Light is subject, like any other wave, to the phenomena of reflection and refraction. In this section we will study the reflection of light and the refraction of light using the ray approach.
When a ray of light propagates in a transparent medium and reaches a separation surface with another, also transparent, a part continues to propagate in the same medium, that is, it reflects. Another part passes to the other medium, that is, it is refracted. In this image the beam of light starts from the focus on the bottom right. When it reaches the surface of the liquid, in addition to continuing its way up through the refracting air, it "bounces", that is, it reflects back down again.
The reflection of light is the change of direction of the light rays that occurs in the same medium after striking the surface of a different medium. It is governed by two principles or laws of reflection:
- The incident ray, the reflected and the normal to the surface at the point of incidence are in the same plane
- The angle of the incident ray i and that of reflection r are equal
The angle formed by the incident ray and the reflected one with the normal to the separation surface (in red color) is the same.
In the reflection does not change the speed of light v, nor its frequency f, nor its wavelength λ.
Considering the irregularities that can exist in the reflection surface, we can distinguish two types of reflections of light:
- Specular reflection: It occurs when the irregularities of the medium are small compared to the wavelength of the incident light and several rays are projected on it.
- Diffuse reflection: It occurs when the irregularities of the medium are of an order of magnitude comparable to the size of the wavelength of the incident light and several rays are projected on this.
Specular and diffuse reflection
On the left, the specular reflection in which the rays remain parallel after reflection occurs. On the right, the diffuse reflection where the rays intersect with each other in all directions.
Refraction
The refraction of light is the change of direction of the light rays that happens after passing these from one medium to another in which the light propagates with different speed. It is governed by two principles or laws of refraction:
The incident ray, the refracted and the normal to the surface at the point of incidence are in the same plane
Snell's law of refraction, which marks the relationship between the angle of incidence i, the refraction r, and the absolute refractive indices of light in media 1 and 2, n1 and n2, according to:
sin (i) sin (r) = n2n1
Refraction
The refraction of light occurs when it passes from a transparent medium with a certain index of refraction to another, also transparent, with a different one. Observe, in the image on the left, that when the velocity of propagation in the new medium is lower, and therefore the refractive index is higher, the ray approaches normal. In the image on the right we see the opposite case, in which the ray moves away from the normal one.
In refraction the frequency of the light f does not change, since it depends on the source, but in doing so its velocity v, its wavelength λ must also change. Since the color with which we perceive light depends on the frequency, it does not change when changing medium.
In refraction the frequency of the light f does not change, since it depends on the source, but in doing so its velocity v, its wavelength λ must also change. Since the color with which we perceive light depends on the frequency, it does not change when changing medium.
Reference
- FISICALAB. 2013. Fernández, José. [En línea] 04 de 2013. [Citado el: 10 de 05 de 2018.] https://www.fisicalab.com/apartado/reflexion-refraccion-luz#contenidos.
- Wikilibros. 2018. Óptica, Reflexión y Refracción. [Online] 03 2, 2018. [Cited: 05 10, 2018.] https://es.wikibooks.org/wiki/F%C3%ADsica/%C3%93ptica/Reflexi%C3%B3n_y_refracci%C3%B3n.
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