This free physics MCQ for BPSC set has 75 questions on the two physics topics that BPSC prelims returns to most often: light and sound. The first 50 questions cover reflection and refraction, mirrors and lenses, total internal reflection, dispersion, the human eye and its defects, microscopes and telescopes, polarisation and diffraction; the last 25 cover sound waves, echoes, ultrasound and the human ear. Every question is tagged to its NCERT source chapter, shows the correct answer and an explanation as soon as you choose an option, and the score bar applies BPSC-style negative marking. For more practice, see our free BPSC mock test series.
What this test covers: Frequency on refraction, critical angle and optical fibres, rear-view and shaving mirrors, hypermetropia and myopia, mirages and twinkling stars, lens power in dioptres, Brewster’s angle, the Cassegrain telescope, diamonds and total internal reflection, LiDAR, sign conventions, speed of sound in solids, echoes and the 17-metre rule, pitch and timbre, Mach number, infrasound and ultrasound, and reverberation.
Physics Special: Light & Sound
75 questions · NCERT Class 9, 10 & 12 · BPSC prelims pattern
How this test works
Click the option you think is right. It locks, turns green if correct or red if not, and the explanation opens underneath. Your score updates in the bar at the top as you go, and Calculate final score at the bottom gives the full breakdown. Choose (E) Not attempted when you would only be guessing, exactly as you would bubble E on the BPSC OMR sheet.
+1 mark — Correct answer
−1/3 mark — Wrong answer
0 — no penalty — (E) Not attempted, the fifth bubble on every question
−1/3 mark — Row left completely blank in the real exam
Light & optics50 questions
1. Which parameter of a light wave remains constant when it travels from air into a glass slab?
Show answer
Answer: (C) Frequency
Frequency is fixed by the source and does not change on refraction; speed and wavelength both fall when light enters glass.
Source: NCERT Class 10, Ch 9
2. Consider the following statements regarding total internal reflection (TIR):
- Light must travel from an optically rarer medium to an optically denser medium.
- The angle of incidence in the denser medium must be greater than the critical angle.
- Optical fibres function on the principle of total internal reflection.
Which of the statements given above are correct?
Show answer
Answer: (B) 2 and 3 only
Total internal reflection needs light going from a denser to a rarer medium at an angle greater than the critical angle, and optical fibres work on it. Statement 1 has the media reversed.
Source: NCERT Class 12, Ch 9
3. The given question consists of two statements, one labeled as Assertion (A) and the other as Reason (R).
Assertion (A): The pupil of the human eye contracts in bright light and expands in dim light.
Reason (R): The ciliary muscles alter the shape of the crystalline lens to focus objects at varying distances on the retina.
Select the correct answer using the code given below:
Show answer
Answer: (B) Both Assertion (A) and Reason (R) are individually true, but Reason (R) is NOT the correct explanation of Assertion (A).
Both statements are true: the pupil adjusts to light intensity and the ciliary muscles change the lens for accommodation. But the pupil is controlled by the iris, so R does not explain A.
4. Match List-I (Optical Instrument / Component) with
List-II (Type of Mirror / Lens Used):
| List-I | List-II |
|---|---|
| A. Rear-view mirror in automobiles | 1. Concave mirror |
| B. Solar cooker reflector | 2. Convex lens |
| C. Simple magnifying glass | 3. Convex mirror |
| D. Dentist’s head-lamp | 4. Concave mirror |
Select the correct option code:
Show answer
Answer: (A) A-3, B-1, C-2, D-4
A convex mirror is used as a rear-view mirror, concave mirrors in solar cookers and dentists’ lamps, and a convex lens as a magnifier. Code A-3, B-1, C-2, D-4.
Source: NCERT Class 10, Ch 9
5. A person suffering from hypermetropia can correct their vision using which type of lens?
Show answer
Answer: (B) Convex lens
Hypermetropia (long-sightedness) is corrected with a convex (converging) lens; myopia needs a concave lens.
Source: NCERT Class 10, Ch 10
6. Consider the following statements regarding image formation by a plane mirror:
- The image formed by a plane mirror is real, erect, and inverted.
