What Is a Withdrawal Reflex?

The "retroreflection" of light is also called re-reflection, which is commonly called back reflection. It is different from the well-known specular reflection and diffuse reflection. It can make the light reflect back along the original path. This phenomenon is widely used in traffic. The roads of the mountainous mountain roads are generally inlaid with some glass balls at equal intervals. When the lights of the car running at night are illuminated, it reminds the driver to pay attention.

The "retroreflection" of light is also called re-reflection, commonly known as retroreflection.

Back Reflection Geometry

If the radius of the small glass sphere is R and the refractive index is 1.73, as shown in the figure, a beam of parallel light is shining on the small glass sphere along the diameter AB direction. The incident light of the test water far from AB is refracted-reflected-refracted After shooting again, return in the original direction, that is, to achieve "reverse reflection".

Only after the incident light is refracted, the light that hits point B is reflected and refracted. After it is emitted, it can return in the original direction, that is, "retroreflection" is achieved. The optical path is as shown in the figure.

From the geometric relationship: i = 2 <r.

It can be solved from the above two formulas: i = 30 °, r = 60 °.

Set the distance from the incident light ray that can achieve "retroreflection" to AB in the original direction to h, then we can know from the geometric relationship

Rear reflection bicycle tail light

Bicycle taillights use the principle of retroreflection of light. The inside of bicycle taillights is a red plexiglass sheet with a number of small protruding triangles (or quadrangles) on the surface. The surfaces of these small triangles (or quadrilaterals) are perpendicular to the surfaces of adjacent small triangles (or quadrilaterals) (see Figure 1). Its function is: one can make the light coming from any direction "reverse back". The surfaces of the small triangles (or quadrilaterals) in the tail light and the adjacent small triangles (or quadrilaterals) are perpendicular to each other, which is equivalent to placing the two plane mirrors M ₁ and M ₂ perpendicular to each other. When a light beam hits one of the plane mirrors (such as M _l ), it is reflected by the other plane mirror (M ₂ ) for secondary reflection. From the law of light reflection and geometric knowledge, it can be seen that the light reflected after the second reflection must be antiparallel to the light emitted by the original person. This special design makes it possible to shoot people from any direction behind the bicycle and reflect it back along the original path when driving at night, so that people behind the car can clearly see the red light reflected by the tail lights, so that The driving situation of the bicycle prevents the danger of collision. The second is the use of red plexiglass for bicycle taillights to reflect red light. Because in visible light, the red light has the longest wavelength, has strong penetrating power, and the light travels far. ^[1]

Back reflection traffic sign symbol

In addition, the various traffic sign signs on both sides of the highway also make use of the principle of retroreflective light, but it is not composed of a flat mirror but a traffic sign substrate and a reflective film attached to it (see figure). The reflective film is composed of a transparent protective layer 1, a single layer of glass microbeads 2, a reflective layer 3, an adhesive layer 4, and the like. During the daytime, drivers can clearly see such traffic signs through the diffuse reflection of sunlight. At night, the lights from a distance toward the reflective film are refracted by glass beads and converge on the reflective surface behind it. Just return in the original direction, so that the car driving at night can use its own light to shine on the road traffic sign, and see the road traffic sign in the dark from the retroreflected light.

The transparent protective layer of this retro-reflective film is usually made of white, yellow, red, orange, green, blue and other colors. Under the illumination of car lights, it can show various traffic signs with beautiful colors. Now such signs have been widely used in railways, car licenses, engineering, commercial advertising, life-saving appliances and other fields. ^[2]