1. Electromagnetic Spectrum Optical &
Properties of Solids (Metals)
Presented By
Dr Azhar Abbas Zaidi
Dr A Waheed Anwar DFT Research Lab,
UET, Lahore
2. Electromagnetic Spectrum
• Wave
– A wave is simply defined as a travelling
disturbance in a medium, however
– A wave is accurately defined as a mechanism to
transfer energy from one place to another.
3. Electromagnetic Spectrum
• Electromagnetic Wave
– An electromagnetic wave is generated by a varying
electric field (with an associated perpendicular
magnetic field). A unique amount of energy of
such waves is also known as photon.
– Photon Energy=hf=hc/λ
– They travel at 3x10^8 m/s in vacuum.
– They are transverse in nature, and hence can be
polarized.
– In a denser medium, they are slowed down and
face a change in wavelength and speed. However,
their frequency remains the same.
– Speed=wavelength x frequency
4.
5. Electromagnetic Spectrum
• A spectrum is defined as a group of wavelengths.
A single unique wavelength is called as a spectral
line.
• The group of wavelengths reaching the earth
from the Sun through our atmosphere is called
Solar Spectrum. Solar Spectrum consists of
– Ultraviolet radiation (wavelengths shorter than 400 nm)*
– Visible light (wavelengths in the range of 400 nm (Violet) to 700
nm (Red).
– Infrared radiation (wavelengths larger than 700 nm)*
6. Electromagnetic Spectrum
There are also radiations with wavelength still
shorter than those of UV, and wavelengths
longer than IR.
The entire family of electromagnetic radiation is
called Electromagnetic spectrum as shown
below.
11. Family of EM Radiation
• Radio: Your radio captures radio waves emitted by radio stations, bringing
your favorite tunes. Radio waves are also emitted by stars and gases in
space.
• Microwave: Microwave radiation will cook your popcorn in just a few
minutes, but is also used by astronomers to learn about the structure of
nearby galaxies.
• Infrared: Night vision goggles pick up the infrared light emitted by our skin
and objects with heat. In space, infrared light helps us map
the dust between stars.
• Visible: Our eyes detect visible light. Fireflies, light bulbs, and stars all emit
visible light.
• Ultraviolet: Ultraviolet radiation is emitted by the Sun and are the reason
skin tans and burns. "Hot" objects in space emit UV radiation as well.
• X-ray: A dentist uses X-rays to image your teeth, and airport security uses
them to see through your bag. Hot gases in the Universe also emit X-rays.
• Gamma ray: Doctors use gamma-ray imaging to see inside your body. The
biggest gamma-ray generator of all is the Universe.
12. Visible Light
• Light that can be detected by the human eye has
wavelengths in the range λ ~ 450nm to 650nm &
is called visible light:
• The human eye can detect light of many different
colors.
• Each color is detected with different efficiency.
3.1eV 1.8eV
Efficiency,
100%
400nm 600nm 700nm
500nm
14. What happens during the photon
absorption process?
Photons interact with the lattice
Photons interact with defects
Photons interact with valence electrons
Photons interact with …..
15. Optical properties
Definition:
• Optical property of a material is defined as its
interaction with electro-magnetic radiation in
the visible range.
• Electromagnetic spectrum of radiation spans
the wide range from γ-rays with wavelength as
10-12 m, through x-rays, ultraviolet, visible,
infrared, and finally radio waves with
wavelengths as along as 105 m.
16. Material – Light interaction
Interaction of photons with the electronic or crystal structure of a material leads to
a number of phenomena.
• The photons may give their energy to the material (absorption);
•Photons give their energy, but photons of identical energy are immediately
emitted by the material (reflection);
•Photons may not interact with the material structure (transmission); or
• During transmission photons are changes in velocity/direction (refraction).
|Refractive Index
• At any instance of light interaction with a material, the total intensity of the
incident light striking a surface is equal to sum of the absorbed, reflected, and
transmitted intensities.
• Where the intensity ‘I ‘is defined as the number of photons impinging on a
surface per unit area per unit time.
T
R
A
o I
I
I
I
17. • Incident light is either reflected, absorbed, or
transmitted:
LIGHT INTERACTION WITH SOLIDS
• Optical classification of materials:
Adapted from Fig. 21.10, Callister
6e. (Fig. 21.10 is by J. Telford,
with specimen preparation by P.A.
Lessing.)
single
crystal
polycrystalline
dense
polycrystalline
porous
Transparent
Translucent
Opaque
T
R
A
o I
I
I
I
18. Optical Materials
Materials are classified on the basis of their interaction
with visible light into three categories.
• Transparent
• Translucent
• Opaque
• Materials that are capable of transmitting light with
relatively little absorption and reflection are called
transparent materials i.e. we can see through them.
• Translucent materials are those through which light is
transmitted diffusely i.e. objects are not clearly
distinguishable when viewed through.
• Those materials that are impervious to the transmission of
visible light are termed as opaque materials. These
materials absorb all the energy from the light photons.
19. Optical Properties – Metals
• Metals consist of partially filled high-energy conduction bands.
• When photons are directed at metals, their energy is used to excite
electrons into unoccupied states. Thus metals are opaque to the visible
light.
• Metals are, however, transparent to high end frequencies i.e. x-rays and γ-
rays.
• Absorption takes place in very thin outer layer. Thus, metallic films thinner
than 0.1 μm can transmit the light.
• The absorbed radiation is emitted from the metallic surface in the form of
visible light of the same wavelength as reflected light. The reflectivity of
metals is about 0.95, while the rest of impinged energy is dissipated as
heat
• The amount of energy absorbed by metals depends on the electronic
structure of each particular metal. For example: with copper and gold
there is greater absorption of the short wavelength colors such as green
and blue and a greater reflection of yellow, orange and red wavelengths.
20. Figure 19.4 (a) Schematic representation of the mechanism of photon absorption for
metallic materials in which an electron is excited into a higher-energy unoccupied state.
The change in energy of the electron E is equal to the energy of the photon. (b)
Reemission of a photon of light by the direct transition of an electron from a high to a low
energy state.
