A Level H2 Physics Waves Sound Light Quiz

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A-Level Physics H2 Quiz - Waves Sound Light

Name: ____________________ Class: ____________________ Date: ____________________ Score: / 55

Duration: 60 Minutes | Total Marks: 55

Instructions:

• Answer all questions in the spaces provided.
• Show all working for calculation questions.
• Use $g = 9.81\text{ m s}^{-2}$ and $c = 3.00 \times 10^8\text{ m s}^{-1}$ unless otherwise stated.

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Section A: Short Answer & Recall (Questions 1–7)

1. State the condition for two sources of light to be said to be coherent. [1]
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2. Define the term threshold frequency in the context of the photoelectric effect. [1]
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3. A wave has a frequency of $500\text{ Hz}$ and a wavelength of $0.68\text{ m}$. Calculate the speed of the wave. [2]
   
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4. State the principle of superposition of waves. [2]
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5. Explain why a laser beam can travel long distances with very little spreading compared to light from a conventional bulb. [2]
   
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6. Distinguish between a longitudinal wave and a transverse wave, providing one example of each. [2]
   
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7. A standing wave is formed in a string fixed at both ends. State the relationship between the length of the string $L$ and the wavelength $\lambda$ for the second harmonic. [2]
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Section B: Structured Application (Questions 8–15)

8. In an X-ray tube, high-speed electrons are decelerated by a target metal. Explain why there is a continuous distribution of wavelengths in the resulting X-ray spectrum. [3]
   
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9. Radiation of wavelength $400\text{ nm}$ gives rise to a maximum photoelectric current $I$ from a metal surface. The intensity of the incident radiation is maintained constant, but the wavelength is reduced to $300\text{ nm}$. (a) State what happens to the stopping potential. [1] (b) Explain your answer to (a). [2]
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10. A double-slit experiment is performed using light of wavelength $600\text{ nm}$. The slits are separated by $0.2\text{ mm}$ and the screen is $1.5\text{ m}$ away. Calculate the fringe separation. [3]
    
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11. An electron is accelerated through a potential difference of $25\text{ kV}$ and strikes a tungsten target. (a) Calculate the maximum energy of the emitted X-ray photons in $\text{keV}$. [2] (b) Calculate the minimum wavelength $\lambda_{\min}$ of the X-rays produced. [3]
    
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12. A sound wave travels from air into water. (a) State which property of the wave remains unchanged. [1] (b) Explain how the wavelength changes as the wave enters the water. [2]
    
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13. Describe the process of "braking radiation" (Bremsstrahlung) and how it relates to the production of X-rays. [3]
    
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14. A metal surface has a work function of $2.3\text{ eV}$. Radiation of wavelength $500\text{ nm}$ is incident on the surface. (a) Calculate the energy of an incident photon in $\text{eV}$. [2] (b) Without calculation, suggest why no photoelectric current would be observed if the wavelength were increased to $700\text{ nm}$. [2]
    
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15. Explain the difference between the continuous X-ray spectrum and the characteristic X-ray spectrum. [3]
    
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Section C: Analysis & Synthesis (Questions 16–20)

16. A diffraction grating has $500\text{ lines per mm}$. A beam of monochromatic light of wavelength $589\text{ nm}$ is incident normally on the grating. Calculate the angle of the second-order maximum. [4]
    
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17. A string of length $0.8\text{ m}$ is fixed at both ends and vibrates in its third harmonic. The speed of the wave on the string is $120\text{ m s}^{-1}$. (a) Calculate the frequency of vibration. [3] (b) Determine the distance between two adjacent nodes. [2]
    
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18. Discuss the effect of increasing the intensity of incident light on: (i) The maximum kinetic energy of the photoelectrons. [2] (ii) The number of photoelectrons emitted per second. [2]
    
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19. A sound source emits a frequency of $1000\text{ Hz}$. An observer moves toward the source at $20\text{ m s}^{-1}$ while the source remains stationary. Given the speed of sound is $340\text{ m s}^{-1}$, calculate the observed frequency. [4]
    
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20. A student uses a ripple tank to study diffraction. He observes that the diffraction effect is more pronounced when the gap width is decreased to be comparable to the wavelength. (a) Explain the physics behind this observation. [3] (b) Suggest one way the student could increase the diffraction effect without changing the gap width. [2]
    
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Answer Key - A-Level Physics H2 Quiz (Waves Sound Light)

1. Coherence: The sources must have a constant phase difference and the same frequency. [1]

2. Threshold Frequency: The minimum frequency of incident radiation required to eject an electron from the surface of a metal. [1]

