Bob

]]>The wave-fronts are perpendicular to the direction of the beam because it is a transverse wave. As CD is shorter than AB, the beam bends towards

the normal. You can work out the usual angle of incidence and refraction connection by doing some trig. using the red line as the hypotenuse.

Bob

]]>The ladder on the left represents a beam of light moving from left to right towards the boundary, shown in red.

The wavelength is represented by AB. Each cross line represents a transverse wave maximum. They are equally spaced and connected by the formula

When the beam reaches the boundary, the speed of light is lower (let's say). But the rate at which wave-fronts arrive at the boundary is unchanged. Maxima must start at the boundary at the same rate and travel through this new medium at the slower rate, let's say d < c. n is unaffected by the transmission through the boundary. CD is the new wavelength; I'll call it mu.

Bob

]]>Bob

]]>Since you cannot get energy from nothing (nor lose it) the maxima (and minima) after passing through the boundary must be conserved, so the frequency must be constant.

Why does wavelength change then?

]]>Bob

]]>Frequency is determined by the distance between wave maxima (or minima).

That is wavelength

]]>Bob

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