A known
1. Light is a wave;
2. Light travels at different speeds in different media.
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As shown in the figure below, light is a medium electromagnetic wave. Taking the cosine wave as an example, take the maximum point of the cosine wave as a reference to establish a plane wave (the wave front is a plane). It can be clearly seen that the direction of light propagation is perpendicular to the wave surface (replaced by black lines) (propagating to the right).
It is not difficult to see that light propagates along a straight line in a homogeneous medium (or along the normal direction of the wave surface line in a non-uniform medium).
Assuming that a beam of light enters the glass from a vacuum, the relative refractive index of the vacuum is 1, and the relative refractive index of the glass is greater than 1 (about 1.5).
The white background in the figure represents the vacuum environment, and the light green background represents the glass material. Due to the slow propagation rate of light in glass, when light enters the glass material, the propagation speed of light decreases (it can be understood that the distance traveled by the next wave peak is shorter in the same time), and the wavelength decreases.
Since the light beam has a certain angle with respect to the interface between the two media, the moment when the same wavefront enters the glass material is different. Although the optical path d of light transmission is the same, the real distance (displacement) L of propagation is different (d = L*n , where n is the refractive index of the substance)
so the displacement of the earliest entry into the glass material is L = d/n; the longest propagation time in the glass has the shortest displacement, and the latest entry into the glass material has the shortest propagation time in the glass , the maximum displacement is L = d; as shown in the figure below:
At this time, the wavefront angle of the light wave changes, and the propagation direction of the light changes. As shown below.
Derivation of the law of refraction
(For a more basic theoretical derivation, refer to the physical optics book, which is derived from Maxwell's equations)
As shown below. According to the relationship of equal optical paths, the propagation path of the light on the far right is equal to that of the light on the far left, assuming that the width of the beam on the interface is x.
but
Generalized to the general form:
Likewise, the reflection of light is only a special case when the index of refraction is -1.
As for why the propagation direction of the beam along the normal direction of the wavefront can be obtained through Huygens' principle, that is, the diffraction effect;