Science & Technology
Physics in Everyday Life
Light and Sound
Reflection, refraction, total internal reflection, scattering, the human eye, and sound: speed, echo, ultrasound, Doppler, decibels and noise rules for CGPSC.
Prerequisites
- Motion and Force, for the idea of speed and the medium a wave travels through.
- Basic idea of a wave: it carries energy from one place to another without carrying matter along.
The Hook: A Rainbow at Chitrakote
Connect
Stand at Chitrakote (चित्रकोट) on the Indravati near Jagdalpur on a sunny monsoon morning with the Sun behind you. A rainbow often hangs in the spray, and the roar of the falls reaches you from far away. Look at the riverbed and it seems shallower than it really is.
The rainbow is light being refracted, dispersed and reflected inside water drops. The shallow-looking water is refraction. The roar is a sound wave travelling through air. Why is the sky blue but the setting Sun red? Why are danger lights red? How does a doctor “see” a baby with sound? This page answers those questions.
Why it matters: Optical phenomena (blue sky, mirage, twinkling stars, diamond sparkle) and sound (ultrasound, echo, Doppler, decibels) are standard general-science MCQs. Mains P-IV asks for short explanations and applications in medicine and technology.
Part A: Light (प्रकाश)
Nature of light
- Light is an electromagnetic wave. It needs no medium and travels through vacuum.
- Speed in vacuum: about \( 3 \times 10^{8} \) m/s (3 lakh km per second). It is slower in water and glass.
- Sunlight takes about 8 minutes 20 seconds to reach Earth.
- Light travels in straight lines in a uniform medium, which is why shadows form and why a pinhole camera works.
Reflection (परावर्तन)
Laws: The angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal lie in the same plane.
| Mirror | Image | Everyday uses |
|---|---|---|
| Plane | Virtual, erect, same size, laterally inverted | Looking glass; periscope. “AMBULANCE” is painted in mirror writing on the front of ambulances so drivers ahead read it correctly in their rear-view mirrors |
| Concave (converging) | Real or virtual, depending on where the object is | Shaving and make-up mirrors (enlarged image), dentists’ mirrors, reflectors in torches and headlights, solar cookers and furnaces |
| Convex (diverging) | Always virtual, erect and diminished | Rear-view mirrors of vehicles (wider field of view); mirrors at blind turns on hill roads and in shops |
Refraction (अपवर्तन)
Light bends when it passes from one medium to another because its speed changes. Going from a rarer to a denser medium (air to water), it bends towards the normal.
Refractive index \( n = \dfrac{c}{v} \) (speed in vacuum ÷ speed in the medium). Typical values: water about 1.33, glass about 1.5, diamond about 2.42.
| Everyday effect | Explanation |
|---|---|
| A pencil in water looks bent; a pond or river looks shallower than it is | Light from the submerged part bends away from the normal as it leaves the water |
| Stars twinkle | Starlight passes through layers of air of changing density, so its path and brightness keep changing. Stars are point sources |
| Planets do not twinkle | They are much nearer and act as extended sources; the flickers from different points average out |
| Advance sunrise and delayed sunset | Atmospheric refraction lets us see the Sun about 2 minutes before it actually rises and about 2 minutes after it actually sets |
| The Sun looks flattened (oval) near the horizon | Unequal atmospheric refraction of its upper and lower edges |
Total internal reflection (पूर्ण आंतरिक परावर्तन)
When light goes from a denser to a rarer medium at an angle greater than the critical angle, it does not leave at all; it is reflected back completely.
| Application | How TIR is used |
|---|---|
| Sparkle of a diamond | Very high refractive index means a small critical angle (about 24°). Light entering a well-cut diamond is reflected many times inside before escaping |
| Optical fibres | Light zig-zags along a thin glass fibre by repeated TIR. Used in internet and telephone cables (for example, the village broadband network) and in endoscopes to see inside the body |
| Mirage in deserts and on hot roads | Air near the hot ground is rarer. Light from the sky bends upwards and is finally totally reflected, so a patch of road looks like water |
| Totally reflecting prisms | Used in binoculars and periscopes instead of mirrors |
Dispersion and the rainbow
- A glass prism splits white light into seven colours, VIBGYOR (violet, indigo, blue, green, yellow, orange, red).
- Violet bends the most, red the least. Red has the longest wavelength in the visible spectrum and violet the shortest.
