How light breaks and reflects gold

Absorption, reflection and refraction

Author:
Publication: 26.7.2008
Origin: Sonnentaler, Berlin

If a beam of light, for example traveling through air, strikes another medium, one of three effects can occur. On the one hand, the light beam can be absorbed, for example when it hits a black object. Or it can be reflected, for example on a mirror. Finally, it can also cross over into the other medium, where its direction changes, for example when it hits a surface of water. The latter is known as refraction.

When dealing with a bundle of light (which consists of a large number of light rays), several of these three effects often occur in parallel: some of the light rays can be absorbed, some can be reflected and the rest can be refracted.

In the following we want to take a closer look at all three effects.

absorption

Observe absorption of light rays every time you look at a colored (non-white) object in sunlight. A red object absorbs all light rays except the red, a green one all light rays except the green etc. A black object absorbs almost all light. The light "disappears" and its energy is in most cases converted into internal energy (the object becomes warmer). This is also the reason why black objects get hot faster when they are exposed to the sun. White and metallic objects, on the other hand, absorb much less light, so they do not heat up quite as quickly in the sun.

reflection

A medium into which light cannot penetrate reflects all light rays that are not absorbed. A car painted red, for example, reflects red light - this is precisely what gives it its color.

The result of the reflection depends on the nature of the surface being viewed. Is this very smooth, such as B. in the case of a highly polished surface, the well-known law applies that the angle of incidence is equal to the angle of reflection, and a mirror image is obtained.

However, if the surface is rough, the incident light is reflected in all directions and one speaks of "diffuse reflection". A typical example is a sheet of white paper. It has a slightly rough surface that appears bright when lit, no matter which direction you look at it from.

Fig. 1: Reflection on a smooth and a rough surface

In order for a mirror image to appear, a surface has to be very smooth. The unevenness of the surface must be smaller than the wavelength of the reflected light, i.e. H. significantly smaller than approx. a thousandth of a millimeter! This can only be achieved with manual techniques through elaborate polishing, which is why mirrors were expensive luxury items until a few centuries ago. It was only when smooth glass surfaces (which were easier to manufacture) could be covered with thin, reflective metal layers that the mirror became a cheap mass product.

refraction

Refraction occurs when light passes from a more or less translucent material (glass, water, air, etc.) into another (translucent) material. When moving from one medium to the other, for example from air to water, the direction of the light rays changes.

Everyone has seen a straw in a glass of water at some point. It looks like the straw is kinked where it goes into the water. However, it is by no means kinked, as you can easily check when you pull it out of the water again. In fact, it is not the straw that is bent, but the rays of light that come from the part of the straw that is in the water. They change their direction as they move from water to air, so that the straw appears kinked. This is because these rays of light hit our eyes at a different angle than those that come from the airborne part of the straw.

To describe this effect, each medium is given a so-called refractive index n to. When a light beam from a medium with a low refractive index n1 into one with a larger refractive index n2 penetrates, it is bent towards the vertical (normal) on the boundary surface of the media (α > β), in the opposite way it is bent away from the vertical.

Fig. 2: Change of direction of a light beam when passing from one medium to another

The refractive index is directly related to the speed of light in a medium: the larger it is, the slower light propagates in this medium. In a vacuum, the refractive index is smallest (exactly 1) and the speed of light is greatest (approx. 300,000 km / s). The refractive index of air is almost as small as that of a vacuum (approx.1,0003), so light spreads almost as quickly in air as in a vacuum. The refractive index of water, on the other hand, is around 1.33 and the speed of light in water is reduced by the corresponding factor, there it is only around 225,000 km / s. Glass even has a refractive index of around 1.5 (with different types of glass having very different refractive indices).

Refraction plays a central role in many optical devices. For example, it is responsible for how lenses work. When entering and exiting the lens, the light is refracted. In combination with a clever shaping of the surface of the lens, you can direct light rays where you want them.

The refractive index of a medium can also depend on the wavelength (color) of the light. As a result, the different colors of light are "refracted" to different degrees when they penetrate the medium. This effect is used, for example, with the prism to split light into its color components. But the consequences of this effect can also sometimes be observed in nature in the form of a rainbow. The rainbow is created when sunlight falls on droplets of water floating in the air. When entering the droplets, the different color components of the white sunlight are refracted to different degrees. The light that emerges from the droplets after reflection on the back of the droplets has a slightly different exit angle depending on the color.

Summary

The effects listed above usually occur together. Pure absorption is only observed with deep black objects, all others reflect at least part of the incident light (e.g. all objects that appear colored to us).

But refraction can also occur at the same time as reflection (and absorption). This can be seen on a pane of glass: it is not completely translucent, but always reflects part of the incident light so that you can see a mirror image (fortunately, because otherwise you would run into glass doors much more often). Typically around 4% of the incident light is reflected on glass panes. In some cases this can be very annoying: In glasses (where the refraction in the lens is important), the surfaces of the lenses are often specially "anti-reflective" in order to reduce reflections.

Last update: 11.2.2014

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