ECAT — Engineering College Admission Test (UET)

ECAT Physics — Physical Optics and Thermodynamics (Set 2)

Take the ECAT Physics — Physical Optics and Thermodynamics (Set 2): 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. A Young's double-slit arrangement gives a fringe width of 0.6 mm in air. What is the fringe width when the whole apparatus is immersed in water of refractive index 4/3?
- A. 0.45 mm ✓
- B. 0.3 mm
- C. 0.8 mm
- D. 0.6 mm
Answer: A. 0.45 mm
Explanation: The wavelength in water is λ/n, and fringe width ∝ λ: 0.6/(4/3) = 0.45 mm.
Q2. In a double-slit experiment with λ = 500 nm, slit separation 0.25 mm and screen distance 1 m, how far from the central maximum is the third bright fringe?
- A. 7.5 mm
- B. 3 mm
- C. 2 mm
- D. 6 mm ✓
Answer: D. 6 mm
Explanation: yₘ = mλL/d = 3 × 500 × 10⁻⁹ × 1/(0.25 × 10⁻³) = 6 × 10⁻³ m.
Q3. Light of wavelength 600 nm falls on two slits 1 mm apart, and the screen is 2 m away. How far from the centre of the pattern is the first dark fringe?
- A. 2.4 mm
- B. 1.2 mm
- C. 0.6 mm ✓
- D. 0.3 mm
Answer: C. 0.6 mm
Explanation: First dark fringe: y = λL/2d = 600 × 10⁻⁹ × 2/(2 × 10⁻³) = 0.6 × 10⁻³ m.
Q4. Two coherent light waves interfere destructively at a point when their phase difference there is:
- A. π/2
- B. an even multiple of π
- C. an odd multiple of π ✓
- D. zero
Answer: C. an odd multiple of π
Explanation: Destructive interference needs a path difference of (m + ½)λ, i.e. a phase difference of π, 3π, 5π, …
Q5. A soap film (n = 1.33) in air is viewed in reflected light of wavelength 532 nm. What is the minimum film thickness that gives strong (constructive) reflection?
- A. 50 nm
- B. 133 nm
- C. 100 nm ✓
- D. 200 nm
Answer: C. 100 nm
Explanation: Reflection at the top surface adds a phase change of π, so constructive reflection needs 2nt = λ/2: t = 532/(4 × 1.33) = 100 nm.

The other 25 questions

Take the timed test (free, no sign-up) to see your score and the answer and explanation for every question.

