An electric bell operated within an evacuated bell jar enables us to infer correctly that

Analyze the question

This is the classic experiment of an electric bell ringing inside a bell jar as the air is pumped out. We need to decide what it lets us infer.

The concept

• Sound is a mechanical wave. It travels by vibrating particles, so it needs a material medium (solid, liquid or gas).
• Light is an electromagnetic wave. It needs no medium and can travel through a vacuum (this is how sunlight reaches Earth).

What happens in the experiment

1. At first the bell rings inside the jar, and we can both see and hear it.
2. As the air is pumped out, the sound gets fainter and eventually becomes inaudible, even though the hammer is still striking the gong.
3. We can still see the bell clearly through the glass jar.

Checking the options

• A. Wrong. We can see the bell in a vacuum, so light does not need a medium.
• B. Wrong. The sound disappears in a vacuum, so sound does need a medium.
• C. Correct. Light is still seen in the vacuum, while sound is lost.
• D. This is a true fact in general, but the experiment does not compare speeds, so we cannot infer it from this.
• E. Wrong. Light and sound travel at very different speeds.

Answer: C

Exam tip: Remember "See but not hear" for the bell jar experiment. If you can still see the bell but cannot hear it, light needs no medium and sound does. Also watch for options that are true in general but not what the experiment shows, like D.