# Holding your hand at arms length you can readily block sunlight from reaching your eyes. Why cant you block sound from reaching your ears this way?

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bandmanjoe | Certified Educator

The speeds of light, which is the electromagnetic wave, and sound, which is a mechanical wave, are two very different finite numbers. Light moves much faster, 186,000 miles per second, while sound is measured at around 700 miles per hour, which is significantly slower.  Light waves are much shorter, which means they are easier to deflect.   Sound waves are much longer, and tend to diffract, or bend around obstructions.  The longer the wavelength, the easier it is to diffract, or bend around barriers or obstructions that try to get in the way.  That is why you can not see someone until they actually enter the room, but you can hear them walking down the hallway that is adjacent to the room.  Our eyes measure the light that enters them, but our ears measure the sound waves that are collected by them.

lfernandino | Student
The main reason is that sound waves have much longer wavelengths than visible light waves. Wavelength is the distance between the top (or peak) of two subsequent waves. Sound is a series of air pressure waves, and its wavelength determines its pitch (smaller wavelength equals higher pitch). Light is a series of electromagnetic waves, and it's wavelength determines its color. Visible light has wavelengths between 400 and 700 nanometers (1 nanometer = 1 billionth of a meter), while audible sound has wavelengths between 17 millimeters and 17 meters. When a traveling wave hits a barrier (say, your hand), it diffracts, that is, the direction of the waves passing through the edges of the barrier changes inward, so the wave has a tendency to "wrap around" the barrier. The larger the wave's wavelength relative to the size of the barrier, the more it diffracts. Light has a small enough wavelength that it won't diffract enough to bend around your hands and reach your eyes, but sound waves will bend around your hand and reach your ears.

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