- The distance of the image behind the mirror is equal to the distance of the object in front of it.
- The minimum height of a plane mirror required for a person to see their full height is equal to half of their height.
Which of the statements given above are correct?
Show answer
Answer: (B) 2 and 3 only
A plane mirror forms a virtual, erect, laterally inverted image as far behind the mirror as the object is in front, and a mirror half your height shows your full image. Statement 1 is wrong.
Source: NCERT Class 10, Ch 9
7. The given question consists of two statements, one labeled as Assertion (A) and the other as Reason (R).
Assertion (A): Polarized sunglasses are effective at reducing glare reflected from horizontal surfaces like water or wet roads.
Reason (R): Light reflected from flat non-metallic surfaces becomes partially or completely plane-polarized parallel to the surface.
Select the correct answer using the code given below:
Show answer
Answer: (A) Both Assertion (A) and Reason (R) are individually true, and Reason (R) is the correct explanation of Assertion (A).
Light reflected from flat non-metallic surfaces such as water or a wet road is partly polarised parallel to the surface, and polarised sunglasses are oriented to block that component, cutting glare. R explains A.
8. Assertion (A): A convex mirror is preferred as a rear-view mirror in motor vehicles.
Reason (R): A convex mirror always forms a virtual, erect, and diminished image, offering a wider field of view to the driver.
Select the correct answer:
Show answer
Answer: (A) Both (A) and (R) are true, and (R) is the correct explanation of (A).
A convex mirror always forms a virtual, erect, diminished image and gives a wide field of view, which is exactly why it is used as a rear-view mirror. R explains A.
9. Which of the following pairs of optical phenomenon and its main physical cause is/are correctly matched?
- Mirage formation in deserts – Total Internal Reflection
- Twinkling of stars – Atmospheric Refraction
- Colour of water in deep sea – Scattering of Light
Select the correct answer using the code given below:
Show answer
Answer: (D) 1, 2, and 3
Mirages arise from total internal reflection in hot air layers, stars twinkle because of atmospheric refraction, and the colour of deep water comes from scattering. All three pairs are correct.
Source: NCERT Class 12, Ch 9
10. If two plane mirrors are placed parallel to each other facing one another, how many images of an object kept between them will be formed?
Show answer
Answer: (C) Infinite
Two parallel mirrors facing each other form an infinite number of images, each image acting as an object for the other mirror.
11. With reference to the power of an optical lens, consider the following statements:
- The SI unit of power of a lens is m-1, which is also known as Dioptre.
- The power of a convex lens is taken as positive, while that of a concave lens is negative.
- If two thin lenses of power +2 D and -4 D are kept in contact, the net power of the combination is +6 D.
Which of the statements given above is/are correct? [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (A) 1 and 2 only
Lens power is measured in dioptres (m^-1); convex lenses have positive and concave lenses negative power. Statement 3 is wrong: +2 D and -4 D in contact give -2 D.
12. When a beam of white light passes through a glass prism, which visible colour undergoes the maximum deviation?
Show answer
Answer: (D) Violet
Violet light has the shortest wavelength and the highest refractive index in glass, so it is deviated the most; red is deviated the least.
Source: NCERT Class 12, Ch 9
13. Assertion (A): An air bubble trapped inside water shines brilliantly like a diamond.
Reason (R): Light entering the air bubble from water suffers total internal reflection at the water-air interface if the angle of incidence exceeds the critical angle.
Select the correct answer:
Show answer
Answer: (A) Both (A) and (R) are true, and (R) is the correct explanation of (A).
Light from water hitting the water-air surface of a bubble beyond the critical angle is totally internally reflected, making the bubble shine. R explains A.
Source: NCERT Class 12, Ch 9
14. A concave mirror forms a real, inverted, and same-sized image of an object when the object is placed at __________.
Show answer
Answer: (B) the Centre of Curvature (C)
A concave mirror gives a real, inverted image of the same size when the object is at the centre of curvature.
