AQA GCSE (9-1) Physics: Waves

Key Takeaways for GCSE Waves

1. Types of Waves

• Transverse Waves: Oscillations perpendicular to energy transfer.
  Examples: Light, water waves.
  • Longitudinal Waves: Oscillations parallel to energy transfer (compressions and rarefactions).
    Examples: Sound, seismic P-waves.
• Key Terms:
  • Amplitude: Maximum displacement from equilibrium (height of wave).
  • Wavelength (λ): Distance between equivalent points (e.g., peak to peak).
  • Frequency (f): Waves per second (Hz).
  • Period (T): Time for one wave: T=1fT=f1​.

Example:
A wave with frequency 5 Hz has a period of T=15=0.2 sT=51​=0.2s.

2. Wave Equation

• Formula: v=fλv=fλ
  • vv = wave speed (m/s), ff = frequency (Hz), λλ = wavelength (m).
• Example:
  Sound travels at 330 m/s. For a 660 Hz tuning fork:
  λ=vf=330660=0.5 mλ=fv​=660330​=0.5m.

Tip: Always convert units to metres, seconds, and Hz before calculations.

3. Reflection & Refraction

• Reflection:
  • Law: Angle of incidence (ii) = Angle of reflection (rr).
  • Specular (smooth surfaces) vs. diffuse (rough surfaces).
• Refraction:
  • Waves change direction when speed changes (e.g., water to shallow water).
  • Frequency stays constant; speed and wavelength change: v1/v2=λ1/λ2v1​/v2​=λ1​/λ2​.

Example:
Water waves slow in shallow water → wavelength decreases, bend towards normal.

4. Electromagnetic Spectrum

• Order (long → short λ): Radiowaves, microwaves, infrared, visible light, UV, X-rays, gamma.
• Properties:
  • Transverse, travel at 3×108 m/s3×108m/s in a vacuum.
  • Hazards: High-frequency waves (UV, X, gamma) are ionising → cell damage.

Example:
Microwave wavelength at 1.8 GHz:
λ=3×1081.8×109=0.17 mλ=1.8×1093×108​=0.17m.

Tip: Use mnemonics: Richard Of York Gave Battle In Vain** for visible spectrum (Red → Violet).

5. Lenses & Ray Diagrams

• Convex Lens:
  • Converges light. Forms real (inverted) or virtual (upright, magnified) images.
  • Principal Focus: Parallel rays meet.
• Concave Lens:
  • Diverges light. Always forms virtual, diminished images.
• Magnification: Magnification=Image heightObject heightMagnification=Object heightImage height​.

Example:
Object height = 2 cm, image height = 4 cm → Magnification = 2.

Exam Tip: Draw two rays (parallel to axis + through lens centre) to locate images.

6. Sound & Ultrasound

• Sound: Longitudinal, requires medium. Speed in air ≈ 330 m/s.
• Ultrasound (>20 kHz):
  • Uses: Sonar (depth calculation), medical imaging.
  • Depth Calculation: Distance=speed×time2Distance=2speed×time​.

Example:
Echo time = 1.2 s, speed = 1500 m/s:
Depth=1500×1.22=900 mDepth=21500×1.2​=900m.

7. Seismic Waves

• P-waves: Longitudinal, travel through solids/liquids.
• S-waves: Transverse, only through solids.
• Earth’s Layers: Liquid outer core stops S-waves → evidence for Earth’s structure.

8. Black Body Radiation

• All objects emit infrared radiation.
• Hotter objects: Emit more radiation, shorter λ (e.g., red → white glow).
• Perfect Black Body: Absorbs all radiation, emits efficiently (e.g., stars).

Example:
Clouds reflect sunlight (cooling Earth by day) and trap IR (warming Earth at night).

9. Practical Tips

• Ripple Tank: Measure λ by averaging multiple waves.
• Leslie Cube: Matt black emits more IR than shiny surfaces.
• Equations to Memorise:
  • v=fλv=fλ
  • T=1fT=f1​
  • Magnification=Image heightObject heightMagnification=Object heightImage height​

Exam Trick: For reflection/refraction questions, always draw the normal at 90° to the surface.

50 GCSE Waves Questions

Section 1: Wave Types and Properties

1. Define transverse wave and give two examples.
2. Explain how longitudinal waves transfer energy.
3. Calculate the period of a wave with a frequency of 25 Hz.
4. A wave has a wavelength of 2 m and travels at 10 m/s. Calculate its frequency.
5. What is the amplitude of a wave? How is it measured?

Section 2: Electromagnetic Spectrum

6. List the electromagnetic spectrum in order of increasing wavelength.
7. Why can’t humans see ultraviolet light?
8. Calculate the wavelength of a radio wave with a frequency of 100 MHz.
9. State two hazards of X-rays and how risks are reduced.
10. Explain why microwaves are used for satellite communication.

Section 3: Reflection and Refraction

11. State the law of reflection.
12. Why does light bend when entering glass from air?
13. Draw a diagram showing refraction of water waves moving from deep to shallow water.
14. A light ray enters glass at an angle of 30° (refractive index = 1.5). Calculate the angle of refraction.
15. Why does a red shirt appear red in white light?

Section 4: Lenses and Ray Diagrams

16. Describe the image formed by a convex lens when the object is beyond the focal point.
17. Draw a ray diagram for a concave lens.
18. Calculate magnification if an object 3 cm tall produces an image 9 cm tall.
19. What is the principal focus of a lens?
20. Why does a convex lens act as a magnifying glass?

