Experiment 14

REFLECTION & REFRACTION

OF LIGHT

PRELAB

PURPOSE

To study the laws of reflection and refraction of light using a ray table. The law of refraction is often referred to as Snell's Law.

EQUIPMENT   Pasco® magnetic optical bench with multi-ray light source and rotatable table, slit mask, slit plate, cylindrical lens, ray optics mirror, prisms, pencil, straight edge.

RELEVANT EQUATIONS

Law of Reflection:                                                       qr = qi

Index of Refraction: 

Law of Refraction: 

  DISCUSSION

Whenever light propagates in a system where the dimensions are much greater than the wavelength of light, an accurate description can be made by assuming that it travels along straight line paths called rays. The only way that the straightness of a light ray can be interrupted is through the processes of reflection and refraction.

A light ray that is incident upon a reflecting surface obeys a very simple relationship. The law of reflection states that the angle of reflection qris equal to the angle of incidence, qi. In applying this law, the two angles are measured from the normal to the surface at the point of reflection.

When light passes from one medium to another, its speed changes. In fact, as an electromagnetic wave, light travels fastest in vacuum---matter slows it down! We can characterize the speed of light in a particular medium with the index of refraction, nm. The index of refraction in a medium is defined as the ratio between the speed of light in vacuum and the speed of light in the medium.

                                                                                      (1)
Since c is the fastest rate at which light can travel, the index of refraction will always be equal to or greater than one.

This change of speed has the effect of bending (refracting) the ray as it passes from one medium to the other. If the ray enters a medium where it slows down, such as in the case of entering glass from the air, then it will be refracted toward the normal. On the other hand, if the ray enters a medium where it travels faster, such as in the case of entering the air from glass, then it will be refracted away from the normal. This simple qualitative relationship can be made more precise by stating the law of refraction, which was first formulated by Snell:

                                                                              (2)
These two laws are illustrated in Fig. 14-1 below:

Figure 14-1: Reflection and Refraction of Light