Source: NCERT Class 10, Ch 9
15. Which phenomenon of light causes oil slicks on wet road surfaces to show vibrant brilliant colors in sunlight?
Show answer
Answer: (B) Interference
Thin oil films show colours because light reflected from the top and bottom surfaces of the film interferes.
16. Consider the following statements regarding the speed of light in different optical media:
- The speed of light is maximum in vacuum and decreases when entering a denser transparent medium.
- The optical density of a medium is directly proportional to its mass density in all conditions.
- Turpentine oil has a lower mass density than water, but its optical density is higher than water.
Which of the statements given above are correct?
Show answer
Answer: (B) 1 and 3 only
Light is fastest in vacuum and slower in denser media, and turpentine is optically denser than water despite its lower mass density. Statement 2 is wrong: optical density is not proportional to mass density.
Source: NCERT Class 12, Ch 9
17. What type of lens is formed by an air bubble submerged inside a glass block or water tank?
Show answer
Answer: (B) Diverging Lens
An air bubble in glass or water is a rarer medium bounded by convex surfaces, so it diverges light and acts as a concave (diverging) lens.
18. Match List-I (Optical Phenomena) with List-II (Optical Principle):
| List-I | List-II |
|---|---|
| A. Endoscopy in medical diagnostics | 1. Total Internal Reflection |
| B. Bending of light around sharp corners | 2. Dispersion |
| C. Splitting of white light by prism | 3. Diffraction |
| D. Anti-reflective coating on camera lenses | 4. Thin Film Interference |
Select the correct option code: [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (D) A-1, B-3, C-2, D-4
Endoscopes use total internal reflection, bending around corners is diffraction, prisms disperse light and anti-reflective coatings work by thin-film interference. Code A-1, B-3, C-2, D-4.
19. Which component of electromagnetic radiation allows night vision goggles to detect objects in total darkness? [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (B) Infrared radiation
Night-vision devices detect the infrared radiation that warm objects emit even in total darkness.
20. Regarding a Cassegrain reflecting telescope, consider the following statements:
- It uses a large concave parabolic mirror as its primary objective.
- It is completely free from chromatic aberration.
- It uses a convex mirror as a secondary reflector to direct light through a hole in the primary mirror.
Which of the statements given above are correct? [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (D) 1, 2, and 3
A Cassegrain telescope has a concave parabolic primary mirror and a convex secondary mirror that sends light through a hole in the primary; mirrors are free of chromatic aberration. All three are correct.
21. What is the focal length of a flat plane glass mirror?[ Source: NCERT Class 10, Ch 9 ]
Show answer
Answer: (B) Infinite
A plane mirror can be treated as a spherical mirror of infinite radius of curvature, so its focal length is infinite.
22. Assertion (A): The focal length of a convex lens decreases when it is immersed completely in water.
Reason (R): The relative refractive index between glass and water is less than the relative refractive index between glass and air.
Select the correct answer:
Show answer
Answer: (C) (A) is false, but (R) is true.
The refractive index of glass relative to water is smaller than relative to air, so the focal length of a convex lens increases in water. A is false and R is true.
Source: NCERT Class 12, Ch 9
23. At what angle of incidence does reflected light become completely plane-polarized when falling on a glass surface?
Show answer
Answer: (B) Brewster’s Angle
At Brewster’s angle the reflected light is completely plane-polarised; the reflected and refracted rays are then perpendicular.
24. Consider the following statements regarding human eye vision and defects:
- Presbyopia occurs due to the gradual weakening of ciliary muscles with aging.
- Astigmatism can be corrected by using a lens with cylindrical curvature.
- Myopia is corrected using a convex lens of appropriate power.
Which of the statements given above is/are correct?
Show answer
Answer: (A) 1 and 2 only
Presbyopia results from weakening ciliary muscles and a stiffer lens, and astigmatism is corrected with cylindrical lenses. Statement 3 is wrong: myopia is corrected with a concave lens.
25. In the Photoelectric Effect, increasing the intensity of the incident light beam without changing its frequency will directly lead to an increase in which of the following?
