Chapter 2 · Topic 2.2 · Prelims + Mains2 of 3
Physics: Sound, light and lamps
Key points
- Waves and sound.
- Light: refraction, dispersion, scattering.
- Lamps, LEDs and displays.
6 PYQs 2013–2026
Prelims box: facts and traps
| Medium (at 15 °C) | Speed of sound |
|---|---|
| Air | 340 m s⁻¹ |
| Water | 1,500 m s⁻¹ |
| Steel | 5,000 m s⁻¹. |
- Source: PIB, 10 years of UJALA (January 2025).
- A CFL produces light using mercury vapour and phosphor, while an LED lamp uses semi-conductor material; the LED is more energy-efficient and also lasts longer (Prelims 2011: 1 and 3).
- The spectrum of visible light from sodium lamps is almost monochromatic (yellow), while LED lamps offer significant colour advantages; LED street-lights also last longer (Prelims 2021: 1 and 3).
- Trap (UPSC Prelims 2017): all the statements are correct.
- Trap (UPSC Prelims 2021, street-lighting): “As street-lights, sodium lamps have longer life span than LED lamps” is incorrect.
On this page (3 sections)
Waves and sound
A wave carries energy without carrying matter along. In a transverse wave (like light, or a wave on a string) the particles move at right angles to the direction of travel; in a longitudinal wave (like sound) they move back and forth along it, making compressions and rarefactions. Wave speed = frequency × wavelength (NCERT Science, Class 9, Chapter 10).
| Medium (at 15 °C) | Speed of sound |
|---|---|
| Air | 340 m s⁻¹ |
| Water | 1,500 m s⁻¹ |
| Steel | 5,000 m s⁻¹ |
Source: NCERT Science, Class 9, Chapter 10, Table 10.1 (rounded textbook values). Sound needs a medium; it cannot travel in a vacuum.
The speed of sound in dry air is about 331 m s⁻¹ at 0 °C and nearly 344 m s⁻¹ at 22 °C; it rises with temperature and humidity. Human hearing roughly spans 20 Hz to 20 kHz. Below 20 Hz is infrasound (elephants can detect it); above 20 kHz is ultrasound (dogs, bats and dolphins can detect it) (NCERT Science, Class 9, Chapter 10).
An echo is heard only if the reflected sound arrives at least 0.1 s after the original. Taking 340 m s⁻¹, sound covers 34 m in 0.1 s, so the reflecting surface must be at least about 17 m away (NCERT Science, Class 9, Chapter 10). SONAR uses ultrasound echoes to find the depth of the sea and underwater objects; an aircraft faster than sound creates a sonic boom.
Light: refraction, dispersion and scattering
Light travels in straight lines and bends (refracts) when it passes from one medium into another at an angle. White light is a mix of colours; a prism spreads them out, which is dispersion, because each colour bends by a different amount (NCERT Science, Class 10, Chapter 10).
A rainbow is a natural spectrum caused by dispersion of sunlight by tiny water droplets. The droplets refract and disperse the incident sunlight, then reflect it internally, and finally refract it again as it comes out; it always forms opposite the Sun (NCERT Science, Class 10, Chapter 10). Rainbow is produced when sunlight falls on drops of rain through dispersion, refraction and internal reflection (Prelims 2013: 1, 2 and 3).
Rainbow is produced when sunlight falls on drops of rain. Which of the following physical phenomena are responsible for this? 1. Dispersion 2. Refraction 3. Internal reflection Select the correct answer using the codes given below.
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer
Answer: (d) · UPSC official key
Atmospheric refraction: stars twinkle because the moving layers of air keep bending their starlight; planets, being much closer, look like extended sources and do not twinkle. The Sun is visible about 2 minutes before actual sunrise and about 2 minutes after actual sunset, and its disc looks flattened near the horizon (NCERT Science, Class 10, Chapter 10).
