Sound waves cannot travel through which of the following environments?

Introduction / Context:
Sound is a mechanical wave that we experience as noise or music. Unlike light, which is an electromagnetic wave, sound requires a material medium to propagate. This basic fact is often tested in physics questions to ensure that students understand the difference between mechanical and electromagnetic waves. The question asks you to identify the environment in which sound waves cannot travel at all.

Given Data / Assumptions:
- Sound waves are mechanical vibrations of particles in a medium.
- The listed environments are a vacuum, water, glass and hydrogen gas.
- We assume idealised conditions where the vacuum is perfect, with no particles at all.
- We consider audible sound frequencies, not other types of waves.

Concept / Approach:
Mechanically, sound propagates when particles of the medium vibrate and transfer energy to neighbouring particles. If there are no particles, there can be no vibration and therefore no sound. A perfect vacuum contains no matter and is the absence of particles. In contrast, liquids like water, solids like glass and gases like hydrogen all contain particles that can vibrate. Therefore, sound can travel in these media, although at different speeds, but it cannot travel in a perfect vacuum.

Step-by-Step Solution:
Step 1: Recall that sound is a mechanical wave and needs a medium such as a gas, liquid or solid. Step 2: Recognise that a vacuum, by definition, contains no particles to vibrate. Step 3: Since there are no particles in a perfect vacuum, sound waves have nothing to compress or rarefy. Step 4: Therefore, sound cannot propagate through a perfect vacuum. Step 5: Consider water, glass and hydrogen gas. All of these are materials made of particles, so sound can travel through them. Step 6: Conclude that the environment in which sound cannot travel is the perfect vacuum.

Verification / Alternative check:
Simple classroom experiments support this concept. If a ringing bell is placed inside a bell jar and the air is gradually pumped out, the sound becomes fainter and eventually disappears, even though the bell continues to move. This shows that as the medium is removed, sound cannot reach the listener. In contrast, sound travels in water, which is why divers can hear underwater, and in solids such as rails, along which train vibrations can be detected.

Why Other Options Are Wrong:
Liquid water: Sound travels quite well in water and often moves faster than in air because particles are more closely packed.
Solid glass: Sound travels in solids, often faster than in liquids and gases, due to strong interactions between particles.
Hydrogen gas: Even though hydrogen gas is very light, it is still a gas with particles, so sound can propagate through it.

Common Pitfalls:
A common mistake is to confuse sound with light and to think that because light can travel through vacuum, sound can as well. Another pitfall is underestimating how well sound travels through solids like metal or glass. Always remember that sound requires matter, so any gas, liquid or solid can carry sound, while a perfect vacuum cannot.

Final Answer:
Sound waves cannot travel through a perfect vacuum.