---
title: "Huygens's Principle and the Laws of Reflection and Refraction"
description: "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."
canonical: https://lightmysky.com/learn/science/huygenss-principle-and-the-laws-of-reflection-and-refraction-mt_xxdniF2__w
source: https://lightmysky.com/learn/science/huygenss-principle-and-the-laws-of-reflection-and-refraction-mt_xxdniF2__w.md
retrieved: 2026-09-12
---

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# 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.

Subject: Science · Area: Waves, Light & Sound · Ages 18 to 19
Page: https://lightmysky.com/learn/science/huygenss-principle-and-the-laws-of-reflection-and-refraction-mt_xxdniF2__w

## Ready when they can

- 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

## Lesson: Wavelets that explain mirrors and bending light

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.

**Example.** 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.

**Tip.** 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.

**Recap.** Wavelets plus arrival times give reflection and refraction, with frequency fixed and wavelength bending to speed.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Superposition and Stationary Waves](https://lightmysky.com/learn/science/superposition-and-stationary-waves-mt_AIxA2TYGJN)
- [Refraction and Lenses](https://lightmysky.com/learn/science/refraction-and-lenses-mt_u8GKuBMmzC)
- [The Doppler Effect and Shock Waves](https://lightmysky.com/learn/science/the-doppler-effect-and-shock-waves-mt_UVfD_coqh6)

## Opens up

- [Curved Mirrors, the Mirror Equation and Real Images](https://lightmysky.com/learn/science/curved-mirrors-the-mirror-equation-and-real-images-mt_7XqB2umthJ)
- [Thin Lenses and the Lens Maker's Equation](https://lightmysky.com/learn/science/thin-lenses-and-the-lens-makers-equation-mt_FXOcC5mpIM)
- [Total Internal Reflection, Optical Fibres and Dispersion](https://lightmysky.com/learn/science/total-internal-reflection-optical-fibres-and-dispersion-mt_ZNSZMLkOD_)