Scattering: very fine particles scatter mainly blue light, so the clear sky is blue; at sunrise and sunset the light crosses more air, the blue is scattered away and the Sun looks red. The Tyndall effect is the visible path of a light beam through smoke or mist (NCERT Science, Class 10, Chapter 10).
UPSC asked which of these are optical illusions: the size of the sun at dusk, the colour of the sun at dawn and the twinkle of stars in the sky are effects of the atmosphere on light, while the Moon at dawn and the Pole Star are real (Prelims 2013: 1, 2 and 4).
Consider the following phenomena : 1. Size of the sun at dusk 2. Colour of the sun at dawn 3. Moon being visible at dawn 4. Twinkle of stars in the sky 5. Polestar being visible in the sky Which of the above are optical illusions?
- 1, 2 and 3
- 3, 4 and 5
- 1, 2 and 4
- 2, 3 and 5
Show answer
Answer: (c) · UPSC official key
Lamps, LEDs and displays
A CFL produces light using mercury vapour and phosphor, while an LED lamp uses semi-conductor material; the LED is more energy-efficient and also lasts longer (Prelims 2011: 1 and 3).
What is the difference between a CFL and an LED lamp ? 1. To produce light, a CFL uses mercury vapour and phosphor while an LED lamp uses semi-conductor material. 2. The average life span of a CFL is much longer than that of an LED lamp. 3. A CFL is less energy-efficient as compared to an LED lamp. Which of the statements given above is/are correct ?
- 1 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer
Answer: (c) · UPSC official key
With reference to street-lighting, sodium lamps produce light in 360 degrees, while LED lamps send it in one direction. The spectrum of visible light from sodium lamps is almost monochromatic (yellow), while LED lamps offer significant colour advantages; LED street-lights also last longer (Prelims 2021: 1 and 3).
With reference to street-lighting, how do sodium lamps differ from LED lamps? 1. Sodium lamps produce light in 360 degrees but it is not so in the case of LED lamps. 2. As street-lights, sodium lamps have longer life span than LED lamps. 3. The spectrum of visible light from sodium lamps is almost monochromatic while LED lamps offer significant colour advantages in street-lighting. Select the correct answer using the code given below.
- 3 only
- 2 only
- 1 and 3 only
- 1, 2 and 3
Show answer
Answer: (c) · UPSC official key
OLED displays use organic compounds that glow when current passes through them; they need no backlight, so they can be fabricated on flexible plastic substrates, built into roll-up displays embedded in clothing, and made transparent (Prelims 2017: 1, 2 and 3).
Organic Light Emitting Diodes (OLEDs) are used to create digital display in many devices. What are the advantages of OLED displays over Liquid Crystal displays ? 1. OLED displays can be fabricated on flexible plastic substrates. 2. Roll-up displays embedded in clothing can be made using OLEDs. 3. Transparent displays are possible using OLEDs. Select the correct answer using the code given below :
- 1 and 3 only
- 2 only
- 1, 2 and 3
- None of the above statements is correct
Show answer
Answer: (c) · UPSC official key
| Achieved, as of 6 India’s LED programmes (launched 5 Jan 2015) | Estimated annual |
|---|---|
| Jan 2025 UJALA (LED bulbs to homes) 36.87 crore LED | savings Rs 19,153 crore; 47,883 |
| bulbs distributed Street Lighting National Programme (EESL) over 1.34 crore | million kWh over 9,001 million |
| LED streetlights | units; peak demand |
| installed | down over 1,500 MW |
Source: PIB, 10 years of UJALA (January 2025). Bulbs and streetlights are measured counts; savings are official estimates.
Blue LEDs, the last missing colour, made white LED light possible; their inventors won the 2014 Physics Nobel (nobelprize.org). Mains 2021 asked: How has this invention impacted the everyday life of human beings? Mains 2021 Cheap, long-life LED bulbs and streetlights (see UJALA above), phone and TV screens, and solar lanterns where there is no grid.
The Nobel Prize in Physics of 2014 was jointly awarded to Akasaki, Amano and Nakamura for the invention of Blue LEDs in 1990s. How has this invention impacted the everyday life of human beings ?
