Light, sound and heat

Why it matters

Why is lightning seen before thunder is heard? Why does steam burn worse than boiling water? Both answers come from this chapter.

• Light is a form of energy that lets us see. It is an electromagnetic wave and needs no medium.
• Sound is energy produced by vibrating objects. It is a mechanical wave and needs a medium (solid, liquid or gas).
• Heat is energy that flows from a hotter body to a colder body because of a temperature difference.

In the Constable paper, most questions here are short facts ("Which lens corrects myopia?", "Sound cannot travel through ___") plus a few quick sums (echo distance, °C to °F, lens power).

Core concept

Level 1 — Beginner: the nature of light and sound

Property	Light	Sound
Type of wave	Electromagnetic, transverse	Mechanical; longitudinal in air and liquids
Needs a medium?	No (travels through vacuum)	Yes (cannot travel through vacuum)
Speed	3 × 10⁸ m/s in vacuum	About 343 m/s in air at 20 °C
Fastest in	Vacuum	Solids

Light is nearly 9 lakh times faster than sound in air, so the lightning flash reaches you before the thunder. Sunlight takes about 8 minutes 20 seconds to reach the Earth.

For every wave: speed = frequency × wavelength (v = f λ).

Level 2 — Intermediate: what light does

1. Reflection — light bouncing back from a surface.

• Laws: angle of incidence = angle of reflection; the incident ray, reflected ray and normal lie in the same plane.
• Plane mirror image: virtual, erect, same size, as far behind the mirror as the object is in front, and laterally inverted (left looks right). That is why "AMBULANCE" is written in reverse on the front of an ambulance.
• To see your full body in a plane mirror, the mirror must be at least half your height.
• Two mirrors at angle θ give (360/θ) − 1 images when 360/θ is even: 3 images at 90°, 5 at 60°, infinitely many when parallel.

Mirror	Nature	Common uses
Concave (converging)	Can form enlarged images	Shaving mirror, dentist's mirror, torch and headlight reflectors
Convex (diverging)	Always virtual, erect, smaller image; wide view	Rear-view mirrors of vehicles, mirrors at blind road turns

For a spherical mirror, focal length = radius of curvature ÷ 2 (f = R/2).

2. Refraction — bending of light when it passes from one medium to another, because its speed changes.

• Going into a denser medium (air → water), light slows down and bends towards the normal.
• Refractive index n = speed of light in vacuum ÷ speed in the medium. Water ≈ 1.33, glass ≈ 1.5, diamond ≈ 2.42. Higher n means light is slower there.
• Effects: a pencil looks bent in water; a pool looks shallower; stars twinkle (atmospheric refraction); the Sun is seen about 2 minutes before actual sunrise and 2 minutes after actual sunset.

3. Total internal reflection (TIR) — when light goes from a denser to a rarer medium at an angle greater than the critical angle, none comes out; it all reflects back. Examples: optical fibres, the sparkle of diamond, and the mirage on hot roads.

4. Dispersion — splitting of white light into seven colours by a prism: VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red). Violet bends the most, red the least. Red has the longest wavelength, violet the shortest. A rainbow forms when sunlight is refracted, dispersed and internally reflected inside raindrops; you see it with the Sun behind you.

5. Scattering — tiny particles in air throw light in all directions. Air molecules scatter short wavelengths (blue) much more than long ones (red).

• The sky looks blue; to astronauts above the atmosphere it looks dark.
• The Sun looks reddish at sunrise and sunset — light crosses a longer path of air, so most blue is scattered away.
• Danger signals are red because red is scattered least and is seen from far away, even in fog.

Level 3 — Advanced: lenses and the human eye

• Convex lens: converging, power positive (+); used as a magnifying glass.
• Concave lens: diverging, power negative (−).

Power of a lens P = 1/f, with f in metres. SI unit: dioptre (D).

Human eye: the eye lens forms a real, inverted image on the retina; the brain makes it upright. Ciliary muscles change the focal length of the eye lens (accommodation). The iris controls the size of the pupil. A normal eye sees clearly from 25 cm (near point) to infinity.

Defect	Problem	Image forms	Correction
Myopia (short-sight)	Cannot see far objects	In front of retina	Concave lens
Hypermetropia (long-sight)	Cannot see near objects	Behind retina	Convex lens
Presbyopia	Old age; weak accommodation	—	Bifocal lens
Cataract	Eye lens becomes cloudy	—	Surgery

Sound in detail

• Speed order: solids > liquids > gases. Roughly air ≈ 343 m/s, water ≈ 1,500 m/s, steel ≈ 5,000–6,000 m/s.