- Rainbow (इंद्रधनुष): formed by refraction, dispersion and internal reflection of sunlight in raindrops. It is seen only with the Sun behind the observer, so morning rainbows are in the west and evening rainbows in the east. In the primary rainbow, red is on the outside and violet on the inside.
Scattering of light (प्रकाश का प्रकीर्णन)
Very small particles (air molecules) scatter shorter wavelengths far more than longer ones. In Rayleigh scattering, the amount scattered varies as \( 1/\lambda^{4} \), so blue is scattered much more than red.
| Observation | Explanation |
|---|---|
| Sky is blue | Air molecules scatter blue light from sunlight in all directions |
| Sun is red or orange at sunrise and sunset | Light crosses a much longer path through the air; most blue is scattered away, leaving red and orange |
| Danger and stop signals are red | Red is scattered least by fog, smoke and dust, so it is visible from the farthest distance |
| Sky looks black to astronauts in space or on the Moon | No atmosphere, so no scattering |
| Clouds are white | Water droplets are large compared with the wavelength and scatter all colours almost equally |
| Tyndall effect | A beam of sunlight is visible in a dusty room, or through forest canopy, because colloidal particles scatter it |
Raman effect: India's Nobel in physics
C.V. Raman discovered in 1928 that light scattered by a substance includes a small part with a changed wavelength, which carries information about the molecules. He received the Nobel Prize in Physics in 1930. National Science Day is observed on 28 February, the day the discovery was announced. Raman spectroscopy is used today to identify chemicals, test drugs and study materials.
Lenses and the human eye
| Lens | Behaviour | Uses |
|---|---|---|
| Convex (converging) | Brings parallel rays to a focus | Magnifying glass, camera, projector, correcting hypermetropia |
| Concave (diverging) | Spreads rays out | Correcting myopia, peepholes in doors |
Power of a lens \( P = \dfrac{1}{f} \), with the focal length \( f \) in metres. Unit: dioptre (D). A convex lens has positive power and a concave lens negative power.
The human eye (मानव नेत्र):
- Most of the bending of light happens at the cornea. The eye lens fine-tunes the focus.
- The ciliary muscles change the lens’s thickness so the eye can focus on near and far objects. This is accommodation.
- The image forms on the retina: real and inverted. The brain turns it upright.
- Rods sense dim light; cones sense colour.
- Near point of a normal eye: about 25 cm. Far point: infinity.
- Persistence of vision: an image stays on the retina for about 1/16 of a second, which is why a film shown as rapid still frames looks continuous.
| Defect | Problem | Correction |
|---|---|---|
| Myopia (short-sightedness, निकट दृष्टि दोष) | Cannot see far objects; image forms in front of the retina | Concave lens |
| Hypermetropia (long-sightedness, दूर दृष्टि दोष) | Cannot see near objects; image forms behind the retina | Convex lens |
| Presbyopia | Age-related loss of accommodation; often both near and far trouble | Bifocal or progressive lenses |
| Astigmatism | Uneven curvature of the cornea; blurred lines in some directions | Cylindrical lens |
| Cataract | Eye lens becomes cloudy | Surgery to replace the lens with an artificial one |
Part B: Sound (ध्वनि)
Nature and speed of sound
- Sound is a mechanical wave. It needs a medium (solid, liquid or gas) and cannot travel through vacuum. Astronauts on the Moon talk by radio.
- In air, sound travels as a longitudinal wave of compressions and rarefactions.
- Speed: solids > liquids > gases. Approximate values: air about 343 m/s at 20 °C (about 331 m/s at 0 °C); water about 1,500 m/s; steel about 5,000–6,000 m/s.
- Speed in air increases with temperature and with humidity.
- We see lightning before we hear thunder because light is almost a million times faster than sound. Counting the seconds between the flash and the thunder gives a rough distance (about 3 seconds per kilometre).
- Children put an ear to a rail to hear a distant train early, because sound travels faster in steel than in air.