3. Calculation: $v = f\lambda = 500 \times 0.68 = 340\text{ m s}^{-1}$. [2]

4. Superposition: When two or more waves overlap, the resultant displacement at any point is the vector sum of the displacements of the individual waves. [2]

5. Laser Spreading: Laser light is highly collimated and monochromatic (low divergence), whereas conventional light is polychromatic and emits in all directions. [2]

6. Waves:

   • Longitudinal: Oscillations parallel to direction of propagation (e.g., sound). [1]
   • Transverse: Oscillations perpendicular to direction of propagation (e.g., light/water). [1]

7. Standing Wave: For the second harmonic, $L = \lambda$. [2]

8. Continuous X-ray Spectrum:

   • High-speed electrons are decelerated by the electric field of target nuclei. [1]
   • They lose kinetic energy in varying amounts depending on the impact parameter. [1]
   • This energy is emitted as photons of a continuous range of frequencies/wavelengths. [1]

9. Photoelectric Effect: (a) Stopping potential increases. [1] (b) Wavelength decreases $\rightarrow$ frequency increases $\rightarrow$ photon energy $hf$ increases. Since $\Phi$ is constant, $E_{k\max} = hf - \Phi$ increases, requiring a higher stopping potential to halt the electrons. [2]

10. Fringe Separation: $w = \frac{\lambda D}{d} = \frac{(600 \times 10^{-9})(1.5)}{0.2 \times 10^{-3}} = 4.5 \times 10^{-3}\text{ m}$ or $4.5\text{ mm}$. [3]

11. X-ray Energy: (a) $E = eV = 25\text{ keV}$. [2] (b) $\lambda_{\min} = \frac{hc}{E} = \frac{(6.63 \times 10^{-34})(3 \times 10^8)}{(25 \times 10^3)(1.6 \times 10^{-19})} = 4.97 \times 10^{-11}\text{ m}$. [3]

12. Sound in Water: (a) Frequency. [1] (b) Speed of sound is higher in water than air. Since $v = f\lambda$ and $f$ is constant, $\lambda$ must increase. [2]

13. Bremsstrahlung:

    • "Braking radiation" occurs when an electron is deflected and slowed down by the nucleus of a target atom. [1]
    • The loss in kinetic energy is emitted as an X-ray photon. [1]
    • Because electrons lose different amounts of energy, a continuous spectrum is produced. [1]

14. Photon Energy: (a) $E = \frac{hc}{\lambda} = \frac{(6.63 \times 10^{-34})(3 \times 10^8)}{500 \times 10^{-9}} = 3.98 \times 10^{-19}\text{ J}$. Convert to eV: $\frac{3.98 \times 10^{-19}}{1.6 \times 10^{-19}} = 2.49\text{ eV}$. [2] (b) Increasing wavelength to $700\text{ nm}$ decreases photon energy. If $hf < \Phi$ (work function), no electrons can be ejected regardless of intensity. [2]

15. X-ray Spectra:

    • Continuous: Produced by deceleration of electrons (Bremsstrahlung); covers a range of wavelengths. [1.5]
    • Characteristic: Produced by transitions of electrons from higher shells to lower shells (e.g., K-shell); appears as sharp peaks at specific wavelengths. [1.5]

16. Diffraction Grating: $d = \frac{1}{500 \times 10^3} = 2 \times 10^{-6}\text{ m}$. $n\lambda = d \sin \theta \rightarrow 2(589 \times 10^{-9}) = (2 \times 10^{-6}) \sin \theta$. $\sin \theta = 0.589 \rightarrow \theta = 36.1^\circ$. [4]

17. String Harmonics: (a) Third harmonic: $L = \frac{3}{2}\lambda \rightarrow \lambda = \frac{2 \times 0.8}{3} = 0.533\text{ m}$. $f = \frac{v}{\lambda} = \frac{120}{0.533} = 225\text{ Hz}$. [3] (b) Distance between nodes = $\frac{\lambda}{2} = \frac{0.533}{2} = 0.267\text{ m}$. [2]

18. Intensity Effects: (i) No effect. $E_{k\max}$ depends only on frequency and work function. [2] (ii) Increases. More photons per second hit the surface, ejecting more electrons per second. [2]

19. Doppler Effect: $f' = f \left( \frac{v + v_o}{v} \right) = 1000 \left( \frac{340 + 20}{340} \right) = 1000 \times 1.0588 = 1059\text{ Hz}$. [4]

20. Diffraction: (a) Diffraction is the spreading of waves. It is most significant when the aperture size is comparable to the wavelength, allowing the wave to bend significantly around the edges. [3] (b) Use a source with a longer wavelength (e.g., red light instead of blue light). [2]