Mains box: analysis and answer points
- White light is a mix of colours; a prism spreads them out, which is dispersion, because each colour bends by a different amount (NCERT Science, Class 10, Chapter 10).
- Atmospheric refraction: stars twinkle because the moving layers of air keep bending their starlight; planets, being much closer, look like extended sources and do not twinkle.
- A wave carries energy without carrying matter along.
- Likely next angle: Echo needs 0.1 s
- Asked: Mains 2021 (GS Paper III, 15 marks): The Nobel Prize in Physics of 2014 was jointly awarded to Akasaki, Amano and Nakamura for the invention of Blue LEDs in 1990s. How has this invention impacted the everyday life of human beings ?
Practice
Prelims PYQs on this topic (6)
Which of the following statements with regard to the National Quantum Mission (NQM) is/are correct ?
- 1. It aims at developing intermediate-scale quantum computers with 50-1000 physical qubits.
- 2. Its implementation includes setting up of four Thematic Hubs (T-Hubs) in academic and national R&D institutes across India.
Select the answer using the code given below :
Answer (c) & explanation
Both statements are correct (c). Statement 1 is right: the National Quantum Mission (2023, under the DST) aims at quantum computers of 50–1,000 physical qubits within eight years. Statement 2 is right: it has four T-Hubs: computing at IISc Bengaluru, communication at IIT Madras, sensing and metrology at IIT Bombay, and materials and devices at IIT Delhi. So options (a), (b) and (d) fail.
Notice the budget: about ₹6,000 crore from 2023-24 to 2030-31. The mission also aims at secure quantum communication over 2,000 km between cities, and satellite-based quantum links. India joined a handful of nations, like the USA, China and France, with such a dedicated mission.
Answer as per the official UPSC answer key.
In the context of digital technologies for entertainment, consider the following statements :
- 1. In Augmented Reality (AR), a simulated environment is created and the physical world is completely shut out.
- 2. In Virtual Reality (VR), images generated from a computer are projected onto real-life objects or surroundings.
- 3. AR allows individuals to be present in the world and improves the experience using the camera of smart-phone or PC.
- 4. VR closes the world, and transposes an individual, providing complete immersion experience.
Which of the statements given above is/are correct?
Answer (b) & explanation
Statements 3 and 4 are correct (b). Statements 1 and 2 swap the definitions. Augmented Reality (AR) keeps you in the real world and adds digital images on top of it, usually through a phone camera or smart glasses (3). Examples are Pokémon Go, Snapchat filters, and apps that show furniture in your room. Virtual Reality (VR) shuts out the real world and places you in a fully simulated one, usually through a headset, giving complete immersion (4).
Mixed reality combines both, letting digital objects interact with the real world, as in Apple Vision Pro. Notice the uses beyond games: training surgeons and pilots, virtual tours of heritage sites, and India's AVGC-XR policy for animation, gaming and extended reality.
Answer as per the official UPSC answer key.
Consider the following pairs :
| Terms sometimes seen in news | Context/Topic
| 1. Belle II experiment | Artificial Intelligence
| 2. Blockchain technology | Digital/Cryptocurrency
| 3. CRISPR-Cas9 | Particle Physics
Which of the pairs given above is/are correctly matched ?
Answer (b) & explanation
Only pair 2 is correct (b). Pair 1 is wrong because Belle II is a particle physics experiment at Japan's SuperKEKB collider (KEK). It studies B-mesons to look for new physics and the matter–antimatter imbalance, and Indian institutions take part. Pair 2 is right because blockchain, a shared, tamper-proof digital ledger, underlies cryptocurrencies such as Bitcoin. Pair 3 is wrong because CRISPR-Cas9 is a gene-editing tool that cuts DNA at a chosen spot, not particle physics.
Notice the news links: Doudna and Charpentier won the Chemistry Nobel (2020) for CRISPR. India released genome-edited rice varieties (2025). The RBI's e-rupee (CBDC) draws on similar ledger ideas, while India taxes crypto gains at 30%.