• Speed in air increases with temperature and with humidity. A change of pressure alone (same temperature) does not change it.

• Human audible range: 20 Hz to 20,000 Hz. Below 20 Hz = infrasonic (earthquakes, elephants, whales). Above 20,000 Hz = ultrasonic (bats, dolphins).

• Uses of ultrasound: sonography (scanning organs and the baby in the womb), SONAR (Sound Navigation And Ranging — sea depth, submarines) and detecting cracks in metal.

• Loudness (unit: decibel, dB) depends on amplitude; pitch (shrillness) on frequency; quality (timbre) on the shape of the wave. A child's voice has a higher pitch than a man's.

• Echo: reflected sound heard separately. Our ear holds a sound for about 0.1 s, so the reflecting surface must be at least about 17 m away.

• Stethoscope and megaphone work on multiple reflection of sound.

• Doppler effect: a horn sounds higher in pitch as a vehicle approaches and lower as it moves away. Police radar speed guns use the Doppler effect with radio waves.

Heat in detail

Heat vs temperature: heat is energy (SI unit joule; 1 calorie ≈ 4.2 J). Temperature tells how hot a body is (SI unit kelvin, K). Heat always flows from higher to lower temperature.

Scales: F = (9/5)C + 32 and K = C + 273.15 (273 in most exam sums).

Point	°C	°F	K
Ice melts	0	32	273.15
Normal body temperature	37	98.6	310.15
Water boils (normal pressure)	100	212	373.15
Absolute zero	−273.15	−459.67	0

−40 °C = −40 °F (the only reading where both scales agree). A clinical thermometer reads 35 °C to 42 °C and has a kink so the mercury does not fall back on its own.

Three ways heat travels:

Mode	How	Examples
Conduction	Particle to particle, mainly in solids	Metal spoon gets hot in tea; metals conduct well, wood and plastic poorly
Convection	Hot fluid rises, cold fluid sinks (liquids and gases)	Sea breeze by day, land breeze by night
Radiation	No medium needed	Sun's heat reaching Earth; warmth felt near a fire

Dark surfaces absorb radiation well; white surfaces reflect it, so we wear white in summer. In a vacuum flask, the vacuum stops conduction and convection, and the silvered walls cut radiation. Woollen clothes trap air, a poor conductor, so they keep us warm.

Specific heat capacity is the heat needed to raise 1 kg of a substance by 1 °C (Q = m c ΔT). Water's is very high (about 4,200 J/kg °C), so it heats and cools slowly. Hence water is used in car radiators, and land heats faster than the sea.

Latent heat is heat taken in or given out during a change of state at constant temperature. Ice: about 3.34 × 10⁵ J/kg (80 cal/g); steam: about 2.26 × 10⁶ J/kg (540 cal/g). Steam at 100 °C burns worse than water at 100 °C because it releases this extra latent heat when it condenses on skin.

Other key facts:

• Evaporation causes cooling (sweating, water in a clay pot). It is faster with more heat, larger surface, wind and dry air.
• Boiling point falls at low pressure (cooking is slow on hills). A pressure cooker raises pressure, so water boils at about 120 °C and food cooks faster.
• Most things expand on heating — gaps are left in railway tracks; a bimetallic strip in thermostats bends when heated.
• Anomalous expansion of water: water is densest at 4 °C. Ice floats, lakes freeze from the top, and fish survive in the water below.

Worked examples

Example 1 — Echo distance. Kiran shouts near a hill and hears the echo after 2 s. Speed of sound = 340 m/s. How far is the hill?
Sound goes to the hill and comes back, so total path = 340 × 2 = 680 m. Distance = 680 ÷ 2 = 340 m. (SONAR depth sums use the same "divide by 2" step.)

Example 2 — Thunder. A constable on night duty sees lightning and hears thunder 3 s later. How far away is the storm? (speed of sound = 340 m/s)
Light arrives almost instantly, so 3 s is the sound's travel time. Distance = 340 × 3 = 1,020 m, about 1 km. No halving, because the sound travels one way.

Example 3 — Fever reading. A thermometer shows 104 °F. What is this in °C?
C = (F − 32) × 5/9 = 72 × 5/9 = 40 °C, a high fever.

Example 4 — Power of a lens. A doctor prescribes a lens of power −2 D. Find its focal length and type.
f = 1/P = 1/(−2) = −0.5 m = −50 cm. The negative sign means a concave lens, so the patient has myopia.