Show answer
Answer: (C) The total number of photoelectrons emitted per second
Raising intensity at a fixed frequency increases the number of photons and hence the number of photoelectrons emitted per second; their maximum kinetic energy depends only on frequency.
26. Which of the following pairs is/are INCORRECTLY matched?
- Convex mirror — Diverging mirror
- Concave lens — Converging lens
- Real image — Can be caught on a screen
Select the correct answer:
Show answer
Answer: (B) 2 only
A convex mirror diverges light and a real image can be caught on a screen, so pairs 1 and 3 are right. A concave lens is a diverging lens, so pair 2 is incorrectly matched.
Source: NCERT Class 10, Ch 9 & NCERT Class 12, Ch 9
27. If the lower half of a convex lens is wrapped in opaque black paper, how does the image of an object change?
Show answer
Answer: (C) The full image is formed, but its intensity decreases.
Covering half the lens still lets the other half form the complete image; only the brightness falls because less light passes through.
Source: NCERT Class 12, Ch 9
28. With reference to a simple microscope, consider the following statements:
- It consists of a single converging lens of short focal length.
- To obtain maximum erect magnification, the object must be kept beyond two focal lengths.
- The final virtual image is formed at or near the least distance of distinct vision.
Which of the statements given above are correct? [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (B) 1 and 3 only
A simple microscope is one converging lens of short focal length giving a virtual image near the least distance of distinct vision. Statement 2 is wrong: the object must be within the focal length.
29. Which phenomenon proves conclusively that light is a transverse wave rather than a longitudinal wave?
Show answer
Answer: (D) Polarization
Polarisation occurs only for transverse waves, so the polarisation of light proves it is transverse.
30. Assertion (A): Primary rainbows are brighter and more vivid than secondary rainbows.
Reason (R): Light undergoes two internal reflections in primary rainbow formation and only one internal reflection in secondary rainbow formation.
Select the correct answer:
Show answer
Answer: (C) (A) is true, but (R) is false.
The primary rainbow is brighter because it involves one internal reflection; the secondary involves two reflections and more light is lost. A is true, R is reversed.
31. What is the standard value of the least distance of distinct vision (near point) for a normal adult human eye? [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (B) 25 cm
The least distance of distinct vision for a normal young adult is taken as 25 cm.
32. Consider the following statements regarding the compound microscope:
- The objective lens has a short focal length and a small aperture.
- The eyepiece lens has a larger focal length than the objective lens.
- The final image formed by a compound microscope is real and erect with respect to the original object.
Which of the statements given above are correct? [ Source:
NCERT Class 12, Ch 9 ]
NCERT Class 12, Ch 9 ]
Show answer
Answer: (A) 1 and 2 only
The objective of a compound microscope has a short focal length and small aperture, and the eyepiece a longer focal length. Statement 3 is wrong: the final image is inverted and virtual.
33. What causes the shimmering, sparkling appearance of a well-cut natural diamond? [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (A) High absolute refractive index and very low critical angle
Diamond’s high refractive index (about 2.42) gives a very small critical angle (about 24 degrees), so light is totally internally reflected many times before escaping, producing the sparkle.
34. Match List-I (Optical Appliance) with List-II (Type of Image Formed):
| List-I | List-II |
|---|---|
| A. Astronomical Telescope (Normal Adjustment) | 1. Virtual, erect, enlarged |
| B. Simple Magnifying Glass | 2. Inverted, magnified, virtual final image |
| C. Convex Mirror | 3. Virtual, erect, diminished |
| D. Compound Microscope | 4. Inverted image formed at infinity |
Select the correct option code: [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (A) A-4, B-1, C-3, D-2
In normal adjustment a telescope forms an inverted image at infinity, a magnifier a virtual erect enlarged image, a convex mirror a virtual erect diminished image and a compound microscope an inverted magnified virtual final image. Code A-4, B-1, C-3, D-2.
35. Which color of light experiences the highest speed when traveling inside a transparent glass prism? [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (C) Red
Red light has the lowest refractive index in glass and therefore the highest speed inside the prism.
36. Consider the following statements regarding a ray of light passing through a rectangular glass slab:
- The emergent ray is parallel to the direction of the incident ray.
