The premise of light propagating along a straight line is that it is in the same homogeneous medium. The straight-line propagation of light is not only in a uniform medium, but also in the same medium. It can be simply called straight-line propagation of light, not straight-line propagation of light. Light will be refracted at the contact surface of two homogeneous media, and then the light will not travel in a straight line.
Wave theory is used to explain the propagation of light: every point in the propagation process is a secondary wave point source, which emits spherical waves. When the light source surface (the area with limited radius) is much larger than the wavelength, the result is approximately a cylinder with the same area and the same direction, that is, linear propagation and actual divergence (except ideal laser). For example, the light emitted by a flashlight has obvious divergence.
The stronger the brightness of light, the closer it is to the illumination reference, the better the monochromaticity of light and the less obvious the divergence. When the radius of the light source is equal to the wavelength, the approximate condition of integration is not established, and the integration result tends to spherical wave, that is, diffraction. Light travels in a straight line (in a uniform medium), but when it meets another medium (in a uniform medium), its direction will change, and it will still travel in a straight line after the change. However, in inhomogeneous media, light usually propagates in a curve.
The propagation path of the light can be determined by Fermat principle. Light travels in all directions, front, back, left, right, up and down. The brighter the light, the less obvious it is. When the light is dark, the light from the emitter to the illumination reference will expand, and the farther the distance, the greater the diffusion, from the initial shape to the disappearance.
When the luminous body is at zero distance from the illumination reference object, the shape of light is the real shape and size of the luminous body, so the propagation direction of light is related to the brightness of light and the distance between light and the illumination reference object.
There are three laws of light propagation in geometrical optics:
1, the straight-line propagation law of light always propagates along a straight line in the same uniform medium; The speed of light propagating in a straight line in the same uniform medium is constant.
2. The Law of Independent Propagation of Light When two beams of light meet in the process of propagation, they do not interfere with each other, but continue to propagate according to their own paths. When two beams of light converge at the same point, the light energy at that point simply adds up.
3. Laws of reflection and refraction of light. When light meets the interface of two different media on the way, some light is reflected and some light is refracted. Reflected light follows the law of reflection and refracted light follows the law of refraction.