Huygens's Principle and the Laws of Reflection and Refraction
Treating every point on a wavefront as a source of new wavelets reproduces reflection and refraction without any new assumption. Snell's law becomes a statement about wave speed in each material.
What a learner can do afterwards
- Constructs a refracted wavefront from wavelets and derives Snell's law
- Relates refractive index to the speed of light in the material
- Explains why the frequency is unchanged on crossing a boundary but the wavelength is not
1 · Read
Huygens pictures each point of a wavefront as a fresh source of wavelets. The next front is simply the surface tangent to all of them.
Matching wavelet arrival times along a boundary forces n1 sine theta1 to equal n2 sine theta2, which is Snell law. The index n equals c over v, and with c near 300 million metres per second, glass near n of 1.5 carries light near 200 million metres per second. Reflection is simpler: incidence angle equals reflection angle, both measured from the normal.
Light entering glass slows down, so crests pack tighter. The source still launches crests at the same rate, which is why frequency holds while wavelength shrinks. Frosted glass and mirrors both reflect point by point, but roughness scrambles the directions while a mirror keeps them ordered.
To sketch refraction, draw slower wavelets inside the second material and lay the new front tangent to them. Measure every angle from the normal, never from the surface.
Wavelets plus arrival times give reflection and refraction, with frequency fixed and wavelength bending to speed.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.