When a wave moves between two different media it changes speed (unless the two media have identical densities). This is refraction and leads to a change in the direction the wave travels in (unless it hits at a zero angle of incidence). See refraction through a rectamgular glass block.

The frequency of the wave does not change - that is determined by the frequency at which the source is vibrating - that is not affected by whatever medium the wave later travels through!

The wave equation:

v = f λ

when the speed changes the wavelength will change too. If speed increases the wavelength gets longer, and if it decreases the wavelength gets shorter..... see the illustration below.

Light Waves

Photons of electromagnetic radiation (of which light is an example) always travel at the speed of light - 3.0 x 10⁸ m/s - that is why the symbol 'c' was given to that speed - it is constant! But as it travels though matter it does so in a series of 'stop and go' movements. It interacts with the particles in matter - gets absorbed and then re-emitted and that takes time. The average velocity at which it travels through a medium is therefore always slower than it would travel through a vacuum.

Light travels faster through 'rare' (less dense) optical media - eg. air - because it does not have to 'waste time' interacting with as many particles in a given space - and fastest it possibly can in a vacuum because there are no particles for it to interact with at all.

Light travels slower through 'dense' optical media such as water or glass because there are more particles to interact with in a given time.

Sound

Sound travels faster in denser media as that is NOT energy travelling as little photons but rather vibrational energy of the particles themselves! Therefore the closer packed they are the easier it is to pass the energy on.... So be careful... it is not the density difference that makes the refraction occur but the overall speed change!

Water Waves

Waves travelling across the surface of water can also be refracted. The change in the speed of water waves when they cross the boundary between two different depths causes a change in their direction (refraction), unless the direction of travel of the waves is along a normal. Water waves travel faster in deep water than in shallow.

How to remember this: Slow in Shallow (therefore the wavelength in shallow water is Small)

When a wavefront changes speed it changes direction (unless the angle of incidence is 0^o - then no change in direction occurs - it is undeviated - but the speed still changes!)

Below is an animation of waves going from one medium to another and slowing down. (It could represent light going from air to glass, or sound going from a solid like rock into air, or water going from a deep to a shallow depth).

The 'lines' represent the crests of the waves as they travel through the medium and the arrowed line represents the direction of travel (it is at right angles to the crests of the waves and is the 'ray' of those waves - it indicates which direction the wave is travelling in).

Slide your mouse-pointer over the diagram to see the wavefronts move.

(Warning! If you are made to feel ill by flashing lights - the diagram animation may make you feel queasy)

Can you see how much slower the wavefronts are in the cyan (lower) medium? They travel more slowly and so the wavelength of the wave is smaller.... and they change direction because they cannot travel as fast.... but the rate at which they move is unaltered as their frequency is unchanged.