Speed of Light
OBJECTIVE-I
EXPLANATION: The speed of light depends on the refractive index of a medium. In the vacuum, the refractive index is 1 for all wavelengths (colors) and they travel exactly with the same velocity = c. In any other medium, the refractive indices for different colors of light are different. Hence in the air µᵣ and µᵧ are different. Therefore the speed of red and yellow lights are not the same in the air.
EXPLANATION: The image is formed by the rays emitted from the object after refraction from the lens. So the rays have to travel a distance from the object to the image. It takes a little time for the light rays to travel this distance from the new position of the object to the image. Hence the image will be shifted a little later than the object. Option (b).
But this time will be very very small because the speed of the light is very high and may not be felt by human eyes.
OBJECTIVE-I
1. Light passes through a closed cylindrical tube containing gas. If the gas is gradually pumped out, the speed of light inside the tube will
(a) increase
(b) decrease
(c) remain constant
(d) first increases and then decreases.
ANSWER: (a)
EXPLANATION: When the gas is gradually pumped out, a vacuum is being created in the tube. The refractive index for vacuum is 1. The refractive index of any other medium is more than 1. The relation between the speed of light in two mediums is given as
v₁/v₂ = µ₂/µ₁
Here µ₁, for vacuum = 1; for gas, µ₂>1, Hence
v₁/v₂> 1
→v₁ > v₂
So the speed of the light in the vacuum will increase. Thus the option (a) is true.
2. The speed of red light and yellow light are exactly the same
(a) in a vacuum but not in the air
(b) in the air but not in a vacuum
(c) in a vacuum as well as in the air.
(d) neither in a vacuum nor in air.
ANSWER: (a)
3. An illuminated object is placed on the principal axis of a converging lens so that a real image is formed on the other side of the lens. If the object is shifted a little
(a) the image will be shifted simultaneously with the object.
(b) the image will be shifted a little later than the object.
(c) the image will be shifted a little earlier than the object.
(d) the image will not shift.
ANSWER: (b)
But this time will be very very small because the speed of the light is very high and may not be felt by human eyes.
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OBJECTIVE-II
1. The speed of light is 299,792,458 m/s
(a) with respect to the earth
(b) with respect to Sun
(c) with respect to a train moving on the earth
(d) with respect to a spaceship going in outer space.
ANSWER: (a),(b),(c),(d).
EXPLANATION: The speed of light in a vacuum is a universal constant. It does not depend on the speed of other things, not even on the speed of the source from which it is emitted. Hence all options are true.
2. Which of the following methods can be used to measure the speed of light in a laboratory?
(a) Roemer method.
(b) Fizeau method.
(c) Foucault method.
(d) Michelson method.
ANSWER: (c)
EXPLANATION: Only the Foucault method can be used to measure the speed of light in the laboratory. Other methods need a large distance outside due to the high speed of the light. Hence the option (c).
3. Which of the following methods can be used to measure the speed of light in water?
(a) Roemer method.
(b) Fizeau method.
(c) Foucault method.
(d) Michelson method.
ANSWER: (c)
EXPLANATION: Since only the Foucalt method can be used in the laboratory, it can easily be used to measure the speed of light in water by keeping the medium between the two mirrors as water. Other methods are not suitable because these are performed in open fields. Hence the option (c).
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Links to the Chapters
Links to the Chapters
CHAPTER- 20 - Dispersion and Spectra
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
CHAPTER- 15 - Wave Motion and Waves on a String
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 20 - Dispersion and Spectra
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
CHAPTER- 15 - Wave Motion and Waves on a String
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 8 - Work and Energy
Click here for → Question for Short Answers
Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → Exercises (1-10)
Click here for → Question for Short Answers
Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → Exercises (1-10)
Click here for → Exercises (11-20)
Click here for → Exercises (21-30)
Click here for → Exercises (31-42)
Click here for → Exercise(43-54)
Click here for → Exercises (21-30)
Click here for → Exercises (31-42)
Click here for → Exercise(43-54)
CHAPTER- 7 - Circular Motion
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Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → EXERCISES (11-20)
Click here for → EXERCISES (21-30)
CHAPTER- 6 - Friction
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Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → EXERCISES (11-20)
Click here for → EXERCISES (21-30)
CHAPTER- 6 - Friction
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Click here for → Questions for Short Answer
Click here for → OBJECTIVE-I
Click here for → Friction - OBJECTIVE-II
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Click here for → Exercises (11-20)
Click here for → EXERCISES (21-31)
Click here for → OBJECTIVE-I
Click here for → Friction - OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → Exercises (11-20)
Click here for → EXERCISES (21-31)
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CHAPTER- 5 - Newton's Laws of Motion
Click here for → QUESTIONS FOR SHORT ANSWER
Click here for → QUESTIONS FOR SHORT ANSWER
Click here for→ Newton's laws of motion - Objective - I
Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
Click here for→ Newton's laws of motion - Objective - I
Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
Click here for→Newton's Laws of Motion,Exercises(Q.No. 13 to 27)
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CHAPTER- 4 - The Forces
The Forces-
"Questions for short Answers"
Click here for "The Forces" - OBJECTIVE-I
Click here for "The Forces" - OBJECTIVE-II
Click here for "The Forces" - Exercises
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CHAPTER- 3 - Kinematics - Rest and Motion
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Click here for "OBJECTIVE-I"
Click here for EXERCISES (Question number 1 to 10)
Click here for EXERCISES (Question number 11 to 20)
Click here for EXERCISES (Question number 21 to 30)
Click here for EXERCISES (Question number 31 to 40)
Click here for EXERCISES (Question number 41 to 52)
CHAPTER- 2 - "Vector related Problems"
CHAPTER- 2 - "Vector related Problems"
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Click here for "OBJECTIVE-II"
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