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Sound & Light (Physics): How are They Different?

Sound received by our human ears and light received by our human eyes may often seem to be two parts of the same phenomenon, but they are actually quite distinct.

While both phenomena are waves, their origins and properties vary considerably. By understanding these differences, we can make sense of our senses and gain a better handle on the underlying physics.

What Is a Wave?

A wave is a disturbance in a medium that propagates from one place to another via oscillations in that medium. The key here is that any point in the medium itself oscillates in place while the disturbance travels, transferring energy in the process.

Consider a crowd in a stadium doing the wave at a ball game. The members of the crowd are the wave medium. They oscillate in place by standing and sitting while the wave itself travels around the stadium.

Waves have wavelength, amplitude and frequency associated with them. The wavelength is the distance between successive wave peaks. The amplitude is the maximum displacement of the medium from equilibrium. The frequency is the number of wavelengths passing by a point per second.

Waves can be transverse or longitudinal. In a transverse wave, the medium oscillates perpendicularly to the direction of propagation. In a longitudinal wave, the medium oscillates along the same direction as propagation.

What Is a Sound Wave?

Sound is an example of a longitudinal wave. When a sound travels through a medium, such as air, it causes compressions (regions of increased density) and rarefactions (regions of decreased density) in the air as it travels.

Sound waves are created by oscillations – either from your vocal chords, a tuning fork, musical instruments or a pile of dishes crashing to the floor. A struck tuning fork, for example, vibrates at a specific frequency. As it moves, it bumps into the air molecules around it, periodically compressing them. The compressed regions transfer this energy to their neighboring air molecules as well, and the disturbance moves through the air.

What Are Light Waves?

Light, also called electromagnetic radiation (EM radiation) or electromagnetic waves, results from oscillations in electric and magnetic fields. If a charge moves back and forth along a wire, it creates a changing electric field, which in turn creates a changing magnetic field, which then self-propagates.

In this way, electromagnetic radiation serves as its own medium and does not require an external medium through which to travel, unlike most classical waves. Because of this, electromagnetic radiation can traverse the vacuum of space. (This is a good thing – otherwise light would never reach us from the sun!)

EM radiation comes in many different forms, depending on its wavelength and frequency, such as radio waves, microwaves, infrared radiation, visible light, ultraviolet light, X-rays and gamma rays. All of these waves travel at the speed of light in a vacuum (approximately 3 × 108 m/s). This is fastest speed in the universe! Faster-than-light travel is not physically possible.

How Are Radio Waves and EM Radiation Different From Sound Waves?

People often confuse these two types of waves because we are so familiar with listening to the radio through a speaker or headphones. And when it comes to orchestrating entertainment events, there are audio visual engineers needed.

But radio waves are a form of electromagnetic radiation. They travel at the speed of light, and they transmit information from the radio station to your radio. However, that information is then converted into the motion of a speaker, which produces sound waves, which are longitudinal waves in the air.

The speed of sound is approximately 343 m/s in air, which is much slower than radio waves, and they require a medium through which to travel.

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