Q6. Light of wavelength 600 nm falls normally on a grating with 5000 lines per cm. What is the highest order of maximum that can be observed?
- A. 5
- B. 4
- C. 2
- D. 3
Q7. A diffraction grating has 4000 lines per centimetre. What is its grating element (spacing d)?
- A. 2.5 μm
- B. 4 μm
- C. 0.25 μm
- D. 25 μm
Q8. If the width of a single slit is made narrower, the central maximum of its diffraction pattern:
- A. stays the same width
- B. becomes narrower
- C. disappears
- D. becomes wider
Q9. Unpolarized light of intensity I₀ passes through a single ideal polaroid. What is the intensity of the transmitted light?
- A. zero
- B. I₀
- C. I₀/4
- D. I₀/2
Q10. Unpolarized light falls on two polaroids placed one after the other with their transmission axes at 90° to each other. The light emerging from the second polaroid is:
- A. half the incident intensity
- B. none at all
- C. the full incident intensity
- D. unpolarized
Q11. X-rays undergo second-order Bragg reflection at a glancing angle of 30° from crystal planes 0.15 nm apart. What is their wavelength?
- A. 0.3 nm
- B. 0.075 nm
- C. 0.15 nm
- D. 0.13 nm
Q12. In a Michelson interferometer illuminated with 550 nm light, 500 fringes cross the field of view as the movable mirror is moved. How far was the mirror moved?
- A. 0.275 mm
- B. 0.055 mm
- C. 0.1375 mm
- D. 0.55 mm
Q13. Spherical wavefronts from a point source of light, far away from the source, become:
- A. cylindrical wavefronts
- B. elliptical wavefronts
- C. smaller spheres
- D. plane wavefronts
Q14. We can hear sound around the corner of a building but cannot see light around it, because:
- A. sound waves are not reflected by buildings
- B. sound wavelengths are comparable to the size of the building, while light wavelengths are far smaller
- C. sound waves are longitudinal and light waves are transverse
- D. light travels much faster than sound
Q15. A gas is compressed: 500 J of work is done on it while it gives out 200 J of heat to the surroundings. What is the change in its internal energy?
- A. +700 J
- B. −300 J
- C. −700 J
- D. +300 J
Q16. On a P–V diagram, how does an adiabatic curve compare with an isothermal curve through the same point?
- A. They have the same slope
- B. The adiabatic curve is a horizontal line
- C. The isothermal curve is steeper
- D. The adiabatic curve is steeper
Q17. A Carnot engine absorbs 1000 J of heat from a source at 500 K and rejects heat to a sink at 300 K. How much work does it do per cycle?
- A. 1000 J
- B. 400 J
- C. 250 J
- D. 600 J
Q18. A Carnot engine has its source at 600 K. If its sink temperature is lowered from 300 K to 200 K, its efficiency changes from:
- A. 50% to 75%
- B. 50% to 33.3%
- C. 33.3% to 50%
- D. 50% to 66.7%
Q19. What is the ratio γ = Cp/Cv for a monatomic ideal gas?
- A. 4/3
- B. 5/3
- C. 3/2
- D. 7/5
Q20. How much heat must be supplied to 1 mole of a monatomic ideal gas to raise its temperature by 20 K at constant pressure? (R = 8.31 J mol⁻¹ K⁻¹)
- A. 249.3 J
- B. 166.2 J
- C. 415.5 J
- D. 581.7 J
Q21. According to the kinetic theory of gases, the absolute temperature of an ideal gas is directly proportional to the:
- A. total number of molecules
- B. average speed of its molecules
- C. average potential energy of its molecules
- D. average translational kinetic energy of its molecules
Q22. What is the total translational kinetic energy of all the molecules in 1 mole of an ideal gas at 300 K? (R = 8.31 J mol⁻¹ K⁻¹)
- A. 1246.5 J
- B. 2493 J
- C. 7479 J
- D. 3739.5 J
Q23. The r.m.s. speed of the molecules of a gas in a closed rigid container is doubled. The pressure of the gas becomes:
- A. four times
- B. unchanged
- C. twice
- D. half
Q24. 600 J of heat flows by conduction from a large reservoir at 600 K to another large reservoir at 300 K. What is the net change in entropy of the two reservoirs?
- A. −1 J/K
- B. zero
- C. +1 J/K
- D. +3 J/K
Q25. In all natural (irreversible) processes, the total entropy of the system and its surroundings:
- A. remains zero
- B. decreases
- C. stays constant
- D. increases
Q26. A gas at 27 °C is heated at constant pressure until its volume is doubled. What is its final temperature?
- A. 54 °C
- B. 573 °C
- C. 600 °C
- D. 327 °C
Q27. If both the pressure and the absolute temperature of a fixed mass of ideal gas are doubled, its volume:
- A. becomes four times
- B. stays the same
- C. doubles
- D. is halved
Q28. Which statement is the Clausius form of the second law of thermodynamics?
- A. Entropy of a perfect crystal is zero at absolute zero
- B. Energy can neither be created nor destroyed
- C. No engine can convert all the heat it absorbs into work
- D. Heat cannot flow by itself from a colder body to a hotter body
Q29. 2 moles of an ideal gas are heated at constant pressure so that their temperature rises by 50 K. How much work does the gas do? (R = 8.31 J mol⁻¹ K⁻¹)
- A. 2077.5 J
- B. 831 J
- C. 1246.5 J
- D. 415.5 J
Q30. A heat engine does 200 J of work per cycle and rejects 600 J of heat to its sink. What is its efficiency?
- A. 25%
- B. 33%
- C. 75%
- D. 20%