Reflection and refraction laws and Fermat’s principle

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Problem 6.3a

Use Fermat’s principle of stationary time to derive the law of reflection.

Background

In the solution of problem 3.1a we showed that the angle of incidence equals the angle of reflection and that, for the angle of refraction $ \theta _{2} $, $ {\rm {\;sin\;}}\theta _{2}=(V_{2}/V_{1}){\rm {\;sin\;}}\theta _{1} $ [see also equation (3.1a)].

These are the laws of reflection and refraction. Fermat’s principle of least time (more accurately, of stationary time) states that wave travel between any two points is along the path for which the traveltime is either a maximum or a minimum value (i.e., the derivative of the traveltime equals zero) compared with the traveltimes along adjacent paths.

Figure 6.3a.  Deriving Snell’s law.

Solution

In Figure 6.3a, the source $ S $ and the receiver $ R $ have coordinates $ (0,\;h_{1}) $ and $ (a,\;h_{2}) $. The traveltime for a wave from $ S $ to $ R $ with reflecting point $ M(x,\;0) $ is

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \begin{align} t=(1/V)\{ (x^{2} +h_{1}^{2} )^{1/2} +[(a-x)^{2} +h_{2}^{2} ]^{1/2} \} . \end{align}

To find the point Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): M for which the value of Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): t is stationary, we differentiate Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): t with respect to Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): x and set the result equal to zero. Thus,

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \begin{align} \frac{{\rm d}t}{{\rm d}x} =(1/V)\left\{ \frac{x}{(x^{2} +h_{1}^{2} )^{1/2} } -\frac{(a-x)}{[(a-x)^{2} +h_{2}^{2} ]^{1/2} } \right\} = 0. \end{align}

The two terms in the brackets are the sines of the angles Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \theta _{1} and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \theta _{1}^{'} ; hence, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\rm \; sin\; }\theta _{1} ={\rm \; sin\; }\theta _{2}^{'}.

Problem 6.3b

Repeat part (a) for the refracted path SMQ, in Figure 5.3a.

Solution

The traveltime for the path SMQ is

$ {\begin{aligned}t=(x^{2}+h_{1}^{2})^{1/2}/V_{1}+[(b-x)^{2}+h_{3}^{2}]^{1/2}/V_{2}.\end{aligned}} $


Differentiation gives

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \begin{align} \frac{{\rm d}t}{{\rm d}x} =\frac{x}{V_{1} (x^{2} +h_{1}^{2} )^{1/2} } -\frac{(b-x)}{V_{2} [(b-x)^{2} +h_{3}^{2} ]^{1/2} } =0, \end{align}

that is, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\rm \; sin\; }\theta_{1} /V_{1} ={\rm \; sin\; }\theta_{2} /V_{2} .

Problem 6.3c

Repeat parts (a), (b) for reflected and refracted converted S-waves.

Solution

If we replace the angles Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \theta with the angles Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \delta and use the S-wave velocities Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): \beta , the foregoing proofs are otherwise unchanged.

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