Characteristics of sound
| Characteristic | Depends on | Example |
|---|---|---|
| Loudness | Amplitude of the wave; measured in decibels (dB) | A shout versus a whisper |
| Pitch (shrillness) | Frequency (hertz, Hz) | A woman’s or child’s voice usually has a higher pitch than a man’s; a mosquito’s buzz is high-pitched, a lion’s roar low-pitched |
| Quality (timbre) | Waveform | You recognise a friend’s voice, or tell a flute from a sitar, even at the same pitch and loudness |
The frequency ranges
| Range | Frequency | Examples and uses |
|---|---|---|
| Infrasound | Below 20 Hz | Produced by earthquakes, volcanoes and large animals such as elephants and whales; some animals sense it |
| Audible range (humans) | 20 Hz to 20,000 Hz (20 kHz) | The upper limit falls with age |
| Ultrasound (पराश्रव्य) | Above 20 kHz | Bats navigate by ultrasound echoes; dolphins use it. Human uses below |
Uses of ultrasound:
- Medical imaging (sonography, ultrasonography): images of the foetus, liver, kidneys, gall bladder; echocardiography of the heart. It uses no ionising radiation, unlike X-rays.
- Breaking small kidney stones into fine grains.
- SONAR (Sound Navigation And Ranging): ships and submarines measure sea depth and detect submarines, shoals of fish and wrecks from ultrasonic echoes.
- Detecting cracks and flaws in metal blocks and rails; cleaning delicate parts and jewellery.
Social angle for Mains
Misuse of ultrasound for sex determination of the foetus is banned under the Pre-Conception and Pre-Natal Diagnostic Techniques (PC&PNDT) Act, 1994, which was amended and renamed in 2003. A Mains answer on ultrasound can note this to show awareness of science’s social side.
Echo, reverberation and resonance
- Echo (प्रतिध्वनि): a reflected sound heard separately. The ear holds a sound for about 0.1 s, so the reflecting surface must be at least about 17 m away (half of 344 m/s × 0.1 s). The usual textbook figure is 17.2 m.
- Reverberation: repeated reflections that make sound persist in a large hall. Auditoriums use curtains, carpets and sound-absorbing panels, and curved ceilings in concert halls spread sound evenly.
- Resonance: a body vibrates strongly when pushed at its own natural frequency. Soldiers are told to break step while crossing a bridge so that their rhythm cannot build up dangerous vibrations.
Doppler effect
The apparent frequency of a sound changes when the source and listener move towards or away from each other. An ambulance siren sounds higher-pitched as it approaches and lower-pitched as it moves away.
Applications: police speed guns, Doppler weather radar (uses radio waves to track the motion of rain and cyclones), Doppler ultrasound to measure blood flow, and astronomy (the “red shift” of receding galaxies, using light).
Supersonic speed and sonic boom
A speed greater than the speed of sound is supersonic. The ratio of an object’s speed to the speed of sound is its Mach number (Mach 2 = twice the speed of sound). A supersonic aircraft produces shock waves heard on the ground as a loud sonic boom.
Decibels and noise pollution
The decibel scale is logarithmic: an increase of 10 dB means about 10 times the sound intensity. Prolonged exposure to loud noise can damage hearing and cause stress, high blood pressure and sleep loss.
Ambient noise standards under the Noise Pollution (Regulation and Control) Rules, 2000 (Environment (Protection) Act, 1986), in dB(A) Leq:
| Area / zone | Day (6 a.m.–10 p.m.) | Night (10 p.m.–6 a.m.) |
|---|---|---|
| Industrial | 75 | 70 |
| Commercial | 65 | 55 |
| Residential | 55 | 45 |
| Silence zone | 50 | 40 |
A silence zone is an area of not less than 100 metres around hospitals, educational institutions and courts, declared as such by the competent authority.
Memory Tricks
Mirror uses: 'Concave Collects, Convex Covers'
Concave collects light to a point: torches, headlights, solar cookers, shaving mirrors. Convex covers a wide view: rear-view mirrors, blind-turn mirrors.
Eye defects: 'My-Con, Hyper-Vex'
Myopia → Concave lens. Hypermetropia → convex lens. Myopia = Missing the Mountains (far objects blur).
Scattering: 'Blue Bounces, Red Runs'
Blue is scattered (bounces) everywhere, so the sky is blue. Red runs straight through the haze, so sunsets and danger signals are red.
Sound speeds: 'Steel, Sea, Sky'
Solids > liquids > gases: Steel (about 5,000+ m/s) > Sea water (about 1,500 m/s) > Sky (air, about 343 m/s).
