### Correct Option: B. The speed of light is constant and approximately 299,792 kilometers per second. #### Detailed Explanation: 1. **Understanding the Speed of Light**: - The speed of light in a vacuum is a fundamental constant of nature, denoted by the symbol \( c \). Its value is approximately \( 299,792,458 \) meters per second, which is often rounded to \( 3.00 \times 10^8 \) m/s for simplicity in calculations. 2. **Why Option B is Correct**: - The speed of light in a vacuum is indeed constant and does not change regardless of the observer's motion or the source of the light. This constancy is a cornerstone of Einstein's theory of relativity, which states that the laws of physics (including the speed of light) are the same for all observers, regardless of their relative motion. 3. **Why the Other Options are Incorrect**: - **Option A: The speed of light increases as it travels through different media.** - This statement is incorrect because the speed of light actually decreases when it travels through different media (like water, glass, etc.) compared to its speed in a vacuum. The speed of light in a medium is given by the formula: \[ v = \frac{c}{n} \] where \( n \) is the refractive index of the medium. Since \( n \) is greater than 1 for any medium other than a vacuum, the speed of light \( v \) in that medium will always be less than \( c \). - **Option C: The speed of light can be faster than the speed of sound in all conditions.** - While it is true that the speed of light is generally much faster than the speed of sound (which is about \( 343 \) m/s in air at room temperature), this statement is misleading. The speed of sound is dependent on the medium (air, water, solids), and while light is faster in a vacuum, the comparison is not meaningful in all conditions. The statement does not accurately reflect the nature of light and sound in different media. - **Option D: The speed of light varies depending on the observer's motion.** - This statement is incorrect because, according to the principles of relativity, the speed of light remains constant at \( c \) for all observers, regardless of their relative motion. This means that if one observer is moving towards a light source and another is moving away, both will measure the speed of light as \( c \). This invariance is a fundamental aspect of how we understand space and time. #### Summary of Key Points: - The speed of light in a vacuum is a constant value of approximately \( 299,792 \) km/s. - Light slows down when passing through different media, contrary to what Option A suggests. - The speed of light is always faster than the speed of sound, but the comparison is not universally applicable (Option C). - The speed of light is invariant and does not change based on the observer's motion (Option D). ### Revision Summary: - The speed of light in a vacuum is constant at approximately \( 299,792 \) km/s. - Light slows down in media with a refractive index greater than 1. - The speed of light is always greater than the speed of sound, but context matters. - The speed of light remains constant for all observers, regardless of their motion.