises the temperature, so pressure rises faster