LIGHT AND TRANSPARENT MATERIALS

Date: 2015-10-07; view: 401.

Reading and Listening

Exercise 11. Read and translate the text below.

Light is energy carried in an electromagnetic wave that is generated by vibrating electric charges. When light strikes matter, electrons in the matter are forced into vibrations that depends on the frequency of the light and the natural frequency of electrons in the material. Light passes through materials whose atoms absorb the energy and immediately reemit it as light. Materials that transmit light are transparent. Glass and water are transparent.

Electrons in glass have a natural frequency in ultraviolet range. Ultraviolet light increases the amplitude of the atoms' vibrations. This energy is in the form of heat. That's why glass is not transparent to ultraviolet.

But when electromagnetic wave has a lower frequency than ultraviolet, as visible light does, the electrons are forced into vibration with smaller amplitudes. The atom holds the energy for less time, with less chance of collision with neighbouring atoms and less energy is transferred as heat. The energy of the vibrating electrons is reemitted as transmitted light. The frequency of reemitted light passed from atom to atom is identical to that of the light that produced the vibration to begin with. The main difference is a slight time delay between absorption and reemission.

Infrared waves, which have frequencies lower than visible light, vibrate not only the electrons, but also the entire structure of glass. This vibration increases the internal energy of the glass and makes it warmer. Thus, glass is transparent to visible light but not to ultraviolet and infrared light.

Exercise 12. Listen to the text and fill in the gaps in it (Track 6.1).