In a movie Rurik is watching, a spaceship explodes far away in space, and the theater booms with the blast's roar. Could that roar really have crossed space?
This lesson builds the sort that answers him, and then drills it: electromagnetic or mechanical, decided by one question.
The test is one question: does the wave need a medium?
A mechanical wave needs a medium. The blast's roar is a sound wave, and sound travels only where there is material to oscillate.
Space holds no material, so the roar could never cross it to a distant ship.
An electromagnetic wave needs no medium. The blast's light crosses the vacuum easily; the explosion would be seen, never heard.
Being in a material does not make a wave mechanical. The question is whether the wave needs the material: light crosses glass, and light also crosses a vacuum, so light is electromagnetic.
Sound, water ripples, and earthquake waves are mechanical. Light, radio waves, and X-rays are electromagnetic.
Sunlight streams through a window's glass into a kitchen. Is the sunlight electromagnetic or mechanical?
The question is whether the wave needs a medium, not whether it is in one.
Light crosses the vacuum from the Sun to Earth, so it needs no medium.
Passing through glass does not change that.
The sunlight is electromagnetic.
A whale's call travels through seawater toward another whale. Is the call electromagnetic or mechanical?
The call is a sound wave, carried by the oscillating water.
Without a medium, the call could not travel at all.
The call is mechanical.
Sound from a drum crosses a school gym. Is the wave electromagnetic or mechanical?
Ripples spread across a pond. Is the wave electromagnetic or mechanical?
An X-ray passes through a patient's arm. Is the wave electromagnetic or mechanical?
A radio signal travels from a Mars rover across space to Earth. Is the wave electromagnetic or mechanical?
An earthquake wave travels through solid rock. Is the wave electromagnetic or mechanical?
Light travels along the inside of a glass fiber. Is the wave electromagnetic or mechanical?
A huge storm rages on the Sun, flashing and roaring where it churns the Sun's gas. Which of the storm's waves could reach Earth across the empty space between?
What decides whether a wave is mechanical?
Sound is traveling through a steel rail, and light is traveling through a glass block. Which of the two could also cross a vacuum?