Don’t Confuse
| This | vs | That |
|---|---|---|
| Light: electromagnetic, travels in vacuum, about \( 3 \times 10^{8} \) m/s | vs | Sound: mechanical, needs a medium, about 343 m/s in air |
| Refraction: bending at a boundary (twinkling, shallow pond) | vs | Total internal reflection: denser to rarer beyond the critical angle (diamond, fibre, mirage) |
| Dispersion: splitting into colours (prism, rainbow) | vs | Scattering: redirection by particles (blue sky, red sunset) |
| Pitch depends on frequency | vs | Loudness depends on amplitude |
| Infrasound: below 20 Hz (earthquakes, elephants) | vs | Ultrasound: above 20 kHz (bats, sonography, SONAR) |
| Myopia: concave lens | vs | Hypermetropia: convex lens |
| Echo: a distinct repeat, needs about 17 m or more | vs | Reverberation: persistence through many reflections |
Common Mistakes
MCQ traps
- “Sound travels fastest in air.” False. It is slowest in gases and fastest in solids.
- “Sound can travel in vacuum, like light.” False. Sound needs a medium.
- “A mirage is caused only by refraction.” Incomplete. Refraction through layers of hot air ending in total internal reflection produces it.
- “Danger signals are red because red is scattered most.” Reversed. Red is scattered least, so it travels farthest through fog and smoke.
- “Pitch depends on loudness.” False. Pitch depends on frequency; loudness on amplitude.
- “A rainbow can be seen while facing the Sun.” False. The Sun must be behind the observer.
- “Planets twinkle like stars.” False. Planets are extended sources and generally do not twinkle.
Practice
All questions below are original practice questions written in CGPSC style. They are not previous-year questions.
Level 1: Foundation
- The phenomenon behind the working of optical fibres is ______.
- The audible range of frequencies for a normal human ear is ______ to ______.
- Myopia is corrected using a ______ lens.
- National Science Day is celebrated on ______ to mark the discovery of the ______ effect.
Answers
- Total internal reflection.
- 20 Hz to 20 kHz (20,000 Hz).
- Concave (diverging).
- 28 February; Raman effect.
Level 2: Prelims MCQs
Q1. Consider the following statements:
- The sky appears blue because air molecules scatter shorter wavelengths of sunlight more than longer wavelengths.
- Stars twinkle because of total internal reflection in the atmosphere.
- The sky appears dark to an astronaut on the Moon because there is no atmosphere to scatter sunlight.
Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer and explanation
(c) 1 and 3 only. Blue light is scattered far more than red by air molecules, so statement 1 is correct. Twinkling is caused by atmospheric refraction through layers of changing density, not by total internal reflection, so statement 2 is wrong. With no atmosphere on the Moon there is no scattering, so the sky looks black.
Q2. Match List I (Phenomenon) with List II (Principle):
| List I | List II |
|---|---|
| A. Sparkling of a diamond | 1. Dispersion |
| B. Formation of the colours of a rainbow | 2. Total internal reflection |
| C. Red colour of the Sun at sunset | 3. Doppler effect |
| D. Change in pitch of a passing siren | 4. Scattering |
- (a) A-1, B-2, C-4, D-3
- (b) A-2, B-1, C-4, D-3
- (c) A-2, B-1, C-3, D-4
- (d) A-2, B-4, C-1, D-3
Answer and explanation
(b) A-2, B-1, C-4, D-3. A diamond’s high refractive index gives a small critical angle and repeated total internal reflection. The colours of a rainbow come from dispersion of light in raindrops (along with refraction and internal reflection). The setting Sun looks red because blue light is scattered away along the long path through the air. A passing siren changes pitch by the Doppler effect.
Q3. Which of the following is not a use of ultrasound?
- (a) Imaging a foetus in the mother’s womb
- (b) Measuring the depth of the sea with SONAR
- (c) Detecting cracks in metal rails
- (d) Imaging fractured bones in the same way as an X-ray plate
Answer and explanation
(d). Fractured bones are normally imaged with X-rays, which pass through soft tissue but are absorbed by bone. Sonography, SONAR and flaw detection in metals are all standard uses of ultrasound.
Q4. Under the Noise Pollution (Regulation and Control) Rules, 2000, the ambient noise limit for a silence zone during the day is:
- (a) 40 dB(A)
- (b) 45 dB(A)
- (c) 50 dB(A)
- (d) 55 dB(A)
Answer and explanation
(c) 50 dB(A). Silence zone limits are 50 dB(A) by day and 40 dB(A) by night. 55 dB(A) is the residential day limit and 45 dB(A) the residential night limit, which are the usual distractors.
