Light and transparent materials

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).

Opaque materials

Materials that _______ light without reemission and thus allow no light through them are opaque. Wood, stone, and people are ________. In opaque materials, any coordinated vibrations given by light to the atoms and molecules are turned into random _______ ______ – that is, into internal energy. The materials become slightly _______.

Metals are also opaque. Interestingly enough, in metals, the outer electrons of atoms are not bound to any _________ ____. They are free to wander with very little restraint throughout the ________. That’s why metals ________ electricity and heat so well. When light shines on metal and sets these free electrons into vibration, their energy does not ‘spring’ from atom to atom in the material but is reemitted as ________ _______. This reemitted light is seen as a _________. That’s why metals are shiny.

Our atmosphere is _________ to visible light and some infrared, but fortunately, almost opaque to high-frequency ___________ ______. The small amount of ultraviolet that does get through is responsible for _________. If it all got through, we wouldn’t dare go out in the sun without protection. Clouds are __________ to ultraviolet, which is why you can get a sunburn on a cloudy day. Ultraviolet also _________ from sand and water, which is why you can sometimes get a sunburn while in the shade of a beach umbrella.

Exercise 13. Complete the table using the information from the texts.

Type of materials	Definition	Examples
Transparent materials
Opaque materials

Writing

Exercise 14. Translate the sentences into English using active vocabulary and paying attention to translation of Participle (words in italics).

Першим великим успіхом у вивченні природи світла було вимірювання швидкості світла. Вперше це здійснив у 1676 р. датський астроном О.Ремер, спостерігаючи затемнення супутника Юпітера Іо. Ремер порівняв результати двох спостережень затемнення, виконаних з інтервалом півроку. Одне виконувалось в той момент, коли відстань між Юпітером і Землею була мінімальною, а друге – коли ця відстань була максимальною. Ремер помітив, що у положенні 2, коли через шість місяців Земля переходить на протилежну частину своєї орбіти, затемнення починається на 22 хв пізніше, ніж це було тоді, коли Земля знаходилась у положенні 1. Знаючи час запізнення затемнення і діаметр орбіти Землі, Ремер знайшов, що швидкість світла перевищує 200 000 км/с. Більш точні виміри показують, що швидкість світла дорівнює 299 792км/с або приблизно 300 000 км/с.