Heat, Light and Sound

When you hold a hot cup of chai on a cold Delhi morning, you are feeling two different physical quantities at once — the energy moving from cup to hand (heat) and the level of hotness your skin senses (temperature). RPF Constable General Science loves this distinction, and every aspirant should treat it as a guaranteed-marks topic.

Definition: Heat is a form of energy that flows from a hotter body to a colder body whenever they are in thermal contact.

Definition: Temperature is a measure of the degree of hotness or coldness of a body. It tells you how hot something is, not how much heat it contains.

Heat vs Temperature — the most-asked one-mark distinction

A small cup of tea and a large bucket of tea, both at 60 °C, are at the same temperature. But the bucket contains far more heat energy because it has many more molecules vibrating. So heat depends on mass + temperature of the body; temperature depends only on the average kinetic energy of the molecules.

This is why the SI unit of heat is the joule (J) — heat is energy. The older unit is the calorie, where 1 calorie = 4.2 J. (Mechanical equivalent of heat: J = 4.2 J/cal, established by James Prescott Joule.)

The SI unit of temperature is the kelvin (K). Other common units are degree Celsius (°C) and degree Fahrenheit (°F). The relationships you should memorise:

• 0 °C = 273 K (more precisely 273.15 K).
• 100 °C = 373 K (boiling point of water at normal atmospheric pressure).
• To convert Celsius to Kelvin: K = °C + 273.
• To convert Celsius to Fahrenheit: °F = (9/5) × °C + 32.
• −40 °C = −40 °F — the only point where the two scales coincide (a popular trick MCQ).

The three modes of heat transfer — Con-Con-Rad

Heat can travel from one place to another in three distinct ways.

1. Conduction.
Heat transfer through direct contact, mainly in solids. The molecules at the hot end vibrate more vigorously and pass their energy on to neighbouring molecules without themselves moving from their place. That is why a metal spoon dipped in hot dal soon feels hot at the handle.
Metals are good conductors (free electrons help). Wood, plastic, and air are poor conductors — which is why the wooden handle of a frying pan stays cool.

2. Convection.
Heat transfer through the bulk movement of particles in fluids (liquids and gases). Hot fluid expands, becomes less dense, and rises; cooler fluid sinks to take its place. The continuous circulation is called a convection current.
Examples: boiling water in a pot, sea breezes near a beach, the working of room heaters and refrigerators.

3. Radiation.
Heat transfer through electromagnetic waves, requiring no medium at all. The Sun's heat reaches Earth through 150 million km of empty space by radiation. Every body above absolute zero radiates infrared waves.
A black body absorbs and radiates well; a polished silver surface barely does — which is why thermos flasks have silvered inner walls (to suppress radiation).

Memory aid: Con-Con-Rad for Conduction, Convection, Radiation.

Special properties of water — favourite exam targets

Water behaves strangely between 0 °C and 4 °C — this is called anomalous expansion.

• From 0 °C to 4 °C, water actually contracts (density increases).
• Above 4 °C, it behaves normally — expands as it gets hotter.
• So water has its maximum density at 4 °C.

Why it matters in nature: in winter, the top layer of a lake cools, becomes denser at 4 °C, sinks to the bottom, while the still-colder water near the surface freezes into ice. Ice (being less dense than water) floats on top, acting as an insulating blanket. Fish and aquatic life survive winter because of this single quirk of water.

Other essentials:

• Freezing point of water = 0 °C = 273 K (at 1 atmosphere pressure).
• Boiling point of water = 100 °C = 373 K (at 1 atmosphere pressure).
• Boiling point rises with pressure (this is why pressure cookers cook dal faster — water boils at ~120 °C inside) and falls with altitude (boiling at the top of a mountain happens below 100 °C, which is why cooking takes longer at hill stations).

Real-world example — your kitchen tells you everything

A pressure cooker on a gas stove uses all three modes of heat transfer at once. The flame heats the metal base by radiation and convection of hot gases, the metal then conducts the heat to the inner contents, and the rising hot water sets up convection currents inside the cooker that cook the rajma evenly. The whistle releases steam to keep the pressure (and hence the boiling point) constant. Anomalous expansion of water explains why, on a Himachal trek in December, you can still find liquid water under a frozen lake surface.

Common misconception: Students often write that "a hotter body has more heat than a colder body." Not necessarily — a very large cold body (like an ocean at 15 °C) contains far more heat energy than a small hot body (a candle flame). Temperature is the intensity of hotness; heat is the amount of energy. Keep them separate.

Another trap: people think radiation needs air. It does not. Sunlight reaches us through the vacuum of space — radiation is the only mode of heat transfer that requires no medium.

