how does electromagnetic waves travel? (transverse waves)?

You seem to be somewhat confused. Refraction is due to a wave (any wave) traveling across a boundary of two different media, because the wave speed is different in each of the media. Thus sound and EM waves can either slow down or speed up during refraction, depending on which material they are coming from and which material they are going to. The speed of sound does depend on the particles through which it travels. A less dense object will have a higher speed of sound, but this is not because it is a longitudinal wave. The same occurs for transverse pressure waves. EM waves also travel at different speeds in different materials, depending on the magnetic permeability and electric permittivity of the material, not because they are transverse waves, per se.

Note:

Longitudinal sound wave speed: sqrt (K / rho)

Transverse sound wave speed: sqrt (B / rho)

Speed of light in a vacuum: sqrt (1 / eps0*mu0)

Speed of light in a material: sqrt (1 / eps*mu)

rho = density of a material

K = bulk modulus of a material

G = shear modulus of a material

eps0 = electric permittivity of free space

mu0 = magnetic permeability of free space

eps = electric permittivity of a material

mu = magnetic permeability of a material

A wave is most simply defined as a traveling packet of energy.

The simplest wave is therefore the photon.

If the wave is traveling and it encounters another wave or medium, then it can refract, reflect, slow-down, speed up, etc... As you know this just means that there is some transfer of energy. How the energy is transferred, the speed, etc... are all measurements you will need to take but the main idea is that different waves are classified, like Hertz or Frequency for example. Each Frequency represents a different orientation or type of wave and each wave has different properties. If you combine two waves, you will most probably get a certain result and you can make things to reflect only specific types of waves as well, such as in the case of an amphitheater or musical auditorium reflecting sound waves. http://duality-rabbithole.blogspot.com/2010/01/mat...

Additionally, the more energy the wave has the faster it travels. And since the wave is just the flow of energy, that means the more energy a system is producing, then the more energy waves are flowing out of it. The sun is a great and simple example. It burns, producing tremendous heat and light energy, and is the shaper and mass attractor of space-time and gravity for the Earth and our solar system, being the main source of energy for all life on earth.

your intial premise that em waves only slow down on refraction is basically incorrect.

Note - em waves cannot exceed the speed in a vacuum = 3 x 10^8 m/s - they do not require a medium to travel.

so they will speed up when going from the Earth's atmosphere into space which of course causes refraction.

The tranverse wave is more free to move (as a particle or a wave) when there is nothing to impede it ( hence in a vacuum) - once in a material medium the particles of the medium impede the wave.

This is opposite to a sound wave which uses compressions to travel so a denser medium assists the passage by speeding it up =

Light is an example of a electromagnetic wave therefore it is a transverse wave - oscillations are perpendicular to the direction of energy transfer. :)

A simple answer is that sound waves need the particles to transmit but to electromagnetic waves the parrticles are an obstacle.

When electromagnetic waves travel in the space they travel with the speed of light (considering space completely empty). Although when they pass from space to another material they slow down. That happens because the electromagnetic waves interact with the materials and their atoms . The atoms absorb quanta of energy and that reduce the speed of the wave...

you can also check: http://physics.bu.edu/~duffy/PY106/EMWaves.html

http://physics.bu.edu/~duffy/PY106/Reflection.html

I apologize for my bad english