Physics (Part-II) (UPSC)

Total Questions: 60

11. Assertion (A): A stick is dipped in water in a slanting position. If observed sideways, the stick appears short and bent at the surface of water. [2001]

Reason (R): A light coming from the stick undergoes scattering from water molecules giving the stick a short and bent appearance.

(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is not a correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

Correct Answer: (c) A is true but R is false

Solution:Statement 1 is true but statement 2 is false. Yes the stick dipped in water appears to be bent and short but refraction of light by water molecules is the phenomena responsible for this observation not because of scattering of light.

12. When light waves pass from air to glass, the variables affected are: [2001]

(a) wavelength, frequency and velocity
(b) velocity and frequency
(c) wavelength and frequency
(d) wavelength and velocity

Correct Answer: (d) wavelength and velocity

Solution:When light enters from rarer (air) to denser (glass) medium wavelength and velocity will get affected. The velocity will decrease. Moreover, as the ray bent in passing through different media which results in decrease of wavelength. As such there is no change in frequency when light enters from air to glass.

13. Consider the following statements: [2002]

1. Light of longer wavelength are scattered much more than the light of shorter wavelength.

2. The speed of visible light in water is 0.95 times the speed in vacuum.

3. Radio waves are produced by rapidly oscillating electrical currents.

4. To detect the over speeding vehicles, police use the Doppler effect of reflected short radio waves.

Which of these statements are correct?

(a) 1 and 2
(b) 1 and 3
(c) 2 and 4
(d) 3 and 4

Correct Answer: (d) 3 and 4

Solution:Radio waves are a form of electromagnetic radiation, created whenever a charged object (in normal radio transmission, an electron) accelerates with a frequency that lies in the radio frequency (RF) portion of the electromagnetic spectrum. Thus statement 3 is correct. The Doppler's Effect shows a change in a wave's frequency received when it is either reflected from a moving object or generated on a moving object. Police radar speed detectors bounce a radio frequency wave off cars and use their internal electronics to determine the speed at which the object is moving by that frequency change. Therefore, statement 4 is also correct.

14. Consider the following natural phenomena: [2002]

1. Terrestrial heating

2. Reflection of light

3. Refraction of light

4. Diffraction of light

Due to which of these phenomena is a mirage formed?

(a) 1 and 2
(b) 2, 3 and 4
(c) 1 and 3
(d) 4 only

Correct Answer: (c) 1 and 3

Solution:Convection currents of hot air rising off a surface have a lower density than the air directly above it. The difference in density cause light passing through the air to refract differently, which causes the formation of mirage.

15. Diffusion of light in the atmosphere takes place due to: [2003]

(a) carbon dioxide
(b) dust particles
(c) helium
(d) water vapours

Correct Answer: (b) dust particles

Solution:Dust particles in the atmosphere are typically larger than the gas molecules but can still be small enough to scatter light efficiently. Therefore, they play a crucial role in the diffusion of light, contributing to both Rayleigh and Mie scattering..

16. Which one of the following statements is not correct? [2003]

(a) The velocity of sound in air increases with the increase of temperature
(b) The velocity of sound in air is independent of pressure
(c) The velocity of sound in air decreases as the humidity increases
(d) The velocity of sound in air is not affected by the change in amplitude and frequency

Correct Answer: (c) The velocity of sound in air decreases as the humidity increases

Solution:The velocity of sound in air is determined by the air itself and is not dependent upon the amplitude, frequency, or wavelength of the sound. The velocity of sound c depends on the temperature of air and not on the air pressure. Thus, velocity of sound in air increases with temperature and remains unaffected with increase of pressure. Therefore, statements 1, 2 and 4 are right. Only statement 3 is wrong. The only other factor that has any effect on the speed of sound in air is the amount of humidity in the air. An increase in the amount of humidity in the air increases the speed by a small amount.

Temperature is also a condition that affects the speed of sound. Molecules at high temperatures have more energy, thus they can vibrate faster. Since the molecules vibrate faster, sound waves can travel more quickly. So the velocity of sound in air decreases with decrease in temperature.

17. In which one among the following is the speed of sound maximum? [2006]

(a) Air at 0°C
(b) Air at 100°C
(c) Water
(d) Wood

Correct Answer: (d) Wood

Solution:The speed of sound varies based on the medium and temperature. This is influenced by the medium's density and elasticity:

1. Air at 0 deg * C : Approximately 331 meters per second (m / s)

2. Air at 100 deg * C : Increases with temperature; at 100 deg * C it's roughly 391.4 m/s.

The formula used is: v = 331.4 + 0.6T

For T = 100

v = 331.4 + 0.6 * 100 = 391.4m / s

3. Water: Higher than in air due to greater density and elasticity, approximately 1482 m/s at 20 deg * C

4. Wood: Varies by type, but generally around 3300 to 4000 m/s.

Thus, the speed of sound is:

Air at 0 deg * C : - 331m / s

Air at 100 deg * C sim 391.4m / s

Water: -1482 m/s

Wood:~3300 to 4000 m/s.

Wood has the maximum speed of sound due to its high density and elasticity.

18. Consider the following statements: [2007]

1. If magenta and yellow coloured circles intersect, the intersected area will have red colour.

2. If cyan and magenta coloured circles intersect, the intersected area will have blue colour.

Which of the statements given above is/are correct?

(a) I only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Correct Answer: (c) Both 1 and 2

Solution:Combination of red and blue colour results into formation of magenta colour. Where as combination of red and green colour results in to formation of yellow colour. Thus on combination of magenta and yellow colour red colour will be highlighted. On the other side combination of blue and green results into formation of cyan colour. Combination of red and blue colour produce magenta colour. Therefore, on combination of cyan and magenta colours blue colour will be more highlighted.

19. Consider the following statements: [2007]

1. A flute of smaller length produces waves of lower frequency.

2. Sound travels in rocks in the form of longitudinal elastic waves only.

Which of the statements given above is/are correct?

(a) I only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Correct Answer: (d) Neither 1 nor 2

Solution:The flute is a musical instrument of the woodwind family. A flute of smaller length produces waves of higher frequency. Sound waves travels through medium rocks in the form of longitudinal as well as transverse waves.

20. The following two items consist of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer these items using the code given below. [2007]

Assertion (A): A jet aircraft moving at Mach Number equal to 1 travels faster at an altitude of 15 km than while moving at Mach Number equal to 1 near the sea level.

Reason (R): The velocity of sound depends on the temperature of the surrounding medium.

(a) Both A are R are true and R is the correct explanation of A
(b) Both 4 and R are true but R is not a correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

Correct Answer: (d) A is false but R is true

Solution:Mach number, a useful quantity in aerodynamics, is the ratio of air speed to the local speed of sound. The speed of sound varies with temperature.

Since temperature and sound velocity normally decrease with increasing altitude, sound is refracted upward. Mach number is a function of temperature at altitude. With decrease in sound velocity Mach number increases.

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