Light travels in straight lines at speed about 3 x 10^8 m/s in vacuum. Reflection: angle of incidence = angle of reflection. A plane mirror forms a virtual, erect, same-size image. Concave mirror can form real or virtual images (used in torches, headlights, shaving mirrors). Convex mirror always forms a small, virtual image and gives a wide view (used as rear-view mirrors in vehicles). Refraction is bending of light when it passes between media of different densities; it causes a pencil to appear bent in water. A convex lens converges light (used for correcting hypermetropia/long-sightedness); a concave lens diverges light (used for myopia/short-sightedness). White light splits into seven colours (VIBGYOR) through a prism: dispersion.

Clap your hands. Something invisible just travelled from your palms to your ear and your brain decoded it as a sharp sound. That something is a sound wave — and RPF Constable, RRB NTPC and SSC routinely build five or six questions around what you are about to read.

Definition: Sound is a mechanical longitudinal wave produced by a vibrating body that travels through a material medium and is detected by the ear.

Two words in that definition do all the work. "Mechanical" means it needs particles to push against — a medium. "Longitudinal" means the particles of the medium vibrate back and forth in the same direction the wave is moving, creating regions of compression (squeezed-together particles) and rarefaction (spread-out particles).

Sound Needs a Medium

The classic proof is the bell-jar experiment. An electric bell is hung inside a glass jar and switched on; you can hear it ring. Now a vacuum pump slowly sucks out the air. As the air thins, the ring grows fainter until it dies away completely, even though the hammer is still visibly hitting the gong. Pump the air back in and the sound returns. Conclusion: sound cannot travel through a vacuum.

This is the single biggest difference between sound and light. Light is an electromagnetic wave and travels happily through the vacuum of space — that is why sunlight reaches us across 150 million km of empty void, while astronauts on a spacewalk must use radios to talk, not shouts.

Why it matters: Almost every General Science paper has a "sound vs light" comparison MCQ. "Which of the following can travel through vacuum?" is asked in a dozen disguises.

Speed of Sound in Different Media

In dry air at around 20 °C, sound travels at roughly 340 m/s (the textbook value is 343 m/s at 20 °C; 332 m/s at 0 °C — RPF questions accept the rounded "340 m/s"). In water, it is about 1,480 m/s, and in steel about 5,100 m/s.

The order is: solid > liquid > gas. The intuition is simple: in a solid the particles are tightly bonded — when one is pushed, it immediately shoves its neighbour. In a gas, the particles are far apart and must travel a longer distance before the push is transmitted. So denser, more rigid media carry sound faster, not slower.

Sound also speeds up as temperature rises, because hotter air molecules move faster and pass on the compression more quickly. The rough rule: +0.6 m/s for every 1 °C rise in air.

Reflection of Sound and the Echo

When sound hits a hard surface — a wall, a hill, the floor of a deep well — it bounces back. A bounced sound that you hear as a separate, repeated sound is called an echo.

Your ear can only distinguish two sounds as separate if they arrive at least 0.1 seconds (one-tenth of a second) apart. Using the speed of sound in air:

Question: What is the minimum distance from a reflecting surface for a distinct echo?

Solution:
Step 1: The sound must travel to the wall and back, so the total path = 2d.
Step 2: 2d = speed × time = 340 × 0.1 = 34 m.
Step 3: Therefore d = 17 m.
Conclusion: The reflecting surface must be at least 17 m away for a clear echo.

This 17 m is a magic number you should write on the inside of your wrist before exam day. RPF and RRB ask it directly.

The same physics gives you a reverberation — multiple overlapping echoes in a hall, which is why concert halls have curtains and corrugated ceilings to absorb stray reflections.

Pitch and Loudness

Two everyday properties of a sound — how high or how low it sounds, and how soft or loud — map onto two physical properties of the wave.

Pitch is determined by frequency (the number of vibrations per second, measured in hertz, Hz). A higher frequency means a higher pitch. A child's voice and a flute have higher pitch than a man's voice or a tabla.

Loudness is determined by amplitude (how big the to-and-fro vibration is). A bigger amplitude packs more energy into the wave, and the ear interprets that as a louder sound. Hit a drum gently and you get a soft thud; hit it hard and the same drum booms — same frequency, larger amplitude.

A third property, quality or timbre, lets you tell a sitar from a violin even when they play the same note at the same loudness; it comes from the mix of overtones in the wave.

The Audible Range and What Lies Beyond

A young, healthy human ear can hear frequencies between 20 Hz and 20,000 Hz (20 kHz) — the audible range.