Answer as per the official UPSC answer key.
Regarding ‘DigiLocker’, sometimes seen in the news, which of the following statements is/are correct?
- 1. It is a digital locker system offered by the Government under Digital India Programme.
- 2. It allows you to access your e-documents irrespective of your physical location.
Select the correct answer using the code given below.
Answer (c) & explanation
Both statements are correct (c). DigiLocker, launched by MeitY in 2015 under the Digital India programme (1), is a cloud-based document wallet linked to Aadhaar. Agencies such as transport departments, CBSE, universities and UIDAI issue documents into it directly, and users can open them from anywhere (2). Under the IT Rules (2016 amendment), documents in DigiLocker are legally valid like physical originals. Police can accept a digital driving licence and registration certificate under the Motor Vehicles rules.
Today it has hundreds of millions of users and links to ABHA health records and the Academic Bank of Credits. Notice that it is part of India's Digital Public Infrastructure stack, alongside Aadhaar, UPI and e-Sign.
Answer as per the official UPSC answer key.
Rainbow is produced when sunlight falls on drops of rain. Which of the following physical phenomena are responsible for this?
- 1. Dispersion
- 2. Refraction
- 3. Internal reflection
Select the correct answer using the codes given below.
Answer (d) & explanation
All three are correct (d). Sunlight is refracted, or bent, as it enters a raindrop (2), and split into its colours because each wavelength bends differently: dispersion (1). The light is then reflected internally at the back of the drop (3) and refracted again as it leaves. Together, these make a rainbow at about 42° from the point opposite the sun. So options (a), (b) and (c) leave out real processes.
Notice the related facts: a rainbow is seen only with the sun behind you; a secondary rainbow, from two internal reflections, has reversed colours and lies outside the main bow. Red is on the outside of the primary rainbow. Newton used a prism to show dispersion. Trap: internal reflection is often forgotten.
Answer as per the official UPSC answer key.
The efforts to detect the existence of Higgs boson particle have become frequent news in the recent past. What is/are the importance/importances of discovering this particle?
- 1. It will enable us to understand as to why elementary particles have mass.
- 2. It will enable us in the near future to develop the technology of transferring matter from one point to another without traversing the physical space between them.
- 3. It will enable us to create better fuels for nuclear fission.
Select the correct answer using the codes given below.
Answer (a) & explanation
Statement 1 only is correct (a). The Higgs boson, discovered at CERN's Large Hadron Collider in July 2012, confirmed the Higgs field, which explains why elementary particles have mass (1). Statement 2 is wrong: it has no link to teleporting matter. Statement 3 is wrong: it does not help make nuclear fuels. So (b), (c) and (d) fail.
Notice the Indian link: the particle is a boson, a class named after Satyendra Nath Bose. Peter Higgs and François Englert won the 2013 Nobel Prize in Physics. India became an associate member of CERN in 2017. The popular name 'God particle' comes from a book title, not from physics. Trap: dramatic applications are attached to basic science.
Answer as per the official UPSC answer key.
Write an answer
In 150 words, explain the significance of Physics with one recent example.
Flashcards (9)
Speed of sound in air, water, steel (NCERT)?
340, 1,500, 5,000 m/s.
Range of human hearing?
About 20 Hz to 20 kHz.
Minimum gap for a distinct echo?
0.1 s (about 17 m to the reflector).
Three effects that make a rainbow?
Dispersion, refraction, internal reflection.
Why do stars twinkle and planets not?
Atmospheric refraction; planets are extended sources.
Why is the sky blue?
Fine particles scatter blue light most.
Sodium lamp vs LED light?
Sodium: 360°, almost monochromatic; LED: directional, better colour.
UJALA LED bulbs distributed by 6 Jan
2025? 36.87 crore.
Visible light wavelength range?
About 700 to 400 nm.
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Next topic · 2.3 →Physics: Waves, lasers and modern physics