
Snell's Law, also known as the Law of Refraction, describes the relationship between the angles of incidence and refraction when light passes through two different media, such as water, glass, or air. It was discovered by the Persian scientist Ibn Sahl in 984 and later by Dutch mathematician Willebrord Snell in 1621. The law is used to calculate the refractive index of a substance and predict the amount of bend when light travels from one medium to another. However, it is important to note that Snell's Law has limitations and cannot determine the angle in certain cases, such as when the angle of incidence is too large, resulting in total internal reflection.
| Characteristics | Values |
|---|---|
| Discovery | Discovered by Persian scientist Ibn Sahl in 984 |
| Named after Willobrord Snell, who discovered the law in 1621 | |
| Definition | The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for the light of a given colour and for the given pair of media |
| Formula | n1/n2 = sin α2/sin α1 |
| n₁sin(θ₁) = n₂sin(θ₂) | |
| Application | Used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air |
| Used to calculate the refractive index of liquids | |
| Used to understand refraction and lenses and their applications | |
| Limitations | Does not account for cases where the sine of the angle of refraction is greater than one, leading to total internal reflection |
| Angles determined by Snell's law depend on frequency or wavelength, causing dispersion in rays of mixed wavelengths |
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What You'll Learn
- Snell's Law is dependent on the medium through which light rays travel
- The law is used to calculate the refractive index of liquids
- It is related to the propagation of the wave, not the details of the wave itself
- The law is used to determine the critical angle
- It was discovered by Persian scientist Ibn Sahl in 984

Snell's Law is dependent on the medium through which light rays travel
Snell's Law, also known as the Law of Refraction, is a formula that describes the relationship between the angles of incidence and refraction when light passes through a boundary between two different media, such as water, glass, or air. This law is named after the Dutch astronomer and mathematician Willebrord Snell, who discovered it in 1621, although it was first discovered by the Persian scientist Ibn Sahl in 984.
The law is expressed as n1/n2 = sin α2/sin α1, where n1 and n2 are the refractive indices of the two media, and α1 and α2 are the angles of incidence and refraction, respectively. The refractive index of a medium is a constant number that represents how much the speed of light is reduced when it passes through that medium, and it depends on the wavelength of light. When light travels from one medium to another, it bends or refracts, and the degree of refraction is given by Snell's Law.
The dependence of Snell's Law on the medium through which light rays travel is made explicit through the use of refractive indices. These are numbers that are constant for given media and represent the speed of light in that medium. As a result, the angles determined by Snell's Law depend on the refractive indices of the media involved. For example, consider a ray of light moving from water to air with an angle of incidence of 50°. The refractive indices of water and air are approximately 1.333 and 1, respectively, so Snell's Law can be used to determine the angle of refraction.
In addition, Snell's Law is also applicable to all isotropic materials and phases of matter. This is because the law is related only to the propagation of the wave and not to the specific details of the wave itself. However, it is important to note that in many wave-propagation media, the velocity of waves changes with frequency or wavelength. This includes light propagation in most transparent substances other than a vacuum. Consequently, the angles determined by Snell's Law also depend on frequency or wavelength, causing rays of mixed wavelengths, such as white light, to spread or disperse.
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The law is used to calculate the refractive index of liquids
Snell's Law, also known as the Law of Refraction, describes the relationship between the angles of incidence and refraction when light or other waves pass through a boundary between two different isotropic media, such as water, glass, or air. It was discovered by Persian scientist Ibn Sahl in 984 and later named after Willobrord Snell, who rediscovered it in 1621.
Snell's Law is particularly useful for understanding refraction, which depends on the media through which light rays are travelling. This relationship is expressed through refractive indices, which are constant for given media. Refractive indices are fundamental physical properties of substances, and they can be used to identify a substance, confirm its purity, or measure its concentration.
In the context of liquids, refractive indices can be used to differentiate between different types of liquids and their concentrations. For example, the refractive index of water is approximately 1.33, while most solids and liquids have refractive indices above 1.3. The refractive index of liquids can be measured using refractometers, which involve shining light through a thin layer of the liquid placed between two prisms and measuring the angle of refraction or the critical angle for total internal reflection.
While Snell's Law is a valuable tool for understanding refraction, it has limitations. In some cases, when light travels from a medium with a higher refractive index to one with a lower refractive index, Snell's Law predicts that the sine of the angle of refraction should be greater than one, which is impossible. This phenomenon, known as total internal reflection, occurs when the angle of incidence is large enough. Therefore, while Snell's Law is useful for calculating refractive indices of liquids, it cannot be applied to all scenarios and has constraints when it comes to determining certain angles.