Real-world connection

• Policing: radar speed guns (Doppler effect), convex mirrors on patrol vehicles, red danger signals (least scattering).
• Daily life: pressure cooker (higher boiling point), clay pot (evaporation), sonography (ultrasound), optical fibre internet (TIR).

How it is asked in the exam

Formats used: direct fact ("SI unit of lens power is ___"), phenomenon → principle ("Mirage is due to ___"), match the following (eye defects with corrections), NOT type ("Which is NOT a use of ultrasound?") and short sums (echo, °C ↔ °F ↔ K, lens power, v = f λ).

Time: about 54 seconds per question; finish fact questions in 15–20 seconds to save time for sums.

• Easy: "Sound cannot travel through: (a) water (b) steel (c) vacuum (d) air" → vacuum.
• Medium: "A lens has power +2.5 D. Its focal length is: (a) 25 cm (b) 40 cm (c) 2.5 m (d) 4 cm" → 40 cm.
• Hard: "Echo heard 1.5 s after a shout at a cliff, speed 330 m/s; distance?" → 330 × 1.5 ÷ 2 = 247.5 m; or "Which pair is wrongly matched?" across four phenomena.

తెలుగు సారాంశం (Telugu summary)

కాంతి ఒక విద్యుదయస్కాంత తరంగం. దీనికి యానకం అవసరం లేదు, శూన్యంలో సెకనుకు 3 × 10⁸ మీటర్ల వేగంతో ప్రయాణిస్తుంది.
ధ్వని ఒక యాంత్రిక తరంగం, దీనికి యానకం తప్పనిసరి. ఇది ఘన పదార్థాల్లో అత్యంత వేగంగా ప్రయాణిస్తుంది; శూన్యంలో అసలు ప్రయాణించదు.
నక్షత్రాలు మిణుకుమిణుకుమనడం వాతావరణ వక్రీభవనం వల్ల జరుగుతుంది. ఆకాశం నీలంగా కనిపించడం, ప్రమాద సంకేతాలు ఎరుపు రంగులో ఉండడం కాంతి పరిక్షేపణం వల్ల.
వజ్రం మెరవడం, ఆప్టికల్ ఫైబర్, ఎండమావి సంపూర్ణాంతర పరావర్తనానికి ఉదాహరణలు. ఇంద్రధనుస్సు కాంతి విక్షేపణం వల్ల ఏర్పడుతుంది.
హ్రస్వదృష్టిని పుటాకార కటకంతో, దీర్ఘదృష్టిని కుంభాకార కటకంతో సరిచేస్తారు. కటక సామర్థ్యం P = 1/f (f మీటర్లలో), దీని ప్రమాణం డయాప్టర్.
మనిషి 20 Hz నుండి 20,000 Hz వరకు ధ్వనులను వినగలడు. శబ్దతీవ్రత కంపన పరిమితిపై, కీచుదనం పౌనఃపున్యంపై ఆధారపడతాయి.
ప్రతిధ్వని వినాలంటే పరావర్తన తలం కనీసం సుమారు 17 మీటర్ల దూరంలో ఉండాలి.
F = (9/5)C + 32, K = C + 273. శరీర సాధారణ ఉష్ణోగ్రత 37 °C = 98.6 °F.
ఉష్ణం వహనం, సంవహనం, వికిరణం అనే మూడు పద్ధతుల్లో ప్రసరిస్తుంది. నీటికి విశిష్టోష్ణం చాలా ఎక్కువ, 4 °C వద్ద నీటి సాంద్రత గరిష్ఠం.
ఆవిరి చర్మంపై సాంద్రీకరణం చెందేటప్పుడు గుప్తోష్ణాన్ని విడుదల చేస్తుంది, అందుకే మరిగే నీటికంటే ఎక్కువగా కాలుస్తుంది.

Key terms (English — తెలుగు):

• Reflection — పరావర్తనం
• Refraction — వక్రీభవనం
• Total internal reflection — సంపూర్ణాంతర పరావర్తనం
• Dispersion — విక్షేపణం
• Scattering — పరిక్షేపణం
• Refractive index — వక్రీభవన గుణకం
• Convex / Concave lens — కుంభాకార / పుటాకార కటకం
• Focal length — నాభ్యంతరం
• Myopia / Hypermetropia — హ్రస్వదృష్టి / దీర్ఘదృష్టి
• Frequency / Amplitude — పౌనఃపున్యం / కంపన పరిమితి
• Heat / Temperature — ఉష్ణం / ఉష్ణోగ్రత
• Conduction / Convection / Radiation — వహనం / సంవహనం / వికిరణం
• Specific heat / Latent heat — విశిష్టోష్ణం / గుప్తోష్ణం