Light, Sound, and Heat
Light
Nature of Light
Light is a form of energy that travels in straight lines (rectilinear propagation). It is a type of electromagnetic radiation that can be detected by the human eye (visible spectrum).
Speed of light: 3 × 10⁸ m/s in vacuum (approximately 300,000 km/s) — the fastest speed in the universe.
Particle vs Wave: Light exhibits both wave-like and particle-like properties (wave-particle duality). For most CTET purposes, wave properties are sufficient to explain optical phenomena.
Reflection of Light
When light hits a surface, it bounces back — this is reflection.
Laws of Reflection:
- The incident ray, reflected ray, and normal all lie in the same plane
- Angle of incidence (∠i) = Angle of reflection (∠r)
Types of reflection:
- Regular (specular) reflection: From smooth, polished surfaces (mirror) — parallel rays remain parallel
- Diffuse (irregular) reflection: From rough surfaces — rays scatter in different directions (enables us to see non-luminous objects)
Mirrors
Plane Mirror:
- Virtual (erect) image
- Same size as object
- Laterally inverted (left and right appear reversed)
- Distance of image from mirror = distance of object from mirror
Spherical Mirrors:
| Type | Shape | Image | Uses |
|---|---|---|---|
| Concave mirror | Inner curved surface | Real (inverted) when object beyond focus; Virtual, enlarged when object within focus | Reflectors (telescopes, torches, headlights), dentist’s mirror |
| Convex mirror | Outer curved surface | Always virtual, diminished (smaller) | Rear-view mirrors in vehicles, shop security mirrors |
Mirror formula (for spherical mirrors): 1/v + 1/u = 1/f (where u = object distance, v = image distance, f = focal length)
Sign conventions (New Cartesian convention):
- Distances measured from the mirror’s pole
- Objects placed in front of mirror → distances are negative
- Focal length: Concave mirror = negative, Convex mirror = positive
Refraction of Light
When light passes from one medium to another (e.g., air to water), it bends — this is refraction.
Laws of Refraction (Snell’s Law): n₁ sin(i) = n₂ sin(r) Where n₁, n₂ = refractive indices of the two media; i = angle of incidence; r = angle of refraction
Refractive Index (n): The ratio of speed of light in vacuum to speed of light in the medium. For water, n ≈ 1.33; for glass, n ≈ 1.5.
Laws:
- Incident ray, refracted ray, and normal all lie in the same plane
- Ratio of sin i to sin r is constant for a given pair of media
Lenses
Convex (Converging) Lens:
- Thicker in the middle than at edges
- Converges parallel rays to a point (focus)
- Used for correcting hypermetropia (long-sightedness) and in magnifying glasses
- Image: Real and inverted when object beyond focal length; Virtual, enlarged when object within focus
Concave (Diverging) Lens:
- Thinner in the middle than at edges
- Diverges parallel rays (rays appear to diverge from focus)
- Used for correcting myopia (short-sightedness)
- Image: Always virtual, diminished, upright
Lens formula: 1/v − 1/u = 1/f (similar to mirror formula but with different sign conventions)
The Human Eye
The human eye functions like a camera:
| Eye part | Function | Camera analogy |
|---|---|---|
| Cornea | Transparent front layer, bends light | Lens (initial bending) |
| Pupil | Adjustable opening that controls light entry | Aperture |
| Iris | Colored part, adjusts pupil size (bright light → pupil contracts; dark → pupil dilates) | — |
| Crystalline lens | Flexible, changes shape (accommodation) for focusing | Focus adjustment |
| Retina | Light-sensitive layer with rods (dim light) and cones (color, bright light) | Film/sensor |
| Optic nerve | Carries signals from retina to brain | — |
Accommodation: The lens changes shape to focus on near and far objects — near objects require lens to become more convex (thicker).
Eye Defects
| Defect | Cause | Far point | Near point | Correction |
|---|---|---|---|---|
| Myopia (Near-sightedness) | Eyeball too long OR lens too convex | Closer than 6m | Less than 25cm | Concave lens (diverging) |
| Hypermetropia (Long-sightedness) | Eyeball too short OR lens too flat | Normal (>6m) | Farther than 25cm | Convex lens (converging) |
| Presbyopia (Age-related) | Lens loses flexibility with age (after ~40 years) | Far point normal | Farther than 25cm | Convex lens for near; bifocal lens |
| Cataract | Lens becomes cloudy/opaque | Blurry vision in all ranges | Blurry vision in all ranges | Surgery to replace lens with artificial IOL (intraocular lens) |
Astigmatism: Irregular curvature of cornea — causes blurred vision at all distances — corrected by cylindrical lenses.
Optical Instruments
- Magnifying glass (simple microscope): Convex lens used to magnify near objects
- Compound microscope: Two convex lenses (objective and eyepiece) — used in biology labs
- Telescope: Used to view distant objects (refracting telescope uses two convex lenses)
- Periscope: Used in submarines to see above water level — uses two mirrors
Sound
Nature of Sound
Sound is a longitudinal wave — particles of the medium vibrate parallel to the direction of wave propagation. It requires a medium to travel (cannot travel through vacuum — unlike light).
Speed of sound (at 22°C in air): ~344 m/s
- In solids (steel): ~5,960 m/s (much faster)
- In liquids (water): ~1,480 m/s
- In gases: Speed ∝ temperature; inversely ∝ molecular weight
Frequency (f): Number of vibrations per second. Unit: Hertz (Hz).