- The ray undergoes lateral displacement without any net angular deviation.
- Lateral displacement increases with an increase in the thickness of the glass slab.
Which of the statements given above are correct? [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (D) 1, 2, and 3
For a rectangular slab the emergent ray is parallel to the incident ray, the ray is only shifted sideways, and the shift grows with thickness. All three are correct.
37. What optical effect causes a swimming pool filled with clear water to appear shallower than its actual physical depth when viewed from above? [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (B) Refraction
Light from the bottom bends away from the normal as it leaves water, so the apparent depth is less than the real depth. This is refraction.
38. Which optical phenomenon occurs when light interacts with molecules of a medium and undergoes inelastic scattering with a change in wavelength?
Show answer
Answer: (D) Raman Effect
In the Raman effect light scattered by molecules changes wavelength (inelastic scattering); Rayleigh scattering keeps the wavelength unchanged.
39. Which of the following pairs regarding sign conventions for spherical mirrors/lenses is/are correctly matched according to Cartesian rules?
- Focal length of a Concave Mirror – Negative
- Focal length of a Convex Lens – Positive
- Distance of real object from optical center – Positive
Select the correct answer: [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (B) 1 and 2 only
With the Cartesian convention a concave mirror’s focal length is negative and a convex lens’s is positive. Statement 3 is wrong: a real object’s distance is taken as negative.
40. What is the nature of the image formed by a concave mirror when an object is placed between its pole and principal focus? [ Source: NCERT Class 10, Ch 9 ]
Show answer
Answer: (C) Virtual, erect, and magnified
With the object between the pole and the focus, a concave mirror gives a virtual, erect and magnified image, as in a shaving mirror.
41. Consider the following statements regarding the Laws of Reflection:
- The angle of incidence is always equal to the angle of reflection.
- The incident ray, normal, and reflected ray lie in different perpendicular planes.
- Laws of reflection hold true for plane surfaces as well as curved spherical surfaces.
Which of the statements given above are correct? [ Source:
NCERT Class 10, Ch 9 ]
NCERT Class 10, Ch 9 ]
Show answer
Answer: (B) 1 and 3 only
The angle of incidence equals the angle of reflection, and the laws hold for curved surfaces too. Statement 2 is wrong: the incident ray, normal and reflected ray lie in the same plane.
42. Which of the following optical instruments uses a combination of two convex lenses where the objective lens has a much larger focal length and larger aperture than the eyepiece? [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (C) Astronomical Refracting Telescope
An astronomical refracting telescope uses two convex lenses, with the objective having a large focal length and large aperture to gather light.
43. Which type of wave or signal does LiDAR technology primarily use to measure distances and create 3D maps?
Show answer
Answer: (C) Laser light beams
LiDAR fires laser pulses and times their reflections to measure distance and build 3D maps.
44. The focal length of a spherical mirror of radius of curvature 40 cm is __________. [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (B) 20 cm
Focal length is half the radius of curvature, so f = 40/2 = 20 cm.
45. What optical phenomenon causes light to spread into the region of geometrical shadow when passing through a narrow slit? [ Source: NCERT Class 12, Ch 10 ]
Show answer
Answer: (C) Diffraction
Diffraction is the bending of light around obstacles or through narrow slits into the geometrical shadow region.
46. Consider the following statements regarding total internal reflection in optical fibers:
- Optical fibers consist of a central core surrounded by a cladding layer.
- The refractive index of the core material is kept strictly lower than that of the cladding material.
- Light signals can propagate through bent optical fibers with minimal intensity loss.
Which of the statements given above are correct? [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (B) 1 and 3 only
An optical fibre has a core surrounded by cladding, and light can follow bends with little loss. Statement 2 is wrong: the core has a higher refractive index than the cladding.
47. Which of the following electromagnetic waves has the highest frequency and shortest wavelength in the light spectrum? [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (C) Gamma rays
Gamma rays have the highest frequency and the shortest wavelength in the electromagnetic spectrum.
