Tag Archives: light

How does a photocopier work?

DADF_(Canon_IR6000)We all know the difference between an insulating and a conductor. Despite that there are also materials which, depending on their conditions, may change their main inner characteristics: photoconductors, for example, are insulating substances which, after absorbing light, turn into conductors. This feature is exploited by a procedure known as xerography, which is at the base of copying machines and laser printers. The working principle of photocopiers consists in creating an electric image of the to-be-copied document over the photoconductor. Some colored pigment particles (toner) are layered on the electric image in order to get stuck to a blank sheet of paper, reproducing the original document.

A thin layer of photoconducting material is applied to a grounded metallic belt.


  1. The free photoconducting surface is electron sprayed by a metallic wire, as if a nebulizer covered it with paint. The electrons polarize the photoconductor and get stuck to it: the effect of the polarization is to form a collection of negative charges on the opposite face of the photoconductor, in contact with the metallic belt. This negative charge attracts a correspondent positive charge which gathers on the upper face of the belt.
  2. At this point, the document is lightened up and its image is projected over the photoconductor. Its exposed areas become conductive and, as this surface has a smaller potential level compared to the ground, the electrons scattered over these regions are immediately grounded. The shadowed areas, instead, keep their charge: over the free photoconductor surface a negative charged copied image is created (whilst a correspondent positively charged picture is kept on the surface facing the metallic belt).
  3. It is now necessary to transfer onto the paper the charged image. By a special brush device, some toner particles (plastic insulating dust containing colored pigments) get positively charged and approached to the photoconductor.  The toner, attracted by charged areas (negative), gets scattered according to the original picture.
  4. In order to let toner particles detach from the photoconductor and attach to the paper, it is necessary to get rid of the electrostatic attraction. The photoconductor gets lightened again and the charged image gets erased. The positive toner particles are kept on the photoconducting layer, in correspondence to the original picture.
  5. After that a previously negatively charged sheet of paper is pressed over the photoconducting layer. The toner particles gets attracted without modifying the original drawing.
  6. The final image gets eventually pressed whilst the toner melted by proper heating. Before a new copy the photoconductor is lightened for a third time to wipe the residual electrons and cleaned, in the very end, from the not transferred toner.

That’s it! Cool, isn’t it?


by Francesco Pochetti

Why is the sky blue?

It is easy to see that the sky is blue.

Have you ever wondered why? A lot of other smart people have, too. And it took a long time to figure it out!

Let’s compose the puzzle starting from the atmosphere. The atmosphere is the mixture of gas molecules and other materials surrounding the earth. It is made mostly of the gases nitrogen (78%), and oxygen (21%). Argon gas and water (in the form of vapor, droplets and ice crystals) are the next most common things. There are also small amounts of other gases, plus many small solid particles, like dust, soot and ashes, pollen, and salt from the oceans.

It is with all this stuff that the light coming from the sun interacts before reaching the Earth’s surface. Well, let’s concentrate a little bit on light then!

Light is a kind of energy that radiates, or travels, in waves. Many different kinds of energy travel in waves. For example, sound is a wave of vibrating air. Light is a wave of vibrating electric and magnetic fields. nasa

The energy of the radiation depends on its wavelength (and frequency), which is the distance between the tops (crests) of the waves.  The longer the wavelength of the light the less energy it contains.

There are plenty of different “kinds of light” (infrared, visible, ultraviolet..), depending on wavelength, and all of them compose the so called electromagnetic spectrum. In particular visible light is the part of the electromagnetic spectrum that our eyes can see. Light from the sun or a light bulb may look white, but it is actually a combination of many colors. We can see the different colors of the spectrum by splitting it with a prism.


Light travels through space in a straight line as long as nothing disturbs it. As it moves through the atmosphere, it continues to go straight until it bumps into a bit of dust or a gas molecule. Then what happens to the light depends on its wavelength and the size of the thing it hits.

Dust particles and water droplets are much larger than the wavelength of visible light. When it hits these large particles, it gets reflected, or bounced off, in different directions.

Gas molecules are smaller than the wavelength of visible light. If it bumps into them, it acts differently. When an electromagnetic wave hits a gas molecule, some of it may get absorbed. After a while, visiblethe molecule releases the light in a different direction. All of the colors can be absorbed. But the lowest wavelengths (blues) are absorbed and released more often than the lower ones (reds). We say that blue light is more scattered than red light.

 As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

However, much of the shorter wavelength waves is absorbed by the gas molecules. The absorbed blue light is then scattered in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue!


by Francesco Pochetti