I have observed in a book that a light ray reflects during refraction, which is a partial reflection, but when does that occur? Is it related to the propagation of light from a greater to lesser refractive medium or from lesser to greater refractive medium?
[Physics] Reflection during refraction
opticsreflectionrefraction
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Leaving aside total internal reflection for now, whenever light crosses a refractive index boundary there is always some reflection and some refraction.
Refraction is easily understood. In an earlier question you established that for any refractive index > 1 the speed of light in the medium $v$ is slower than $c$. However the frequency $f$ of the light remains the same. Since the wavelength $\lambda$ of the light is given by $\lambda = v/f$ this means the wavelength of the light decreases.
As this diagram (shamelessly cribbed by Googling) shows, if the wavelength decreases the direction of the light ray has to change. The change of direction is called refraction.
For reflection, I can't think of a simple diagrammatic way to show why there is always reflection. When we're calculating the reflection we use the principle that the electric field of the light must be continuous at the boundary. If you add up the field on the two sides of the bounday you find that whenever there is refraction there is always some reflection to balance it out.
Total internal reflection only occurs when light is passing from a higher refractive index medium to a lower refractive index medium. The easiest way to see why it happens is described below.
In the diagram above Snell's law tells us that:
$$ \sin\theta_{air} = \frac{n_{water}}{n_{air}} \space \sin\theta_{water} $$
As light passes from the water into the air the light ray is always bent away from the normal and towards the surface of the water. The problem is that there is a value of $\theta_{water}$ at which the light is bent right round until it's parallel with the surface of the water i.e. $\sin\theta_{air} = 1$. This happens when:
$$ \frac{n_{water}}{n_{air}} \space \sin\theta_{water} > 1 $$
For values of $\theta_{water}$ greater than this critical value no light can escape from the water so all the light is reflected back into the water. This is the total internal reflection.
The corpuscular model says that light is composed of tiny discrete particles.
It can explain reflection if we assume that the particles are so small that they're very unlikely to collide with each other, or that they don't interact with each other for some other reason.
For example, if the corpuscles in a beam of light take up only one trillionth of the volume of the beam, then in a light beam reflecting back on itself there will be hardly any collisions. This would also explain why two beams of light can pass through each other without interference. (If the theory had held up over time, we'd probably have experiments designed to demonstrate these rare collisions.)
Or we could consistently assume that the corpuscles interact with ordinary matter, but can freely pass through each other.
The wave model can also explain reflection. Both sound waves and waves on the surface of a body of water can reflect off hard surfaces; the reflected waves pass through the incoming waves without being disrupted.
Best Answer
Everything is about electromagnetic waves.
At the border of medium one of the components of electric field have to be continuus and other component of electric displacement field. Such calculations leads to Fresnel equations