48. Match List-I (Optical Device) with List-II (Key Function / Feature):
| List-I | List-II |
|---|---|
| A. Periscope | 1. Direct light signal transmission via TIR |
| B. Optical Fiber | 2. Uses two plane mirrors inclined at 45 degrees |
| C. Prism Binoculars | 3. Bends light rays by 90 or 180 degrees using TIR |
| D. Solar Cooker | 4. Uses concave mirror to focus sunlight |
Select the correct option code: [ Source: NCERT Class 12, Ch 9 ]
Show answer
Answer: (A) A-2, B-1, C-3, D-4
A periscope uses two plane mirrors at 45 degrees, optical fibres carry signals by total internal reflection, prism binoculars bend light by 90 or 180 degrees, and solar cookers use concave reflectors. Code A-2, B-1, C-3, D-4.
49. What happens to the focal length of a concave mirror when it is submerged inside a tank of clear water? [ Source:
NCERT Class 12, Ch 9]
Show answer
Answer: (C) Remains completely unchanged
A mirror’s focal length depends only on its radius of curvature, not on the surrounding medium, so it stays the same under water.
50. Consider the following statements regarding the dispersion of light:
- Dispersion occurs because different wavelengths of light travel at different speeds in a material medium.
- In vacuum, light of all colours travels at the exact same speed.
- Red light bends more than violet light while passing through a glass prism.
Which of the statements given above are correct? [ Source:
NCERT Class 12, Ch 9]
NCERT Class 12, Ch 9]
Show answer
Answer: (A) 1 and 2 only
Dispersion happens because different colours travel at different speeds in glass, while all colours travel at the same speed in vacuum. Statement 3 is wrong: violet bends more than red.
Sound25 questions
51. Which type of physical wave is a sound wave propagating through gaseous media such as air?
Show answer
Answer: (A) Longitudinal mechanical wave
Sound in air is a longitudinal mechanical wave: the particles oscillate along the direction of propagation and a material medium is needed.
Source: NCERT Class 9, Ch 10
52. Why are astronauts performing spacewalks unable to hear each other directly through outer space without electronic communication devices?
Show answer
Answer: (B) Sound is a mechanical wave requiring a material medium, which is absent in a vacuum.
Sound needs a material medium; there is none in the vacuum of space, so astronauts must use radio communication.
Source: NCERT Class 9, Ch 10
53. Why does sound travel faster in humid air than in completely dry air at the same temperature and pressure?
Show answer
Answer: (C) Humid air is less dense than dry air
Water vapour is lighter than the nitrogen and oxygen it displaces, so humid air is less dense and sound travels faster in it.
54. Which wave property determines the objective human perception of pitch?
Show answer
Answer: (D) Frequency
Pitch is determined by frequency: a higher frequency gives a higher pitch. Amplitude determines loudness.
Source: NCERT Class 9, Ch 10
55. What is the lower frequency boundary for ultrasonic sound waves?
Show answer
Answer: (D) 20,000 Hz
Ultrasonic sound has frequencies above 20,000 Hz (20 kHz), the upper limit of human hearing.
56. What minimum distance must exist between a sound source and a reflecting surface to hear a distinct echo at 22°C in air (speed of sound ~ 344 m/s)?
Show answer
Answer: (B) 17.2 m
The reflected sound must arrive at least 0.1 s after the original, so the total path is 344 x 0.1 = 34.4 m and the surface must be at least 17.2 m away.
57. Which anatomical structure converts mechanical pressure variations into electrical nerve impulses in the human ear?
Show answer
Answer: (A) Cochlea
The cochlea in the inner ear converts pressure variations into electrical signals that travel to the brain along the auditory nerve.
58. What acoustic property allows a listener to distinguish two musical notes played at equal pitch and volume on different instruments?
Show answer
Answer: (D) Timbre
Timbre, or quality, lets us tell apart notes of the same pitch and loudness played on different instruments.
59. What is the primary operational mechanism behind the medical diagnosis technique known as Echocardiography?
Show answer
Answer: (C) Reflection of ultrasonic waves from cardiac tissue boundaries
Echocardiography sends ultrasonic waves into the chest and uses their reflections from heart tissue to form an image of the heart.
