Christian Huygens (1629-1695) was a Dutch Physicist who proposed the wave theory of light long before Maxwell's incredible discoveries. In 1678, while living in Paris (indeed, many Frenchmen seem to have studied the nature of light quite extensively), Huygens published his "Traité de la lumiere" in which he described this wave theory, and the principle named after him comes from this text.

Huygens's Principle:
Every point on a propagating wavefront serves as the source of spherical secondary wavelets, such that the wavefront at some later time is the envelope of these wavelets. If the propagating wave has a frequency, f, and is transmitted through the medium at a speed, v, then the secondary wavelets will have the same frequency and speed.

This principle is quite useful, for from it can be derived the laws of reflection and refraction [the latter often referred to as Snell's Law].

 

 

There are several types of light bulbs and they emit light through different mechanisms. I'll try to explain how incandescent light bulbs work.

A regular incandescent light bulb relies on the fact that all bodies with a temperature greater than absolute zero emit radiation. This radiation is logically named thermal radiation and the intensity of radiation emitted at a given wavelength is a function of the temperature of the body. At low temperatures a body emits very low intensity radiation, mostly of lower energy than visible light. The visible light that is emitted is of far to low intensity to be seen by human eyes. However at high temperature the intensity of visible light (and other radiation) emitted by the body increases. Depending on its specific physical properties a body may emit more or less visible light at a given temperature. Incandescent light bulbs use a thin wire called a filament (traditionally made of tungsten, perhaps currently made of a different material) as a thermal radiation emitting body. The filament has a very tiny axial diameter (it is skinny) and therefore a fairly high resistance to current. When AC current flows through the filament it is heated very rapidly to a very high temperatures (that is why incandescent light bulbs are hot) and it emits lots of thermal radiation in the visible spectrum.

 

 

This glowing effect is caused by the re-radiation or fluorescence of light at longer (visible) wavelengths from scattered ultraviolet, violet and blue light in the atmosphere. Remember that the daytime sky is blue because of the preferential scattering of blue light known as Rayleigh scattering.

Many flowers fluoresce, and also reflect in near-ultraviolet light. This is beneficial because pollinating insects can see these wavelengths. It is particularly noticeable on flowers of the Geranium family, both the wild species and the cultivated pelargoniums.

Because there is no other source of light present, the fluorescence shows up proportionately more, giving rise to the extra-luminous effect described. It is not an optical illusion; it is a real phenomenon.

 

 

The greying of the inner surfaces of incandescent bulbs is the result of gradual evaporation of tungsten from the filament while the light is on. This evaporation eventually makes the filament so thin that it burns out.

Various methods have been developed to reduce greying. Filaments of the first incandescent lamps burnt in a vacuum, but it was soon found that introducing inert gas to the bulb reduced the rate of greying. A mixture of nitrogen and argon is used today. In addition, "getters"--reactive metals such as tantalum and titanium--can be placed near the filament to attract the tungsten so that it is not deposited on the glass. Alternatively, a small amount of abrasive tungsten powder can be placed in the bulb. Shaking it occasionally will remove the grey coating from the surface of the glass.

Greying can be almost eliminated by introducing a small amount of the halogens iodine and bromine. As tungsten evaporates from the filament, it reacts with the halogens which then redeposit the tungsten on the filament. This keeps the bulb wall clean. To prevent the tungsten halides from condensing on the bulb and breaking the cycle, the temperature of the bulb wall must be at least 500 °C. This is too hot for glass bulbs, which normally operate at about 150 °C, so fused quartz (silicon dioxide) must be used instead.

Compared with ordinary incandescent lamps, quartz-halogen lamps have longer lives and maintain their light output over time. For example, a quartz-halogen lamp with a 2000-hour life will have dimmed by less than 5 per cent by the time it burns out. When an incandescent lamp with a 1000-hour life burns out, it will have dimmed by more than 15 per cent.

 

No mirror reflects 100 per cent of the light falling on it. If your mirror is very good and reflects 99 per cent of the light, after some 70 reflections only 50 per cent of the light is left, after 140 reflections only 25 per cent of the light is left, and so on until none of the light remains to travel between the two mirrors. In fact, most mirrors reflect some colours of light much better than others and some colours are absorbed better by the glass, so the multiple reflections that you see not only get darker but also more colour-distorted as they recede towards infinity.

Even with perfect reflection of all colours you could never see infinite reflections, for geometric reasons.

First, the faces of the two mirrors would need to be perfectly parallel. This is actually impossible to achieve. There will always be a slight disparity in their positions relative to one another. Hence the curving nature of such reflections, until eventually the reflection is lost "around the bend".

Secondly, even if the mirrors are perfectly reflective, perfectly parallel, and really huge, your eyes are in the middle of your head, not on its edge. Therefore, at some point, the receding and hence apparently more distant mirror images would become smaller than, and hidden behind, the first reflected image of your head. Even with a tiny camera and a giant pair of mirrors, the reducing reflection size would eventually be smaller than the first reflected image of the camera apparatus.

 

 

The reflected light from the newspaper is diffused. Where do these diffused rays go, is not sure and certain. But on those positions where the letters are imprinted, the color is generally black, so light is absorbed there. So the brain makes a distinction there - locations from where light is being reflected (in a diffused manner) and locations where light is absorbed and from where it is not coming out.

To see the image of an object in the newspaper requires a regular and strong reflection. They should collect in enough quantity after reflection to be able to give and impression to the brain. This is possible only in an extremely smooth surface, not in a rough surface like newspaper.

 

 

Yes, plane mirror can form a real image when the converging rays of the virtual object strike on the plane mirror, a real image of the object is formed.

 

The image of an object is produced only if the surface gives the regular reflection, in order to have a good collection of energy and make an impression on the brain. In the case of the plane and polished glasses, this is possible while the reflection from the ground glasses is irregular or diffused. So energy do not accumulate after reflection. So there is not a good impression reaching the brain. Hence, ground glasses do not produce any images but plane and polished glasses do.