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## Physics library

### Course: Physics library>Unit 8

Lesson 3: Introduction to mechanical waves

# Introduction to waves

Transverse and longitudinal waves are two types of mechanical waves, which involve the transfer of energy through a medium (e.g. water, air, a solid). Learn about transverse and longitudinal waves through the examples of a shaken rope and a sound wave. Finally learn about the difference between a single wave pulse and periodic waves. Created by Sal Khan.

## Want to join the conversation?

• How does sound travel through wires?
• Andrew M's answer is definitely correct if you meant sound through an electrical speaker; however, if you are referring to a tin can telephone, then here is an answer. The classic kid's toy works by vibration. When you speak into one can, your voice causes the back of the can to vibrate. These vibrations transfer to the string, which functions as a conduit all the way to the other can. As long as the string is pulled tightly, when the vibrations reach the back of the other can, it resonates much like a drum, producing a sound nearly identical to the original vibration. Here's an article from Wikipedia on the toy, if you were curious: https://en.wikipedia.org/wiki/Tin_can_telephone
• How radio waves pass through walls, while light waves do not?
• But then why do gamma waves, which have a high frequency, pass through walls as well?
• how would you define energy?
• It's actually quite hard to define energy, but I would say @Ethan Dlugie gives the best definition here.
• this is true that light has properties of wave and matter.Then why doesn't it exerts pressure?
• Could anyone give a few examples each of transverse and longitudinal waves?
• Transverse waves: Electromagnetic waves(light waves, Radio waves),wave in a string etc.
Longitudinal waves: compression waves in a spring, sound waves etc.
• because sound is made by a vibration and because atoms always vibrate, do they make even the tiniest amount of sound?
• yes they make the tiniest amount of sound,according to the no of vibrations,called hertz,the sound is inaudible below 20hz
• what kind of waves are used in our microwaves
• In microwaves, electromagnetic waves are used to heat up your food. The waves in your microwave are the same sort of waves that make up visible light.
• What happens to the wave if it is travelling through different mediums?
Example: From a medium of high density to low density
• Great question! Unfortunately, the answer is kind of complicated. In general, if you keep EVERYTHING ELSE the same, the speed of sound in a gas (like air) is slower when the density is high, and faster when the density is low. But more commonly, what we measure about air is the temperature, and the speed of sound is faster in hot air and slower in cold air. If waves hit a boundary between two media with different speeds, the wave will be refracted, which means it will change direction. This is what causes a mirage in the desert, because the air near the ground is much hotter than the air higher up, and so the light bends (you look just below the horizon but you see the bright light from the sky coming from that direction). If the boundary between the media is sudden, like between air and water, then the refracted wave turns a sharp corner, but there will also be a partial reflection of the wave.