60. How does the sound intensity change when the acoustic amplitude of a sound wave is tripled?
Show answer
Answer: (C) Increases by a factor of 9
Intensity is proportional to the square of the amplitude, so tripling the amplitude multiplies the intensity by nine.
61. Which metal listed below exhibits the highest speed of sound propagation at room temperature (25°C)?
Show answer
Answer: (A) Aluminum
Sound travels at about 6,420 m/s in aluminium, faster than in iron (5,950), copper (3,750) or lead (1,210).
62. What sound frequency range is produced by rhinoceroses for long-distance social communication?
Show answer
Answer: (D) Infrasonic frequencies around 5 Hz
Rhinoceroses communicate using infrasound of around 5 Hz, below the range of human hearing.
63. Assertion (A): Sound waves cannot propagate through the vast vacuum of outer space.
Reason (R): Sound waves are mechanical waves that require a material medium to transfer energy.
Choose the correct option:
Show answer
Answer: (A) Both A and R are true, and R is the correct explanation of A
Sound is a mechanical wave that needs a medium, so it cannot travel through the vacuum of space. R explains A.
Source: NCERT Class 9, Ch 10
64. Assertion (A): The speed of sound in air increases when atmospheric air temperature rises.
Reason (R): Increasing temperature increases the density of dry air at constant atmospheric pressure.
Show answer
Answer: (C) A is correct while R is false
The speed of sound in air rises with temperature (A is true), but warmer air at constant pressure is less dense, not denser, so R is false.
Source: NCERT Class 9, Ch 10
65. Assertion (A): Ultrasonic waves are used to detect internal structural flaws in thick steel blocks.
Reason (R): Ultrasonic waves have long wavelengths that easily bend around internal structural obstacles.
Show answer
Answer: (C) A is correct while R is false
Ultrasound is used to detect flaws in metal blocks because its short wavelength lets it be reflected by small defects; it does not bend around them. R is false.
Source: NCERT Class 9, Ch 10
66. Consider the following statements regarding sound wave propagation:
Statement I: Particles of the transmitting medium travel permanently from the source to the listener’s ear.
Statement II: Sound propagation represents the transport of energy and momentum through density variations.
Show answer
Answer: (D) Statement I is false, but Statement II is true.
The particles of the medium only oscillate about their mean positions; it is the disturbance, carrying energy and momentum, that travels. Statement I is false and II is true.
Source: NCERT Class 9, Ch 10
67. Consider the following statements regarding acoustic properties in different media:
Statement I: The speed of sound in steel is higher than its speed in pure water.
Statement II: The speed of sound in water is higher than its speed in sea-level dry air.
Show answer
Answer: (A) Both Statement I and Statement II are true.
Sound travels fastest in solids and slowest in gases: about 5,960 m/s in steel, 1,480 m/s in water and 340 m/s in air. Both statements are true.
Source: NCERT Class 9, Ch 10
68. Consider the following statements regarding animal auditory capabilities:
Statement I: Bats navigate and capture prey in total darkness using ultrasonic echolocation.
Statement II: Earthquakes emit low-frequency infrasonic waves prior to severe main shock waves.
Show answer
Answer: (A) Both Statement I and Statement II are true.
Bats use ultrasonic echolocation and earthquakes produce infrasound before the main shock, which some animals can sense. Both statements are true.
Source: NCERT Class 9, Ch 10
69. Match the items in List-I with their definitions in List-II:
| List-I (Term) | List-II (Definition) |
|---|---|
| (A) Wavelength | (1) Number of oscillations per unit time |
| (B) Frequency | (2) Distance between consecutive compressions |
| (C) Amplitude | (3) Maximum displacement from equilibrium |
| (D) Time Period | (4) Time taken for one complete oscillation |
Options:
Show answer
Answer: (C) A-2, B-1, C-3, D-4
Wavelength is the distance between consecutive compressions, frequency the number of oscillations per unit time, amplitude the maximum displacement and time period the time for one oscillation. Code A-2, B-1, C-3, D-4.