- Audible range for humans: 20 Hz to 20,000 Hz (20 kHz)
- Infrasound: < 20 Hz (elephants, whales use this for communication)
- Ultrasound: > 20 kHz (bats, dolphins, medical imaging — echocardiography, pregnancy scanning)
Characteristics of Sound
| Property | What it affects | Higher value = |
|---|---|---|
| Loudness | How loud sound is | Amplitude (decibel/dB) |
| Pitch | How high or low | Frequency (Hz) — higher Hz = higher pitch |
| Quality/Timbre | Recognizability of a voice/instrument | Waveform (overtones) |
Decibel (dB): Logarithmic unit of sound intensity. 0 dB = threshold of hearing; 120 dB = threshold of pain; Normal conversation ~60 dB; Jet engine ~140 dB. Prolonged exposure >85 dB causes permanent hearing damage.
Echo
An echo is a reflected sound wave heard after a minimum delay (~0.1 seconds) from the original sound. For echo to be heard distinctly, the reflecting surface must be at least 17.2 m away in air (calculation based on speed of sound ~344 m/s and round trip).
Applications of echo:
- SONAR (Sound Navigation and Ranging): Used by ships and submarines to detect underwater objects, measure depth
- Ultrasound imaging: Used in pregnancy scanning (monitoring fetus), echocardiography (heart imaging), kidney stone detection
Musical Sounds vs Noise
Musical sound: Regular, periodic waveforms — pleasing to hear (violin, tuning fork) Noise: Irregular, non-periodic waveforms — unpleasant (car horn, construction noise)
Note: Both can be measured in decibels — but noise at lower intensity can still cause stress.
Heat
Heat vs Temperature
Heat: Total kinetic energy of all molecules in a substance (depends on mass, type of substance, temperature). Unit: Joule (J) or Calorie (1 Cal = 4.184 J).
Temperature: Average kinetic energy of molecules (doesn’t depend on mass). Unit: Celsius (°C), Kelvin (K), Fahrenheit (°F).
Conversion:
- °C to K: Add 273 (0°C = 273 K)
- °C to °F: (°C × 9/5) + 32
- °F to °C: (°F − 32) × 5/9
Kelvin (K) is the SI unit of temperature — 0 K (absolute zero) is the lowest possible temperature (−273°C) where all molecular motion stops.
Transfer of Heat
| Mode | How it Works | Example |
|---|---|---|
| Conduction | Heat transfers through a substance without movement of the substance (touch) | Metal spoon in hot water (metals are good conductors), wooden spoon (poor conductor) |
| Convection | Heat transfers through movement of the heated substance itself (currents) | Hot water rises, cold water sinks (water heating); sea breezes |
| Radiation | Heat transfers as electromagnetic waves (infrared) — no medium needed | Sun’s heat reaching Earth; microwave oven; heating coil of a room heater |
Conductors: Metals (copper, iron, aluminum) — good thermal conductors. Insulators: Wood, plastic, rubber, air — poor thermal conductors.
Convection in nature:
- Land and sea breezes: Day — land heats faster → hot air rises → sea breeze. Night — land cools faster → reverse.
- Mountain and valley winds: Day — valley air heats, rises → upslope winds. Night — mountain air cools, descends → downslope winds.
Thermometers
| Type | Based on | Range and Use |
|---|---|---|
| Mercury thermometer | Expansion of mercury | Clinical (35–42°C), lab use. Mercury is toxic — replaced gradually |
| Alcohol thermometer | Expansion of alcohol | Low temperature measurements (-115°C to 78°C) |
| Digital thermometer | Electronic sensor (thermistor) | Fast, safe, no mercury |
| Infrared thermometer | Infrared radiation from body | No-contact — used during COVID for forehead temperature |
| Maximum-minimum thermometer | Two markers locked at max/min | Weather stations (Seismic instrument) |
Clinical thermometer: Range 35–42°C (94–108°F), only has a constriction to prevent mercury from going back down — so it records the highest temperature.
Latent Heat
When a substance changes state (melting or boiling), heat is added but temperature does not change — the heat is used to change the state. This is called latent heat (hidden heat).
Latent heat of fusion (melting): Heat needed to change 1 kg of solid to liquid at its melting point (without temperature change).
- Ice melting at 0°C: 334 kJ/kg (latent heat of fusion of ice)
Latent heat of vaporization (boiling): Heat needed to change 1 kg of liquid to gas at its boiling point (without temperature change).
- Water boiling at 100°C: 2260 kJ/kg (latent heat of vaporization of water)
This is why sweating cools you down — sweat absorbs heat from the body as it evaporates.
CTET Exam Focus
- Laws of reflection: ∠i = ∠r
- Concave vs convex mirrors: Image type, size, use
- Refraction: Snell’s law, refractive index
- Convex vs concave lenses: Corrections for hypermetropia and myopia
- Human eye: Cornea, pupil, iris, lens, retina, accommodation
- Eye defects: Myopia (concave lens), hypermetropia (convex lens), cataract (cloudy lens)
- Sound: Longitudinal wave, speed in different media, audible range (20 Hz – 20 kHz)
- Ultrasound: >20 kHz, used in sonar and medical imaging
- Heat transfer: Conduction (metals), convection (fluids), radiation (no medium needed)
- Temperature scales: °C, K, °F; conversion formulas
- Latent heat: Fusion (ice melting) and vaporization (water boiling)
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