• Below 20 Hz → infrasonic (or infrasound). Elephants communicate using infrasonic rumbles that travel kilometres through the ground. Earthquakes generate strong infrasonic waves.
• Above 20,000 Hz → ultrasonic (or ultrasound). Bats and dolphins navigate using ultrasonic echolocation. Humans use ultrasound for SONAR (Sound Navigation And Ranging, to map the sea bed and detect submarines), in medical imaging (pregnancy scans, gall-stone detection), in non-destructive testing of metals, in ultrasonic cleaners for jewellery, and in lithotripsy to shatter kidney stones.

As you age, the upper end of your audible range falls — many adults cannot hear above 15 kHz. This is normal, not a disease.

Real-world example: A submarine's SONAR sends out a short ultrasonic ping; some of it bounces off a hidden object and returns. By timing the round trip and using the speed of sound in seawater (~1,500 m/s), the submarine calculates the distance — exactly the echo formula above, just under the sea. This is also how fishermen in Kerala find shoals of fish with handheld fish-finders.

Common misconception: "Sound travels faster in a vacuum because there is nothing to slow it down." Completely wrong. Sound cannot travel through a vacuum at all — it needs particles to pass the disturbance along. The misconception comes from mixing up sound (mechanical wave, needs a medium) with light (electromagnetic wave, does not need a medium).

Another tripper: "Higher pitch means louder sound." No. Higher pitch means higher frequency, not greater loudness. A whisper can be high-pitched; a roar can be low-pitched.

Cover the answer, recall, then check. 13 cards on heat, light and sound for RPF Constable.

Q1. What is the SI unit of heat, and how does a calorie relate to it?
A1. SI unit is the joule (J). 1 calorie = 4.18 J. Heat is a form of energy.

Q2. How do you convert Celsius to Kelvin, and Celsius to Fahrenheit?
A2. K = °C + 273; °F = (9/5)°C + 32. Water freezes at 0°C = 32°F = 273 K.

Q3. Name the three modes of heat transfer.
A3. Conduction (in solids), convection (in liquids and gases), and radiation (needs no medium — how the Sun's heat reaches us).

Q4. What is the speed of light in vacuum?
A4. About 3 × 10⁸ m/s (3 lakh km/s). Light travels in a straight line.

Q5. State the laws of reflection of light.
A5. (1) The angle of incidence equals the angle of reflection. (2) The incident ray, reflected ray, and normal all lie in the same plane.

Q6. What is refraction of light?
A6. The bending of light when it passes from one transparent medium to another due to a change in its speed (e.g. a pencil looks bent in water).

Q7. What is dispersion of light?
A7. The splitting of white light into its seven colours (VIBGYOR) when it passes through a prism. This causes a rainbow.

Q8. Which type of mirror is used as a vehicle rear-view mirror and why?
A8. A convex mirror, because it gives an erect, diminished image and a wider field of view.

Q9. What is the least distance of distinct vision for a normal human eye?
A9. About 25 cm. The part of the eye where the image forms is the retina.

Q10. Does sound need a medium? What is its approximate speed in air?
A10. Yes — sound needs a material medium and cannot travel in vacuum. Its speed in air is about 340–343 m/s.

Q11. What is the audible range of frequency for humans, and what lies beyond it?
A11. 20 Hz to 20,000 Hz. Below 20 Hz is infrasonic; above 20,000 Hz is ultrasonic (ultrasound is used in medical imaging and in SONAR to measure sea depth).

Q12. What is an echo?
A12. The repetition of a sound caused by reflection from a distant surface. It is heard when the reflecting surface is far enough (about 17 m or more).

Worked Example

Problem: An object is placed 30 cm in front of a concave mirror whose focal length is 10 cm. Using the mirror formula, find the position of the image and state its nature.

Solution:
Step 1 — Write the mirror formula: 1/v + 1/u = 1/f, where u = object distance, v = image distance, f = focal length.
Step 2 — Apply sign convention (distances measured against the incoming light are negative). For a concave mirror: f = −10 cm and u = −30 cm.
Step 3 — Substitute and solve for v:
1/v = 1/f − 1/u = (1/−10) − (1/−30) = −1/10 + 1/30.
Take LCM 30: −3/30 + 1/30 = −2/30 = −1/15.
So v = −15 cm.
Step 4 — Interpret. v is negative, so the image forms 15 cm in front of the mirror — it is real and inverted.
Magnification m = −v/u = −(−15)/(−30) = −0.5, so the image is half the size of the object (diminished).

Answer: The image forms 15 cm in front of the mirror; it is real, inverted and half the size of the object.