Refractiongiveslight is the phenomenon that consists in the change of propagation speed of the electromagnetic wave when it crosses different optical media. During the refraction, the length of wave of light changes, while your frequency remains constant. The refraction may or may not be accompanied by a changeatdirection of the propagation of light.
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Introduction to light refraction
THE refraction occurs when light crosses the interface between two optical and transparent media, like air and water. When this happens, the velocityinpropagationgiveslightchanges, since this speed depends on a characteristic of each optical medium called indexinrefractionabsolute.
The absolute refractive index is a greatnessdimensionless, that is, a quantity that has no unit of measure, calculated by the ratio between the speed of light in vacuum and the speed of light in that medium.
no - refractive index
ç – speed of light in vacuum (c ≈ 3.0.108 m/s)
v - speed of light in the middle (m/s)
The higher the refractive index of a medium, the slower the speed at which lightifspreads inside it, in other words, we say that the middle is more refringent. Since there is no optical medium in which light travels faster than in a vacuum, the absolute refractive index is always greater than or equal to 1.
Check the refractive index of some known optical media in the table below:
optical medium |
Refractive index |
Atmospheric air (25°C) |
1,00029 |
Water (25°C) |
1,33 |
Ethyl alcohol |
1,36 |
table salt |
1,54 |
Glycerin |
1,90 |
Acrylic |
1,49 |
Diamond |
2,42 |
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Light refraction and its properties
THE refractiongiveslight always occurs when the relative refractive index between two means is differentof 1. The formula for the relative refractive index is shown below, note:
no1,2 – relative refractive index of media 1 and 2;
no1 and not2 – refractive index of the light source medium and the light destination medium, respectively;
v1 and you2 – speed of propagation of light in the medium in which light emerges and immerses, respectively.
Unlike the absolute refractive index, which measures the relationship between the speed of light in that medium and the speed of light in a vacuum, the Relative refractive index measures the relationship between the speed of light propagation in the two media and, therefore, it can assume values greater or less than 1.
The change in the speed of light passing from medium 1 to medium 2 can cause the appearance of a lateral displacement of the light beam. This change happens if the light ray in question hits perpendicularly to the surface (90º), in the same direction as the straightnormal to the plan. The normal line, in turn, is used as a reference for the measurements of the incidence angles and refraction, as shown in the following figure:
θi and θr – incidence and refraction angles
no1 and not2– refractive index of media 1 and 2
Another important fact about the refraction of light concerns the dependency between the refractive index and the light frequency incident. The change in the speed of light depends, among other factors, on the "color" of the light: the higher the frequency of the light wave, the lower the absolute refractive index of the medium. That's why the white light scattered in multiple colored bands when passing through a prism: each of its components has a specific index of refraction and this causes each of them to undergo a specific change of direction. Want to better understand the different frequencies of light? Access our text: electromagnetic spectrum.
Laws of refraction
Once we know the main concepts of refraction, we can understand how the laws of refraction:
→ 1st law of refraction
THE first law of refraction states that the light raysincident and refracted, as well as the normal line, are straight coplanar, that is, they must be contained in the same plane.
→ 2nd law of refraction - Snell-Descartes Law
THE second law of refraction, also known as Snell-Descartes' law, is used to calculate the Detourangular suffered by the refracted ray of light. According to this law, the ratio between the sines of the angles of incidence and refraction is equal to the ratio between the speeds of light in the incident and refracted media, respectively. The formula for the 2nd law of refraction is shown below, note:
Solved exercises on the refraction of light
Question 1) Determine the absolute refractive index of an optical medium in which light propagates at a speed of 2.4.108 m/s.
Data: c = 3.0.108 m/s
a) 1.75
b) 1.50
c) 1.25
d) 2.50
e) 1.45
Template: Letter C. To solve the exercise, let's use the absolute index of refraction formula:
Question 2) A ray of light that comes from a medium in which its propagation speed is 1.5.108 m/s falls on the interface of another optical medium, in which light propagates at a speed of 2.0.108 m/s. Calculate the relative refractive index between these optical media.
a) 1.33
b) 1.40
c) 0.72
d) 2.57
e) 0.63
Template: Letter a. We will use the relative refractive index formula:
By Rafael Hellerbrock
Physics teacher
Would you like to reference this text in a school or academic work? Look:
HELERBROCK, Rafael. "Light refraction"; Brazil School. Available in: https://brasilescola.uol.com.br/fisica/a-refracao-luz.htm. Accessed on June 27, 2021.
