In the question given below, two statements are marked as Assertion (A) and Reason (R). Mark your answer according to the code given in the alternatives. Assertion (A) : When lightning strikes, the light appears shortly after the sound is heard. Reason (R) : The velocity of light is greater than the velocity of sound.
A is false but R is true.
Understanding Lightning, Light, and Sound Speeds
This question asks us to analyze an Assertion (A) and a Reason (R) related to lightning strikes and the speeds of light and sound.
Analyzing Assertion (A) and Reason (R)
Let's break down each statement:
Assertion (A): When lightning strikes, the light appears shortly after the sound is heard.
- Think about your experience during a thunderstorm. Do you see the flash of lightning before you hear the rumble of thunder, or the other way around?
- Common observation shows that the flash of light from lightning is seen almost instantly, while the sound of thunder is heard seconds later.
- Therefore, Assertion (A), which claims the light appears after the sound is heard, is incorrect.
- Conclusion for Assertion (A): False.
Reason (R): The velocity of light is greater than the velocity of sound.
- Light and sound are both forms of energy that travel as waves, but they travel at very different speeds, especially through air.
- The speed of light in a vacuum is approximately \(3 \times 10^8\) meters per second (\(m/s\)). Its speed in air is slightly less but still incredibly fast.
- The speed of sound in air is much slower, approximately 343 meters per second (\(m/s\)) at room temperature (\(20^\circ C\)).
- Clearly, the speed of light is vastly greater than the speed of sound.
- Therefore, Reason (R), which states that the velocity of light is greater than the velocity of sound, is correct.
- Conclusion for Reason (R): True.
Evaluating the Options
We have concluded that Assertion (A) is False and Reason (R) is True.
Let's look at the given options:
- Option 1: A is true but R is false. (Incorrect, A is false, R is true)
- Option 2: Both A and R are true and R is the correct explanation of A. (Incorrect, A is false)
- Option 3: A is false but R is true. (Correct, A is false, R is true)
- Option 4: Both A and R are true but R is not the correct explanation of A. (Incorrect, A is false)
Based on our analysis, only Option 3 correctly describes the truthfulness of both statements.
Final Answer
Assertion (A) is false, and Reason (R) is true.
Revision Table: Key Facts about Lightning and Wave Speeds
| Aspect | Light (from Lightning) | Sound (Thunder) |
|---|---|---|
| Nature | Electromagnetic wave | Mechanical wave |
| Speed in Air (approx.) | Very high (\(\sim 3 \times 10^8\) m/s) | Much lower (\(\sim 343\) m/s) |
| Detection during Lightning | Seen almost instantly | Heard seconds later |
| Assertion (A) says... | Appears after sound | Heard before light |
| Reality is... | Appears before sound | Heard after light |
| Reason (R) says... | Speed > Sound Speed | Speed < Light Speed |
| Reason (R) is... | True | True (relative to light) |
Additional Information on Speed of Light vs Speed of Sound
The significant difference in the speeds of light and sound is a fundamental concept in physics and explains several everyday phenomena, not just lightning.
- Seeing Fireworks Before Hearing Them: Similar to lightning, you see the burst of a firework in the sky before you hear its boom. This is because light from the explosion reaches your eyes much faster than the sound reaches your ears.
- Calculating Distance to a Thunderstorm: You can estimate how far away a thunderstorm is by counting the seconds between seeing the lightning flash and hearing the thunder. Sound travels about 343 meters per second (or roughly 1 kilometer every 3 seconds, or 1 mile every 5 seconds). By multiplying the time difference by the speed of sound, you can get a rough estimate of the distance. Light travels so fast that the time it takes to reach you is negligible for this calculation.
- Speed of Light: The speed of light in a vacuum (\(c\)) is a universal constant. It is the fastest speed at which conventional matter, energy, or any signal carrying information can travel through space.
- Speed of Sound: The speed of sound varies depending on the medium it travels through (solid, liquid, or gas) and the properties of the medium, such as temperature, density, and pressure. Sound travels faster in denser media like water or solids compared to air.
Important Questions from Assertions and Reasons
In this question, two statements are given – one labeled as Assertion (A) and the other labeled as Reason (R) respectively. Assuming that all the information given in the assertion is true, analyze both the reasons along with it and decide which of them is/are correct and whether R is the correct description of A.
Assertion: Oral polio vaccine should be given to all children below the age of 5 years; But children over the age of 5 do not need to take the dose.
Reason: In India, almost all complications of poliomyelitis are found in children below the age of 5.
For the Assertion (A) and Reason (R) given below, choose the correct option from the following :
Assertion (A): Buddhism considers suicide as the best way to attain salvation.
Reason (R): Buddhism claims that the world is full of sorrow and pain.
In the question given below, two statements are marked as Assertion (A) and Reason (R). Mark your answer according to the code given in the alternatives.
Assertion (A) : There has been a huge loss of property in Japan last year.
Reason (R) : Japan was hit by Tsunami last year.
In the question given below, there are two statements marked as Assertion (A) and Reason (R). Mark your answer as per the codes in options.
Assertion (A): Iron rusts when exposed to water.
Reason (R): Iron reacts with the oxygen molecules present in water.
An assertion (A) and a reason (R) are given below.
Assertion (A): Forest cover in the country has gradually decreased.
Reason (R): Encroachment by humans is one of the concerns for the forest department.
Choose the correct option.
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 the correct explanation of A
C. A is true, but R is false
D. A is false, but R is true
