What is the primary condition for light to undergo refraction?

The primary condition for light to undergo refraction is that it must pass from one transparent medium into another medium with a different refractive index. Additionally, the light ray must strike the interface between the two media at an angle other than $0^circ$ (i.e., not perpendicular to the surface). If the angle of incidence is $0^circ$, the light ray passes straight through without bending, although its speed still changes.

How does the speed of light change when it refracts?

When light refracts, its speed changes. If light travels from an optically rarer medium (lower refractive index) to an optically denser medium (higher refractive index), its speed decreases. Conversely, if it travels from an optically denser medium to an optically rarer medium, its speed increases. The frequency of light remains constant during refraction, but its wavelength changes proportionally to the change in speed.

What is the difference between absolute and relative refractive index?

The absolute refractive index ($n$) of a medium is its refractive index with respect to vacuum (or air, approximately). It's defined as $n = c/v$, where $c$ is the speed of light in vacuum and $v$ is the speed of light in the medium. The relative refractive index ($n_{21}$) describes the refractive index of medium 2 with respect to medium 1, calculated as $n_{21} = n_2/n_1$. It indicates how much light bends when going from medium 1 to medium 2.

Can Snell's Law be applied to other waves besides light?

Yes, Snell's Law is a general principle that applies to any wave phenomenon that changes speed when passing from one medium to another. This includes sound waves, water waves, and seismic waves. For example, sound waves bend when they pass from air into water, or when they encounter layers of air with different temperatures (and thus different sound speeds), following a similar relationship.

What happens if the angle of incidence is $0^circ$?

If the angle of incidence is $0^circ$, it means the light ray is incident normally (perpendicularly) to the interface between the two media. In this specific case, the light ray does not bend or deviate from its path. It passes straight through, although its speed and wavelength will still change according to the refractive index of the new medium. Mathematically, $n_1 \sin 0^circ = n_2 \sin \theta_2 \implies 0 = n_2 \sin \theta_2$. Since $n_2 \neq 0$, it implies $\sin \theta_2 = 0$, so $\theta_2 = 0^circ$.

Why does a prism split white light into colors?

A prism splits white light into its constituent colors (spectrum) due to a phenomenon called dispersion, which is an extension of Snell's Law. The refractive index of a material is slightly different for different wavelengths (colors) of light. Generally, violet light has a slightly higher refractive index than red light in glass. Consequently, violet light bends more than red light when passing through the prism, causing the different colors to separate and form a spectrum.

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Snell's Law

Physics

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Version 1Updated 22 Mar 2026

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Snell's Law, also known as the law of refraction, quantitatively describes the relationship between the angles of incidence and refraction, and the refractive indices of the two media involved, when light or other waves pass through the boundary between two different isotropic media. Formally stated, it dictates that the ratio of the sines of the angles of incidence and refraction is equivalent to…

Quick Summary

Snell's Law, or the law of refraction, describes how light bends when it passes from one transparent medium to another. This bending occurs because light changes its speed in different media. The optical density of a medium is quantified by its refractive index ( $n$ ), defined as the ratio of the speed of light in vacuum ( $c$ ) to its speed in the medium ( $v$ ), i.

e., $n = c/v$ . Snell's Law states that for a given pair of media, the product of the refractive index and the sine of the angle of incidence (or refraction) in that medium is constant: $n_1 \sin \theta_1 = n_2 \sin \theta_2$ .

Here, $\theta_1$ is the angle of incidence in medium 1 (refractive index $n_1$ ), and $\theta_2$ is the angle of refraction in medium 2 (refractive index $n_2$ ). Angles are always measured with respect to the normal (perpendicular line) to the interface.

If light enters an optically denser medium ( $n_2 > n_1$ ), it bends towards the normal ( $\theta_2 < \theta_1$ ). If it enters an optically rarer medium ( $n_2 < n_1$ ), it bends away from the normal ( $\theta_2 > \theta_1$ ).

This law is fundamental to understanding lenses, prisms, and phenomena like apparent depth and total internal reflection.

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Key Concepts

Refractive Index and Speed of Light

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Direction of Bending and Optical Density

Snell's Law, $n_1 \sin \theta_1 = n_2 \sin \theta_2$ , directly shows how the direction of bending relates to…

Critical Angle and Total Internal Reflection (TIR)

The critical angle ( $\theta_c$ ) is a special angle of incidence that occurs when light travels from an…

Snell's Law: —

n_1 \sin \theta_1 = n_2 \sin \theta_2

Refractive Index: —

n = c/v

(where

c

is speed of light in vacuum,

v

in medium)

Relative Refractive Index: —

n_{21} = n_2/n_1 = v_1/v_2

Bending: — Rarer to denser (

n_1 < n_2

)

\implies

bends towards normal (

\theta_2 < \theta_1

). Denser to rarer (

n_1 > n_2

)

\implies

bends away from normal (

\theta_2 > \theta_1

Critical Angle (for TIR): —

\sin \theta_c = n_{\text{rarer}}/n_{\text{denser}}

(occurs when light goes from denser to rarer medium).

Apparent Depth: —

n = \text{Real Depth} / \text{Apparent Depth}

(for normal viewing).

Unchanged Property: — Frequency (

f

) remains constant during refraction.

Changed Properties: — Speed (

v

), Wavelength (

\lambda

), Direction (unless

\theta_1 = 0^circ

N-S-T-A: No Speed Through Air. (Meaning, frequency is constant, speed changes, direction changes, angles are from normal). Or, Snell's Law: Sin Light Angles With Normals. ( $n_1 \sin \theta_1 = n_2 \sin \theta_2$ )

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