O Level Physics Waves Sound Light Quiz

O-Level Physics Quiz - Waves Sound Light (Answer Key)

Total Marks: 40

Section A: Multiple Choice & Short Concepts (Questions 1-5)

1. C
Reasoning: In transverse waves, oscillations are perpendicular to energy transfer. In longitudinal waves, they are parallel. [1]

2. B
Reasoning: Frequency is determined by the source and does not change when a wave changes medium. Speed and wavelength change. [1]

3. A
Reasoning: $x$ is the maximum displacement from equilibrium (amplitude). $y$ is the distance between two identical points on consecutive waves (wavelength). [1]

4. D
Reasoning: Gamma rays have the shortest wavelength and therefore the highest frequency ($c = f\lambda$). [1]

5. 340 m/s
Working:
Distance traveled by sound = $2 \times 170 \text{ m} = 340 \text{ m}$ (to wall and back).
Time = 1.0 s.
Speed = Distance / Time = $340 / 1.0 = 340 \text{ m/s}$.
Marking:

• Distance = $2 \times 170$ [1]
• Calculation $340/1$ [1]

Section B: Wave Properties & Sound (Questions 6-10)

6. Sonar / Depth sounding / Cleaning delicate instruments / Detecting cracks in metals.
Marking: Any valid non-medical application. [1]

7. Sound is a mechanical wave / requires a medium (particles) to vibrate/transfer energy.
Marking: Must mention medium or particles. Vacuum has no particles. [1]

8. 1.51 (or 1.5)
Working:
$n = \frac{\sin i}{\sin r}$
$n = \frac{\sin 45^\circ}{\sin 28^\circ}$
$n = \frac{0.7071}{0.4695} \approx 1.506$
Marking:

• Formula [1]
• Correct substitution and answer (1.50 - 1.52 accepted) [1]

9. The angle of incidence in the denser medium for which the angle of refraction in the less dense medium is $90^\circ$.
Marking: Must mention incidence in denser medium and refraction angle of 90 degrees. [1]

10. B
Reasoning: Real images can be projected onto a screen. Virtual images cannot. [1]

Section C: Structured Questions - Optics & EM Spectrum (Questions 11-15)

11. (a) 4 V
Working: Amplitude is peak voltage. Height is 2 divisions. Scale is 2 V/div. $2 \times 2 = 4 \text{ V}$. [1]

(b) 0.02 s
Working: Period is time for one cycle. Horizontal length is 4 divisions. Scale is 5 ms/div.
$T = 4 \times 5 \text{ ms} = 20 \text{ ms} = 0.02 \text{ s}$. [2]

(c) 50 Hz
Working: $f = 1/T = 1 / 0.02 = 50 \text{ Hz}$. [2]

(d) Amplitude increases (trace becomes taller); Wavelength/Period remains the same (width of cycle unchanged).
Marking:

• Louder = Higher Amplitude [1]
• Same pitch = Same Frequency/Period [1]

12. (a) Ray Diagram:

• Ray 1: Parallel to principal axis, refracts through focal point on the other side.
• Ray 2: Through the optical center, passes undeviated.
• Image formed where rays intersect (beyond 2F on the other side).
  Marking:
• Correct ray paths [2]
• Correct image position and label [1]

(b) Characteristics:

1. Real
2. Inverted
3. Magnified
   Marking: 1 mark for each correct characteristic. [3]

(c) The image becomes virtual (or upright / magnified more / on the same side as object).
Reasoning: Object is now inside the focal length ($u < f$). [1]

13. (a) Total Internal Reflection (TIR) [1]

(b) Conditions:

1. Light must travel from a denser medium to a less dense medium (e.g., glass to air/cladding).
2. The angle of incidence must be greater than the critical angle.
   Marking: 1 mark for each condition. [2]

(c) Advantages (Any one):

• Higher bandwidth / carries more data.
• Less signal attenuation / loss over long distances.
• Immune to electromagnetic interference.
• Lighter and thinner than copper.
  Marking: Statement [1] + Brief explanation/context [1]. [2]

14. (a) Visible Light [1]

(b) Satellite communications / Radar / Cooking food (Microwave ovens). [1]

(c) Hazards:

• Gamma rays are ionizing radiation.
• They can damage DNA / kill cells / cause cancer / mutations.
  Marking:
• Ionizing nature [1]
• Biological effect (cell damage/cancer) [1] [2]

15. (a) Graph:

• Axes labeled correctly with units ($\sin i$ and $\sin r$ are unitless).
• Points plotted correctly.
• Best fit straight line drawn through the origin.
  Marking:
• Labels [1]
• Plotting [1]
• Line of best fit [1] [3]

(b) Gradient Calculation:

• Select two points on the line, e.g., $(0,0)$ and $(0.508, 0.766)$.
• Gradient $m = \frac{\Delta y}{\Delta x} = \frac{0.766 - 0}{0.508 - 0} \approx 1.51$.
  Marking:
• Substitution from graph [1]
• Correct value (1.50 - 1.52) [1] [2]

(c) Refractive Index ($n$) of the glass. [1]

(d) Speed of light in glass:

• Formula: $n = \frac{c}{v}$
• Rearrangement: $v = \frac{c}{n}$
• Calculation: $v = \frac{3.0 \times 10^8}{1.51} \approx 1.99 \times 10^8 \text{ m/s}$.
  Marking:
• Formula/Substitution [1]
• Correct answer ($1.98 - 2.0 \times 10^8 \text{ m/s}$) [1] [2]

(e) Source of Error & Improvement:

• Error: Parallax error when reading the protractor / Width of the light ray makes it hard to judge the center.
• Improvement: Use a thinner ray (slit) / View protractor directly from above / Repeat readings and average.
  Marking:
• Valid error [1]
• Corresponding improvement [1] [2]

Section D: Additional Concepts & Applications (Questions 16-20)

16. D
Reasoning: Radio waves are electromagnetic waves and can travel through a vacuum. Sound, water, and seismic waves are mechanical and require a medium. [1]

17. 300 m/s
Working:
$v = f \times \lambda$
$v = 200 \times 1.5$
$v = 300 \text{ m/s}$
Marking:

• Formula [1]
• Correct answer [1]

18. (a) Dispersion [1] (b) Violet [1]

19. Real images are formed where light rays actually converge/meet. Virtual images are formed where light rays appear to diverge from (they do not actually meet).
Marking:

• Real: rays converge/meet [1]
• Virtual: rays appear to come from/diverge [1]

20. Ultrasound pulses are sent into the metal. If there is a crack, the ultrasound reflects (echoes) back from the crack sooner than it would from the far end of the metal. The time taken for the echo to return is measured to locate the crack.
Marking:

• Reflection/Echo from crack [1]
• Time measurement/detection of earlier return [1]