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It is related to the propagation of the wave, not the details of the wave itself
Snell's Law, also known as the Law of Refraction, describes the relationship between the angles of incidence and refraction and the refractive indices of two media. It is a formula used to describe the relationship between the angles of incidence and refraction when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air. When a light ray enters a different medium, its speed and wavelength change, causing it to bend towards or away from the normal of the two media boundaries. This change in velocity and wavelength is dependent on the medium through which the light rays are travelling.
Snell's Law is related to the propagation of the wave, not the details of the wave itself. This means that it can be applied to all isotropic materials, regardless of the phase of matter. The law allows us to predict the amount of bend when light travels from one medium to another, helping us understand how refraction works.
The refractive indices, represented as n1 and n2, are constant for given media and are used to calculate the unknown refractive index of a substance by measuring angles and applying Snell's Law. The angles of incidence and refraction, denoted as θ1 and θ2, are measured from the normal to the surface at the point of contact. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given pair of media, and this constant value is the refractive index of the second medium concerning the first.
Snell's Law is derived from Fermat’s principle, which states that light travels in the shortest path that takes the least time. It is an important concept in optics, providing insights into the path taken by a ray of light when crossing the boundary between two contacting substances.
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The law is used to determine the critical angle
Snell's Law, also known as the Law of Refraction, is a formula used to describe the relationship between the angles of incidence and refraction when light or other waves pass through two different isotropic media, such as water, glass, or air. The law was discovered by Persian scientist Ibn Sahl in 984 and was later named after Willobrord Snell, who rediscovered it in 1621.
The law is particularly useful in determining the critical angle, which is the largest possible angle of incidence that results in a refracted ray. At this angle, the refracted ray travels along the boundary between the two media. For example, consider a ray of light moving from water to air with an angle of incidence of 50°. Using Snell's Law, we can determine that the refractive indices of water and air are approximately 1.333 and 1, respectively.
The critical angle phenomenon occurs when the angle of refraction becomes 90 degrees. At this point, the light exhibits total internal reflection and is reflected back into the medium without passing through or undergoing refraction. This is particularly important in optical microscopy when attempting to image specimens with a medium other than air between the cover glass and the objective front lens.
The critical angle can be calculated using Snell's Law by rearranging the formula to make the critical angle the subject. Trigonometry is often used in this process to determine unknown angles. The refractive indices of the two media involved must be known to calculate the critical angle.
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It was discovered by Persian scientist Ibn Sahl in 984
Although Snell's Law bears the name of the Dutch astronomer and mathematician Willebrord Snell (also known as Snellius), who discovered it in 1621, it was first discovered by the Persian scientist Ibn Sahl in 984. Snell's Law, also known as the Snell-Descartes Law, the ibn-Sahl law, and the law of refraction, is a formula used to describe the relationship between the angles of incidence and refraction. It is used when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.
In his manuscript 'On Burning Mirrors and Lenses', Ibn Sahl used the law to derive lens shapes that could focus light without geometric aberration. However, it was not until over a century later, in 1021, that Alhazen came close to rediscovering the law of refraction in his 'Book of Optics'. The law was then rediscovered by Thomas Harriot in 1602, although he did not publish his findings. Snell derived a mathematically equivalent form in 1621, but his work also remained unpublished during his lifetime.
René Descartes independently derived the law in 1637, using heuristic momentum conservation arguments in terms of sines in his essay 'Dioptrique'. He used his derivation to solve a range of optical problems. However, Descartes assumed that the speed of light was infinite and that the denser the medium, the greater the speed of light. Pierre de Fermat disagreed with these assumptions and arrived at the same solution based on his principle of least time, assuming instead that the speed of light is finite and slower in a denser medium.
Snell's Law is more complicated than the law for reflection, but an understanding of refraction is necessary for discussing lenses and their applications. The law of refraction also depends on the media through which the light rays are travelling, and the angles determined by Snell's Law depend on frequency or wavelength. This means that a ray of mixed wavelengths, such as white light, will spread or disperse.
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Frequently asked questions
Snell's law only applies when light travels from one medium to another and refracts.
Snell's law describes the relationship between the angles of incidence and refraction when light passes through two different media.
The formula for Snell's law is n1sin(θ1) = n2sin(θ2), where n1 and n2 are the refractive indices of the two media, and θ1 and θ2 are the angles of incidence and refraction, respectively.
When the angle of incidence is larger than the critical angle, total internal reflection occurs, and all light is reflected from the boundary.
Snell's law applies to all isotropic materials and phases of matter because it depends only on the propagation of the wave, not its details. However, the angles determined by Snell's law depend on the frequency or wavelength of the waves.











































