Rami Arieli: "The
Laser Adventure" Laser Safety Appendix, page 3
Laser Light propagation
To prevent a laser beam from entering the eye, the main
factors determining the laser beam path need to be understood:
-
As long as the beam propagates in a homogeneous
medium (such as air) it moves in a
straight line.
-
When a laser beam comes to the border surface
between two mediums, its straight path can change as a result
of one of two processes:
-
Reflection.
-
Refraction.
-
Reflection
There are two types of reflections:
-
Specular (ordered) Reflection
- Such as reflection from a mirror, or other shiny surfaces. In this case
the reflection law is valid, meaning:
The angle of reflection is the same as the incidence angle (as can be seen
in figure 1).
Figure 1: Specular Reflection.
-
Diffusive (scattered) Reflectance
- In diffusive reflectance the light is scattered to all directions, and
loose the property of directionality (see figure 2).
Figure 2: Diffusive Reflection.
It is evident that in diffusive reflectance the reflected light has
lower power density, and the risk hazards are much lower.
-
Refraction - When
a beam of light comes from one transparent medium to the border surface
with another transparent medium, its direction changes according to Snell,s
Law of Refraction:
n1sinq1 = n2sinq2
You can play with an interactive simulation to see the behavior of a light
beam according to this law, by clicking on Snell
Applet.
The refraction process is responsible for focusing light in a lens,
and the direction changes of light beams passing through prisms.