Optical Phenomena and Properties of Materials

Textbook chapter 11. Pages 11-1 to 11-8

1.Transmission and Scattering of Light

Light is a form of (electromagnetic) radiation. As with other radiation, when light strikes the surface of an object, some of it bounces back (we say it is reflected) and some of it passes through (we say it is transmitted).

In both cases, changes may occur in the properties of the reflected or transmitted beam. For instance, it may not be as bright or it may become polarised.

Objects that transmit some light are said to be translucent. Think, for example, of lampshades, mist and frosted light bulbs. The more light they transmit, the more transparent they are – transparent objects transmit all of the light that strikes them.

Objects that reflect all of the light are said to be opaque. The mirrors that we are familiar with are typical examples, as are metals, wood, etc. Almost anything that is thick enough will be opaque.

Scattering of Light

The deflection of radiation is called scattering – this occurs frequently, especially when the surface struck is not a plane surface, but is uneven. Both the reflected ray and the transmitted ray may be scattered.

This is not to be confused with diffusing, which is what happens when the beam spreads out and its intensity is diminished.

Blue Sky

The sky looks blue because of what happens to the sunlight that passes through the atmosphere to us.

Visible light has wavelengths in the range of about 350nm to 750nm. As this part of the sun’s radiation comes into the Earth’s atmosphere, some of it is scattered by air molecules.

Most of the scattering (30-40%) takes place with the photons of higher energy, that is, it occurs at the higher frequencies or shorter wavelengths (blue/violet) – almost none takes place at the red end of the spectrum. Solar radiation is most intense in the blue region (about 500nm) and, because most of the scattering occurs here, the sky appears blue.

From the Moon or from spacecraft, the sky is black because the Moon has no atmosphere.

Sunset: the sky can appear red or orange at sunset as a result of more scattering occurring. This happens because the light has to travel through more of the atmosphere to get to the observer. In addition the atmosphere has more dust in it at lower levels and this results in a greater scattering of the longer wavelengths..

Energy of Light

Under the topic of Electromagnetic Radiation we looked at many types of radiation, e.g. infra-red, ultraviolet, etc. You will know that these different radiations have different frequencies (and therefore different energies and wavelengths.)

Although usually expressed in joules the energy of photons is often expressed as electron volts (eV). An electron volt is a unit of energy equal to the work done by an electron accelerated through a potential difference of 1 volt. (1 electron volt = 1,60 × 10-19 joules)

So, since visible light is electromagnetic, it has a range of energies associated with it. Blue light has a frequency of about 6.25 x 1014Hz. The energy of a photon of blue light can be calculated from the formula you have used before, E = hf, where h is Planck’s constant, which is 6.63 x 10-34J.s.

E = hf = 6.63 x 10-34 x 6.25 x 1014 = 4.14 x 10-19J

e.g.What is the wavelength of light where each photon has an energy of 6,64 x 10-19 J?

Photo-electric Effect(Page 11-1 to 11-5)

The emission of electrons from the surface of a material following the absorption of electromagnetic radiation.

When light or other electromagnetic radiation falls on certain metals or semiconductors, they release or eject electrons. But the light must have a certain minimum frequency (called the threshold frequency), and hence energy. The photoelectric effect occurs when radiation (usually light) of sufficient energy causes electrons to be ejected from a metal or semiconductor.

Remember that a photon is the elementary particle of electromagnetic radiation, a quantum or package of electromagnetic energy. The fact that the energy in light comes in packages or quanta is established by the photoelectric effect.

Einstein explained it like this: he reasoned that an electron is not released as a result of the cumulative effect of photons striking the material, but only when a single photon has enough energy (called the threshold frequency or Work Function) to dislodge the electron from its atom. No matter how many photons strike the metal, if no single one has enough energy, the forces keeping the electrons in place will prevent ejection.

For example, a photon of red light does not have the energy to remove an electron from a potassium atom. But a photon with (at least) the threshold energy will dislodge it. On striking the metal, the photon’s quantum of energy E = hf is transferred to the electron – it is emitted, and the photon no longer exists.

Thus the photo-electric effect supports the particle theory of light.

When doing calculations involving the photo electric effect, we use:

Energy of photon = work function + kinetic energy of electron

E = W + ½ mv2

E is the energy of the photon given by the formula E = hf
W is the work function of the metal, given by the formula E = hfthreshold where fthreshold is the threshold frequency of the metal
½ mv2 is the kinetic energy given to the ejected electron. It is zero if E = W. It is the energy that is in excess.

Worked Example:

Calculate the energy of a photon of red light of wavelength 7,5 x 10-7m.
The work function of a particular metal is 1,60 x 10-19J. Calculate the kinetic energy of an electron ejected from the metal when it is illuminated by red light.

Answers:

Activity 1The Photo-electric Effect

1.How does the photo-electric effect illustrate the particle nature of light?

2.The frequency of a photon of light is 1,6 x 1015 Hz. It is shone onto nickel, which has a work function of 8 x 10-19J. Calculate the kinetic energy of the ejected electron.

3.The photo-electric threshold frequency of copper is 9,4 x 1014 Hz.

(a)What is the work function of copper?

(b)With what maximum kinetic energy will electrons be ejected when light of frequency 2,0 x 1015 Hz is shone onto copper?

4.The work function of aluminium is 6,8 x 10-19J.

(a)What is the threshold frequency?

(b)Will light of wavelength 3 x 10-7m be able to eject photo-electrons from aluminium? Give a reason.

THE DUALITY OF LIGHT

The wave/particle duality principle of quantum physics holds that matter and light exhibit the behaviours of both waves and particles, depending upon the circumstances of the experiment. It is a complex topic, but among the most intriguing in physics.

For example:

light acts as a wave in refraction, diffraction and interference but

light acts as a particle is the photoelectric effect, emission and absorption spectra.

Emission and Absorption Spectra(pages 11-6 and 11-7)

Absorption Spectra

Atoms will only absorb energy when the size of the ‘energy package’ or quantum is just right to promote or excite electrons from lower to higher levels in the atom, no more and no less. Thus not all of the available energies will be absorbed from the range on offer, which are often those of visible and UV light. If the remnant of energies is examined (i.e. the absorption spectrum) it is seen as a rainbow of colours with gaps. The gaps are the missing energy values, energies that have been absorbed from the continuum.

Emission Spectra

The spectra that arise when energy is emitted from excited atoms are remarkable – these are atomic emission spectra. They consist of a series of lines, a unique fingerprint for each element, that correspond to electron transitions from higher to lower levels in the atoms of the element. They are a kind of inverse of absorption spectra.

They can be produced and investigated by

sealing the gaseous form of an element into a glass tube (called a discharge tube) at very low pressure.
applying a high voltage across two electrodes embedded in the glass.
observing the light from the electric discharge or arc that results (use a diffraction grating).

An atomic emission spectra is the single most important piece of evidence for the quantisation of energy levels. They point to the existence of discrete energy levels

clustered together in shells around the nucleus,
of similar composition from element to element, yet
significantly different to the extent that each element can be identified by the particular pattern of its spectral lines.

Emission spectra in the different regions were observed and studied. Thus, for example, a spectrum with the prominent lines in the visible and near UV region (the Balmer series), like the one below, would identify an element under examination as hydrogen.

Each line corresponds to a particular electron transition from a higher to a lower energy level. In this case, the Balmer series, all of the transitions end at the second energy level, n = 2. The Lyman series, in the UV region, results from transitions ending at the lowest level, n = 1.