Section 5: Sound and Ultrasound

21. Why can’t sound travel in a vacuum?
22. Calculate the depth of the seabed if an ultrasound pulse takes 0.8 s to return (speed = 1500 m/s).
23. What frequency range defines ultrasound?
24. Explain how a microphone converts sound into electrical signals.
25. Why is ultrasound safer than X-rays for foetal scans?

Section 6: Seismic Waves

26. Why do S-waves not travel through the Earth’s outer core?
27. Compare P-waves and S-waves.
28. How do seismic waves provide evidence for the Earth’s structure?
29. Calculate the speed of a P-wave travelling 2000 km in 400 s.
30. Why are transverse waves more destructive in earthquakes?

Section 7: Black Body Radiation

31. What is a perfect black body?
32. Explain why a blacksmith judges metal temperature by its colour.
33. Why do clouds cool the Earth during the day but warm it at night?
34. How does temperature affect infrared radiation emission?
35. Why does a matt black surface emit more radiation than a shiny one?

Section 8: Practical Experiments

36. Describe how to measure wave speed using a ripple tank.
37. What is the purpose of a Leslie cube?
38. Explain how to calculate wavelength from a ripple tank pattern.
39. Why is a white screen used in ripple tank experiments?
40. How does tension affect wave speed on a stretched string?

Section 9: Calculations and Equations

41. Convert 5 GHz to Hz.
42. Rearrange v=fλv=fλ to find λλ.
43. A wave has a period of 0.02 s. Calculate its frequency.
44. Light travels at 3×108 m/s3×108m/s. Calculate the frequency of red light (λ=650 nmλ=650nm).
45. Calculate wave speed if 10 waves pass a point in 2 s, each with λ=0.5 mλ=0.5m.

Section 10: Application and Analysis

46. Explain why a pencil appears bent in water.
47. How do mirrors form virtual images?
48. Why are acoustic panels textured?
49. Describe how fibre optics use total internal reflection.
50. Explain why gamma rays are used to treat cancer.

Answers

1. Transverse wave: Oscillations perpendicular to energy transfer (e.g., light, water waves).
2. Longitudinal waves transfer energy via compressions and rarefactions (e.g., sound).
3. T=1f=125=0.04 sT=f1​=251​=0.04s.
4. f=vλ=102=5 Hzf=λv​=210​=5Hz.
5. Amplitude: Maximum displacement from equilibrium. Measured from peak to equilibrium.
6. Radiowaves → Microwaves → Infrared → Visible → UV → X-rays → Gamma.
7. Human eyes detect only visible light (400–700 nm). UV has shorter λ.
8. λ=vf=3×108100×106=3 mλ=fv​=100×1063×108​=3m.
9. Hazards: Cell damage, cancer. Reduction: Lead shielding, limited exposure.
10. Microwaves penetrate Earth’s atmosphere and are reflected by satellites.
11. Law of reflection: Angle of incidence=Angle of reflectionAngle of incidence=Angle of reflection.
12. Light slows in glass, bending towards the normal.
13. Waves bend towards normal in shallow water (shorter λ, lower speed).
14. sin⁡(r)=sin⁡(i)n=sin⁡(30)1.5=0.333sin(r)=nsin(i)​=1.5sin(30)​=0.333. r=19.5∘r=19.5∘.
15. Red shirt reflects red light; absorbs other colours.
16. Real, inverted, diminished image.
17. Rays diverge; virtual image forms on same side as object.
18. Magnification=93=3Magnification=39​=3.
19. Principal focus: Where parallel rays converge after refraction.
20. Object inside focal length → virtual, magnified image.
21. Sound needs a medium (e.g., air) for vibrations.
22. Depth=1500×0.82=600 mDepth=21500×0.8​=600m.
23. Ultrasound: f>20 kHzf>20kHz.
24. Sound vibrates diaphragm → electrical signal via coil/magnet.
25. Ultrasound is non-ionising; X-rays damage DNA.
26. S-waves are transverse; cannot travel through liquid outer core.
27. P-waves: Longitudinal, faster. S-waves: Transverse, slower.
28. S-wave shadow zones indicate liquid outer core.
29. v=distancetime=2000×103400=5000 m/sv=timedistance​=4002000×103​=5000m/s.
30. Transverse waves cause sideways shaking → structural damage.
31. Perfect black body: Absorbs all radiation, emits maximally.
32. Colour correlates with temperature (red → white as temp ↑).
33. Clouds reflect sunlight (cooling) and trap infrared (warming).
34. Hotter objects emit more radiation and shorter λ.
35. Matt black has rough surface → more emission; shiny reflects.
36. Measure λ (distance between peaks) and frequency (waves/sec). v=fλv=fλ.
37. Compare infrared emission from different surfaces.
38. λ=Total lengthNumber of wavesλ=Number of wavesTotal length​.
39. White screen reflects light → clearer wave pattern.
40. Higher tension → higher wave speed (v∝Tv∝T​).
41. 5 GHz=5×109 Hz5GHz=5×109Hz.
42. λ=vfλ=fv​.
43. f=1T=10.02=50 Hzf=T1​=0.021​=50Hz.
44. f=vλ=3×108650×10−9=4.6×1014 Hzf=λv​=650×10−93×108​=4.6×1014Hz.
45. f=102=5 Hz;v=fλ=5×0.5=2.5 m/sf=210​=5Hz;v=fλ=5×0.5=2.5m/s.
46. Light bends (refracts) at water-air boundary → apparent displacement.
47. Mirrors reflect light; rays appear to come from behind (virtual image).
48. Textured surfaces scatter sound (diffuse reflection) → reduce echoes.
49. Light reflects internally at critical angle → minimal signal loss.
50. Gamma rays ionise cancer cells → destroy DNA, preventing division.