Q5. A person cannot read a newspaper held at normal distance but can see distant hills clearly. The defect and its correction are:
- (a) Myopia, concave lens
- (b) Hypermetropia, convex lens
- (c) Myopia, convex lens
- (d) Astigmatism, cylindrical lens
Answer and explanation
(b) Hypermetropia, convex lens. Difficulty with near objects is long-sightedness (hypermetropia). The image falls behind the retina, and a converging (convex) lens brings it forward.
Level 3: Mains Answer Writing
Q1. (60 words) Why does the sky appear blue while the Sun appears reddish at sunrise and sunset?
Model answer outline
- Principle: Scattering by air molecules is much stronger for short wavelengths (blue) than long ones (red).
- Blue sky: Scattered blue light reaches our eyes from every direction.
- Red Sun at the horizon: Sunlight crosses a much longer stretch of atmosphere; most blue is scattered away, so mainly red and orange reach us.
- Link: The same reason makes red the colour of danger signals.
Q2. (150–200 words) Discuss the applications of ultrasound in medicine and industry. What are the concerns associated with its misuse?
Model answer outline
- Intro: Ultrasound is sound above 20 kHz, beyond human hearing. It reflects from boundaries between tissues or materials, and the echoes are turned into images or measurements.
- Medicine: sonography of the foetus and abdominal organs; echocardiography; Doppler ultrasound for blood flow; breaking small kidney stones. It is non-invasive and uses no ionising radiation.
- Industry and navigation: SONAR for sea depth, submarines and fish shoals; detecting cracks in rails, pipes and metal blocks; ultrasonic cleaning.
- Concerns: misuse for sex determination, which contributes to an adverse child sex ratio; it is banned under the PC&PNDT Act, 1994 (amended 2003). Other concerns are unregistered clinics and over-reliance on scans without trained interpretation.
- Chhattisgarh angle: Expanding diagnostic access in district hospitals and remote tribal areas makes regulation of clinics and trained sonologists important.
- Conclusion: Ultrasound is a safe, cheap and versatile tool. Strong regulation keeps it a tool for health, not discrimination.
Quick Revision Table
| Fact | Detail |
|---|---|
| Speed of light in vacuum | About \( 3 \times 10^{8} \) m/s |
| Refractive index | Water about 1.33; glass about 1.5; diamond about 2.42 |
| Total internal reflection | Denser to rarer, angle above critical angle: diamond, optical fibre, mirage |
| Twinkling of stars | Atmospheric refraction; planets do not twinkle |
| Rainbow | Refraction + dispersion + internal reflection; Sun behind observer |
| Blue sky, red sunset | Scattering, stronger for shorter wavelengths |
| Danger signals | Red: least scattered |
| Raman effect | C.V. Raman, 1928; Nobel 1930; National Science Day 28 February |
| Lens power | \( P = 1/f \) (f in metres); unit dioptre |
| Myopia / hypermetropia | Concave / convex lens |
| Near point | About 25 cm |
| Persistence of vision | About 1/16 s |
| Speed of sound in air | About 343 m/s at 20 °C; solids > liquids > gases |
| Audible range | 20 Hz – 20 kHz |
| Infrasound / ultrasound | Below 20 Hz / above 20 kHz |
| Minimum distance for an echo | About 17.2 m |
| Doppler effect | Apparent change in frequency with relative motion |
| Silence zone limits | 50 dB(A) day, 40 dB(A) night; 100 m around hospitals, schools, courts |
Key Takeaways
Key Takeaways
- Refraction explains twinkling, shallow-looking water and the early sunrise. Total internal reflection explains diamonds, optical fibres and mirages.
- Scattering is stronger for short wavelengths: a blue sky, red sunsets and red danger lights.
- Myopia → concave; hypermetropia → convex. The near point is about 25 cm.
- Sound needs a medium and is fastest in solids. Pitch follows frequency; loudness follows amplitude.
- Ultrasound powers sonography and SONAR. Its misuse is checked by the PC&PNDT Act. Noise limits are set by the 2000 Rules.
What’s Next
- Electricity and Magnetism, the next topic in this chapter
- Motion and Force, the previous topic
- Computers and Information Technology, where optical fibres carry data
- Environmental Pollution and Waste Management, which covers noise pollution in more depth
- Life Science and Human Health, for the eye and ear as organs
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