ECAT — Engineering College Admission Test (UET)

ECAT Physics — Physical Optics and Thermodynamics (Set 1)

Take the ECAT Physics — Physical Optics and Thermodynamics (Set 1): 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 Young's double-slit experiment, light of wavelength 600 nm is used with slits 0.5 mm apart and a screen 1 m away. What is the fringe spacing?
- A. 0.6 mm
- B. 0.3 mm
- C. 1.2 mm ✓
- D. 2.4 mm
Answer: C. 1.2 mm
Explanation: Δy = λL/d = (600 × 10⁻⁹ × 1)/(0.5 × 10⁻³) = 0.001 m = 1.2 mm.
Q2. In Young's double-slit experiment, if the separation between the slits is doubled (all else unchanged), the fringe spacing:
- A. is unchanged
- B. is halved ✓
- C. is doubled
- D. becomes four times
Answer: B. is halved
Explanation: Δy = λL/d, so Δy ∝ 1/d.
Q3. For constructive interference of light from two coherent sources in phase, the path difference must be:
- A. zero only
- B. an odd multiple of λ/4
- C. an integral multiple of λ ✓
- D. an odd multiple of λ/2
Answer: C. an integral multiple of λ
Explanation: Bright fringes occur where path difference = mλ (m = 0, 1, 2, …); dark fringes at (m + ½)λ.
Q4. The central spot of Newton's rings seen in reflected light is dark because:
- A. reflection from the glass plate introduces an extra phase change of π ✓
- B. the air film is thickest at the centre
- C. no light reaches the centre
- D. the lens absorbs light at the centre
Answer: A. reflection from the glass plate introduces an extra phase change of π
Explanation: At the contact point the film thickness is zero, but one reflection (from the denser medium) adds a λ/2 path change, giving destructive interference.
Q5. A diffraction grating has 5000 lines per cm. Light of wavelength 500 nm falls normally on it. At what angle is the second-order maximum observed?
- A. 14.5°
- B. 30° ✓
- C. 60°
- D. 45°
Answer: B. 30°
Explanation: d = 1/5000 cm = 2 × 10⁻⁶ m; sin θ = 2λ/d = 2 × 500 × 10⁻⁹/(2 × 10⁻⁶) = 0.5, so θ = 30°.

