WAEC Physics 1991 past questions

Checked by the Skolra question team · Last updated 15 October 2026

Ten questions from the 1991 WAEC Physics paper, with answers and explanations. We hold 41 questions from this paper, and the Skolra app has all of them.

WAEC Physics 1991 questions and answers

1. 1.The intensity of light falling on the film in a camera depend on the brightness of the object, diameter of the aperture stop, and speed of the shutter.
   1. A
      Brightness of the object and diameter of the aperture only
      Correct
   2. B
      Speed of the shutter only
   3. C
      Diameter of the aperture and speed of the shutter only
   4. D
      Brightness of the object, diameter of the aperture, and speed of the shutter
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   Answer: A

   The instantaneous intensity (irradiance, W/m²) of light reaching the film depends on:

   • The brightness of the object — a brighter object emits/reflects more light
   • The diameter of the aperture stop — a larger aperture collects more light; intensity ∝ d²

   The speed of the shutter determines the exposure time — the total energy (= intensity × time) reaching the film — but does not affect the instantaneous intensity of light falling on the film.

2. 2.In which of the following is a stationary wave produced?
   1. A
      Sound waves emitted by a moving vehicle
   2. B
      A single pulse sent along a long rope
   3. C
      Waves spreading outward on the surface of a pond
   4. D
      A stretched string vibrating between two fixed points
      Correct
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   Answer: D

   A stationary (standing) wave is formed by the superposition of two progressive waves of equal frequency, wavelength, and amplitude travelling in opposite directions.

   In a stretched string vibrating between two fixed points, the forward wave and the wave reflected from the fixed end superpose to produce nodes (zero displacement) and antinodes (maximum displacement) at fixed positions — the hallmark of a stationary wave.

3. 3.A ray of light is incident at an angle of 30° on a glass prism of refractive index 1.5. Calculate the angle through which the ray is minimally deviated in the prism.
   1. A
      37.2°
   2. B
      30.0°
   3. C
      21.1°
      Correct
   4. D
      47.1°
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   Answer: C

   At minimum deviation, the ray passes symmetrically, so the angle of incidence equals the angle of emergence: i₁ = i₂ = 30°. At this condition, the refracted ray inside the prism is parallel to the base, and r = A/2.

   Applying Snell's law at the entry face:
   n = sin(i) / sin(A/2)
   1.5 = sin(30°) / sin(A/2) = 0.5 / sin(A/2)
   sin(A/2) = 1/3 → A/2 = 19.47° → A = 38.94°

   Minimum deviation: D_m = 2i − A = 2(30°) − 38.94° ≈ 21.1°

4. 4.Which of the following is not a mechanical wave?
   1. A
      Sound wave
   2. B
      Seismic wave
   3. C
      Light wave
      Correct
   4. D
      Water wave
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   Answer: C

   Light waves (electromagnetic waves) are not mechanical waves. Mechanical waves require a material medium for propagation. Light is an electromagnetic wave that can travel through a vacuum. Sound waves, water waves, and seismic waves all require a physical medium to travel through.

5. 5.The work function of a metal is 4.65ev and the metal is illuminated with a radiation of 6.86ev. What is the kinetic energy of the electrons ejected from the surface of the metal?
   1. A
      11.51 eV
   2. B
      1.12 eV
   3. C
      2.21 eV
      Correct
   4. D
      4.65 eV
   Show answerHide answer

   Answer: C

   Using Einstein's photoelectric equation: KE = hf − φ

   KE = 6.86 eV − 4.65 eV = 2.21 eV

   The kinetic energy equals the energy of the incident photon minus the work function of the metal.

6. 6.A wire, 20m long is heated from a temperature of 5°C to 55°C. If the change in length is 0.020m, calculate the linear expansivity of the wire
   1. A
      2.0 × 10⁻⁴ K⁻¹
   2. B
      1.0 × 10⁻⁵ K⁻¹
   3. C
      2.0 × 10⁻⁵ K⁻¹
      Correct
   4. D
      4.0 × 10⁻⁵ K⁻¹
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   Answer: C

   Linear expansivity α is defined by:

   α = ΔL / (L₀ × ΔT)

   where ΔL = 0.020 m, L₀ = 20 m, ΔT = 55 − 5 = 50°C (= 50 K)

   α = 0.020 / (20 × 50) = 0.020 / 1000 = 2.0 × 10⁻⁵ K⁻¹

7. 7.Which of the following remains unchanged as light travels from one medium to the other? i. speed. ii. wavelength. iii. frequency.
   1. A
      ii only
   2. B
      iii only
      Correct
   3. C
      i and ii only
   4. D
      i only
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   Answer: B

   When light travels from one medium to another, frequency remains unchanged. The relationship between speed, frequency and wavelength is v = fλ. Since the speed changes at a boundary, the wavelength also changes proportionally, but frequency is determined by the source and stays constant across all media.

8. 8.Which of the following is stored by dry Leclanche cell?
   1. A
      Heat energy
   2. B
      Chemical energy
      Correct
   3. C
      Mechanical energy
   4. D
      Electrical energy
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   Answer: B

   A dry Leclanché cell stores chemical energy. When connected in a circuit, an electrochemical reaction converts this chemical energy into electrical energy. The cell consists of a zinc anode, a carbon cathode, and an ammonium chloride electrolyte paste.

9. 9.Which of the following statements about viscosity are correct? When a ball falls through a viscous liquid, several forces act upon it.
   1. A
      Weight and upthrust only
   2. B
      Weight, upthrust, and viscous drag
      Correct
   3. C
      Weight and viscous drag only
   4. D
      Weight, upthrust, viscous drag, and magnetic force
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   Answer: B

   When a ball falls through a viscous liquid, exactly three forces act on it:

   1. Weight (W = mg) — acts downward
   2. Upthrust / Buoyancy (U) — acts upward (Archimedes' principle: U = ρ_liquid × V_ball × g)
   3. Viscous drag (F_v) — acts upward, opposing the downward motion (Stokes' law: F_v = 6πηrv)

   At terminal velocity, W = U + F_v, and the ball moves at constant speed with zero net force.

10. 10.What type of motion does the skin of a 'talking' drum perform when it is struck with a drum stick?
    1. A
       Simple harmonic motion
       Correct
    2. B
       Circular motion
    3. C
       Random Brownian motion
    4. D
       Translational motion
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    Answer: A

    When a drum skin is struck, it vibrates back and forth about its equilibrium position. This is simple harmonic motion (SHM) — an oscillatory motion where the restoring force is proportional to the displacement from the equilibrium position. The vibrating drum skin produces longitudinal pressure waves in the air that we perceive as sound.

The questions come from past JAMB, WAEC and NECO papers. We use software to help write the answer options and explanations, and every answer is checked twice before it goes on the site. How we check

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Practise all 41 questions we hold from the 1991 paper, on a timed CBT screen

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