Source: NCERT Class 9, Ch 10
70. Match the Sound Frequency Classifications in List-I with their appropriate Frequency Ranges in List-II:
| List-I (Classification) | List-II (Frequency Range) |
|---|---|
| (A) Infrasonic Sound | (1) 20 Hz to 20,000 Hz |
| (B) Human Audible Range | (2) Below 20 Hz |
| (C) Ultrasonic Sound | (3) Up to 25,000 Hz |
| (D) Dog Hearing Limit | (4) Above 20,000 Hz |
Options:
Show answer
Answer: (B) A-2, B-1, C-4, D-3
Infrasonic sound is below 20 Hz, the audible range is 20 Hz to 20,000 Hz, ultrasonic sound is above 20,000 Hz and dogs hear up to about 25,000 Hz. Code A-2, B-1, C-4, D-3.
Source: NCERT Class 9, Ch 10
71. Match the Medical/Industrial Applications in List-I with their underlying Acoustic Phenomena in List-II:
| List-I (Application) | List-II (Acoustic Principle) |
|---|---|
| (A) Stethoscope | (1) Multiple sound reflections inside tubes |
| (B) SONAR | (2) Echo-ranging with ultrasonic pulses |
| (C) Ultrasonic Cleaning | (3) High-frequency detachment of dirt particles |
| (D) Echocardiography | (4) Ultrasonic reflection from heart tissues |
Options:
Show answer
Answer: (D) A-1, B-2, C-3, D-4
A stethoscope works by multiple reflections in its tubes, SONAR by echo-ranging with ultrasonic pulses, ultrasonic cleaning by high-frequency vibration detaching dirt, and echocardiography by ultrasonic reflection from the heart. Code A-1, B-2, C-3, D-4.
Source: NCERT Class 9, Ch 10
72. The ratio of the speed of an object moving through a fluid medium to the speed of sound in that same medium is called the ______ Number.
Show answer
Answer: (C) Mach
The Mach number is the ratio of an object’s speed to the speed of sound in the same medium.
Source: NCERT Class 9, Ch 10
73. In an electronic hearing aid, the external device that receives incoming sound waves and converts them into corresponding electrical signals is called the ______.
Show answer
Answer: (B) Microphone
The microphone of a hearing aid receives sound and converts it into electrical signals, which are amplified and sent to the speaker.
Source: NCERT Class 9, Ch 10
74. The persistence of auditory sensation in the human brain lasts for approximately ______ second.
Show answer
Answer: (D) 0.1
The sensation of sound persists in the brain for about 0.1 second, which is why an echo must arrive after that interval to be heard separately.
Source: NCERT Class 9, Ch 10
75. What acoustic wave phenomenon causes the prolonged persistence of sound in large untreated auditoriums?
Show answer
Answer: (A) Reverberation
Reverberation is the persistence of sound due to repeated reflections, which is why auditorium walls are lined with sound-absorbing material.
Source: NCERT Class 9, Ch 10
Your result
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1 C · 2 B · 3 B · 4 A · 5 B · 6 B · 7 A · 8 A · 9 D · 10 C · 11 A · 12 D · 13 A · 14 B · 15 B · 16 B · 17 B · 18 D · 19 B · 20 D · 21 B · 22 C · 23 B · 24 A · 25 C · 26 B · 27 C · 28 B · 29 D · 30 C · 31 B · 32 A · 33 A · 34 A · 35 C · 36 D · 37 B · 38 D · 39 B · 40 C · 41 B · 42 C · 43 C · 44 B · 45 C · 46 B · 47 C · 48 A · 49 C · 50 A · 51 A · 52 B · 53 C · 54 D · 55 D · 56 B · 57 A · 58 D · 59 C · 60 C · 61 A · 62 D · 63 A · 64 C · 65 C · 66 D · 67 A · 68 A · 69 C · 70 B · 71 D · 72 C · 73 B · 74 D · 75 A
Scoring: +1 correct, −1/3 wrong, 0 for (E) Not attempted, −1/3 for a row left blank in the real exam.
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