The other 25 questions

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Q6. A diffraction grating gives the first-order maximum of 500 nm light at 30°. How many lines per centimetre does the grating have?
- A. 20000 lines/cm
- B. 10000 lines/cm
- C. 5000 lines/cm
- D. 2500 lines/cm
Q7. Which phenomenon shows that light waves are transverse?
- A. Reflection
- B. Diffraction
- C. Polarization
- D. Interference
Q8. In a Michelson interferometer using light of wavelength 600 nm, the movable mirror is displaced by 0.15 mm. How many fringes cross the field of view?
- A. 500
- B. 250
- C. 1000
- D. 2500
Q9. X-rays are diffracted by crystal planes 0.2 nm apart. The first-order Bragg reflection occurs at a glancing angle of 30°. What is the X-ray wavelength?
- A. 0.4 nm
- B. 0.2 nm
- C. 0.35 nm
- D. 0.1 nm
Q10. Light of wavelength 600 nm falls on a single slit of width 1.2 μm. At what angle from the centre does the first minimum occur?
- A. 30°
- B. 60°
- C. 45°
- D. 15°
Q11. Two light sources are said to be coherent if they have:
- A. the same amplitude but different frequencies
- B. a randomly changing phase difference
- C. the same intensity only
- D. the same frequency and a constant phase difference
Q12. Huygens' principle states that:
- A. every point on a wavefront acts as a source of secondary wavelets
- B. the angle of incidence equals the angle of refraction
- C. light consists of particles called photons
- D. light travels in straight lines only
Q13. Thin soap films show bright colours in white light mainly because of:
- A. dispersion of light by the soap
- B. polarization of the reflected light
- C. total internal reflection in the film
- D. interference between light reflected from the two surfaces of the film
Q14. A cylinder of gas is given 800 J of heat while it pushes a piston outward, doing 300 J of work. By how much does the internal energy of the gas change?
- A. +300 J
- B. +500 J
- C. −500 J
- D. +1100 J
Q15. For an isothermal expansion of an ideal gas, which statement is correct?
- A. No heat is exchanged with the surroundings
- B. The heat absorbed equals the work done by the gas
- C. The internal energy of the gas increases
- D. The work done by the gas is zero
Q16. When an ideal gas expands adiabatically:
- A. its temperature stays constant
- B. its internal energy increases
- C. its temperature rises because heat is absorbed
- D. its temperature falls because work is done at the expense of internal energy
Q17. A Carnot engine works between a source at 527 °C and a sink at 127 °C. What is its efficiency?
- A. 24%
- B. 50%
- C. 40%
- D. 76%
Q18. A Carnot engine with its sink at 300 K has an efficiency of 40%. What is the temperature of its source?
- A. 500 K
- B. 180 K
- C. 420 K
- D. 750 K
Q19. For an ideal gas, the difference between the molar specific heats at constant pressure and constant volume is:
- A. R
- B. γR
- C. 3R/2
- D. zero
Q20. A gas expands at a constant pressure of 2 × 10⁵ Pa and its volume increases by 3 litres. How much work does it do?
- A. 600 J
- B. 6 × 10⁵ J
- C. 60 J
- D. 1500 J
Q21. If the absolute temperature of an ideal gas is doubled, the root-mean-square speed of its molecules becomes:
- A. √2 times
- B. 1/√2 times
- C. 4 times
- D. 2 times
Q22. At the same temperature, what is the ratio of the r.m.s. speed of hydrogen molecules (M = 2) to that of oxygen molecules (M = 32)?
- A. 8 : 1
- B. 4 : 1
- C. 16 : 1
- D. 1 : 4
Q23. A gas has density 1.2 kg/m³ and its molecules have an r.m.s. speed of 500 m/s. What pressure does it exert?
- A. 2 × 10⁵ Pa
- B. 1 × 10⁵ Pa
- C. 3 × 10⁵ Pa
- D. 600 Pa
Q24. 1 kg of ice at 0 °C melts to water at 0 °C. Taking the latent heat of fusion as 3.36 × 10⁵ J/kg, what is the change in entropy?
- A. 336 J/K
- B. 1.23 J/K
- C. 1231 J/K
- D. 9.17 × 10⁷ J/K
Q25. Which is a statement of the second law of thermodynamics (Kelvin's form)?
- A. Heat flows spontaneously from a cold body to a hot body
- B. Energy can neither be created nor destroyed
- C. The entropy of an isolated system always decreases
- D. No engine working in a cycle can convert all the heat absorbed into work
Q26. A gas occupies 2 L at 300 K. It is heated at constant pressure to 450 K. What is its new volume?
- A. 4.5 L
- B. 1.33 L
- C. 2.5 L
- D. 3 L
Q27. For a gas taken through a complete cycle, the net work done is represented on a P–V diagram by:
- A. the area enclosed by the cycle
- B. the slope of the isothermal curve
- C. the length of the cycle's perimeter
- D. the area under the highest curve only
Q28. In an isochoric (constant-volume) process:
- A. the pressure stays constant
- B. no heat is exchanged
- C. no work is done and all heat supplied increases the internal energy
- D. the temperature stays constant
Q29. How much heat is needed to raise the temperature of 2 moles of a monatomic ideal gas by 10 K at constant volume? (R = 8.314 J mol⁻¹ K⁻¹)
- A. 166 J
- B. 83 J
- C. 416 J
- D. 249 J
Q30. A gas at 1 atm occupies 6 L. It is compressed isothermally to 2 L. What is its new pressure?
- A. ⅓ atm
- B. 3 atm
- C. 2 atm
- D. 12 atm