Grade 9 Advanced Physics — Term 3 Revision Notes
1. Waves
A wave transfers energy from one place to another without transferring matter permanently.
Types of waves
- Transverse wave: particles vibrate perpendicular to the direction of travel. Examples: rope waves, light waves.
- Longitudinal wave: particles vibrate parallel to the direction of travel. Example: sound in air.
- Surface wave: occurs at the boundary between two media, such as water ripples.
Important wave quantities
- Amplitude: maximum displacement from equilibrium. Larger amplitude usually means greater energy.
- Wavelength, \(\lambda \): distance between two consecutive crests or compressions.
- Period, T: time for one complete oscillation.
- Frequency, f: number of oscillations per second, measured in hertz.
- Wave speed: speed of wave travel.
Key equations:
- \(f = 1/T\)
- \(v = \lambda f\)
Reflection of pulses
- At a fixed end, a reflected pulse is inverted.
- At a free end, a reflected pulse remains upright.
Superposition
When two waves overlap, the resultant displacement is the algebraic sum of the individual displacements.
- Same direction: constructive interference.
- Opposite direction: destructive interference.
Standing waves
Standing waves form when two waves of the same frequency travel in opposite directions and interfere.
- Node: point of zero/minimum displacement.
- Antinode: point of maximum displacement.
- For a string fixed at both ends: \(\lambda_n = 2L/n\).
2. Sound
Sound is a mechanical longitudinal wave. It travels through matter as pressure oscillations.
Sound properties
- Sound travels as compressions and rarefactions.
- Sound needs a medium; it cannot travel through a perfect vacuum.
- Sound generally travels fastest in solids, slower in liquids, and slowest in gases.
- In air, sound speed increases as temperature increases.
Pitch and loudness
- Pitch depends on frequency. Higher frequency means higher pitch.
- Loudness depends mainly on amplitude/intensity. Larger amplitude means louder sound.
- Sound level is measured in decibels, dB.
Doppler effect
The Doppler effect is the change in detected frequency due to relative motion between source and observer.
- Approaching source/observer: detected frequency increases.
- Moving apart: detected frequency decreases.
Resonance
Resonance occurs when a system is driven at its natural frequency, causing a large increase in amplitude. Musical instruments use resonance in strings and air columns.
3. Light and Illumination
Light sources and media
- Luminous source: produces its own light, such as the Sun or a lamp.
- Non-luminous object: seen by reflected light, such as the Moon or a book.
- Transparent: allows clear transmission of light.
- Translucent: allows light through but scatters it.
- Opaque: blocks most light.
Light quantities
- Luminous flux, P: total visible light output, unit lumen (lm).
- Illuminance, E: light falling on unit area, unit lux (lx).
- Luminous intensity: light emitted in a direction, unit candela (cd).
Useful equations:
- \(E = P/A\)
- For a point source: \(E = P/(4\pi r^2)\)
Illuminance follows an inverse-square relationship:
- Double distance → illuminance becomes one-quarter.
- Triple distance → illuminance becomes one-ninth.
4. Diffraction and Color
Diffraction
Diffraction is the bending or spreading of waves as they pass an edge or opening. It is most noticeable when the gap size is similar to the wavelength.
Color of light
Color depends on wavelength and frequency.
- Red light: longer wavelength, lower frequency.
- Violet light: shorter wavelength, higher frequency.
Additive color mixing
Primary colors of light:
Combinations:
- Red + green = yellow
- Green + blue = cyan
- Red + blue = magenta
- Red + green + blue = white
Object color depends on which wavelengths are present in incident light and which wavelengths are reflected or absorbed.
Pigments and dyes
Pigments use subtractive color mixing.
- Cyan absorbs red.
- Magenta absorbs green.
- Yellow absorbs blue.
Mixing pigments usually absorbs more wavelengths and gives darker colors.
5. Polarization and Malus's Law
Polarization is the restriction of light vibrations to one direction. Only transverse waves can be polarized.
Polarization by filtering
A polarizing filter allows one vibration direction to pass and blocks the perpendicular component.
Polarization by reflection
Reflected glare can be partially polarized. Polarizing sunglasses reduce glare by blocking much of this reflected polarized light.
Malus's law
For polarized light passing through an analyzer:
\(I_2 = I_1 \cos^2\theta \)
Where:
- \(I_1\) is intensity before the analyzer.
- \(I_2\) is intensity after the analyzer.
- \(\theta \) is the angle between the polarizer and analyzer axes.
Special cases:
- \(\theta \) = 0° → maximum transmitted intensity.
- \(\theta \) = 90° → zero/minimum transmitted intensity.
- For unpolarized light passing through the first ideal polarizer: \(I_1 = I_0/2\).
Exam Tips
- Always check units: Hz, m/s, m, s, dB, lm, lx, cd.
- For formula questions, write the formula first, substitute values, then calculate.
- For wave diagrams, mark the direction of travel and direction of particle vibration.
- For superposition, add displacements with signs.
- For Malus's law, decide whether the light entering the first polarizer is unpolarized or already polarized.
