Chapter 7
Light
Some definitions
• Luminous: Something that produces light.
The sun is a luminous object that provides
almost all the natural light on the earth.
• Incandescent: Visible light given off as a
result of high temperatures. e. g. light bulb,
a flame.
Electromagnetic Wave
When an electric charge is accelerated by some external force the
acceleration produces a wave consisting of electric and magnetic fields that
become isolated from the accelerated charge.
As the wave moves through space, the two fields exchange energy back and
forth, continuing on until they are absorbed by matter and give up their
energy.
These waves move at a speed of 300,000,000 meters per second, known as
the speed of light, or “c”.
The units for
wavelength are m or
cm.
Fixed point in space
Frequency (f) is how many times per
second the crest of a wave passes
through a fixed point in space. The units
are in cycles per second.
Electromagnetic
Spectrum
Increase
Frequency (f)
Decrease
Wavelength (λ)
Frequency and Wavelengths
• Speed of EM wave 3x108m/s (c). This is a
constant value.
• Relationship between wavelength and
frequency:
• λxf=c
Wavelength and frequency
inversely related
λ (wavelength) = c / f
f (frequency) = c / λ
Energy of an EM wave
• Higher frequencies (shorter wavelengths)
means more energy.
• E=hxf
• h is Planck’s constant.
• Since frequency (f) and wavelength (λ) are
inversely proportional, then higher energy
means lower or shorter wavelengths.
Radiation is given off by matter at any temperature. This is called black body
radiation.
Near absolute 0 (0K),there is little energy available and no radiation is given off.
As the T of an object is increased, more energy is available, and this energy
is distributed over a range of values, so more than one frequency of radiation
Is emitted.
As the T increases the peak frequency (more abundant) becomes higher, since
there is more energy available.
Radiation
• At room temperature, the radiation given off from an object is in the
infrared region (heat), invisible to the human eye. When the
temperature of the object reaches about 700oC, (about 1300oF), the
peak radiation is still in the infrared region, but the peak has shifted
enough toward the higher frequencies that a little visible light is
emitted as a dull red glow. As the temperature of the object
continues to increase, the amount of radiation increases, and the
peak continues to shift toward shorter wavelengths. The object
begins to glow brighter and the color changes from red, to orange, to
yellow, and eventually to blue and then white.
• The incandescent flame of a candle or fire (approx. 2700 oF ) results
from the blackbody radiation of carbon particles in the flame. The
tungsten filament of an incandescent light bulb is heated to a higher
temperature by an electric current (~ 4000 oF), and as a result the
visible light emitted is brighter (more white).
• The radiation from the sun, or sunlight, comes from the sun’s
surface, which has even higher temperatures (approx. 10000 oF).
Blackbody Radiation
• Electromagnetic radiation is given off from
matter at any temperature. This is called
blackbody radiation.
• The radiation originates from the
acceleration of charged particles near the
surface of an object. The frequency of the
radiation is determined by the temperature
of the object.
Energy of an EM wave
•
Sun’s temperature
6000K,
•
UV, Visible and IR
enter our
atmosphere.
The sun’s radiation has a broad
Spectrum centered near the
yellow-green frequency. Your eye
Is most sensitive to this frequency of
sunlight. The spectrum of sunlight
before it travels through the earth’s
Atmosphere is:
Infrared ~ 51%
Visible light ~ 40%
UV light (ultraviolet) ~9%
Sunlight originates as energy released during nuclear reactions in the sun’s core.
This energy requires about a million years to work its way up to the surface. At
the surface, the energy from the core accelerates charged particles, which
then emit light like tiny antennas. The sunlight requires about eight minutes to
travel the distance from the sun’s surface to the earth.
Fig. 7.5
Properties of Light
• You can see luminous objects from the
light they emit and you can see non
luminous objects from the light they reflect.
• You cannot see the path of light itself. You
cannot see a flashlight beam unless you
fill the air with chalk or smoke. The dust or
smoke reflect light, so you see the path of
the beam.
A ray of light travels in a straight line from a
source until it encounters an object or
particles of matter.
If the surface is perfectly smooth the rays of
light undergo reflection, as in a mirror.
If the surface is not smooth, the light rays are
reflected in random directions, like dust in the
air.
Some materials allow much of the light that
falls on them to move through the material
without being reflected. Materials that allow
transmission of light through them are called
transparent, like water and glass.
Materials that do not allow transmission
of any light are called opaque. They reflect
light, absorb light, or some combination thereof.
The reflected light gives rise to the perception
of color.
Color
• Visible light is the part of the electromagnetic
spectrum that your eyes can detect. It is a narrow
range of wavelength. This can be subdivided into
ranges of wavelengths that you perceive as colors.
These are the colors of the rainbow, and there are six
distinct colors, red, orange, yellow, green, blue, and
violet.
• Light can be interpreted to be white if it has the same
mixture of colors as the solar spectrum. It is a mixture
of all the independent colors.
• Isaac Newton found that a beam of sunlight falling on
a glass prism in a darkened room produced a band of
colors he called a spectrum.
• Black is the absence of all colors.
Why is the sky blue?
• A clear cloudless day-time sky is blue because molecules in the air
scatter blue light from the sun more than they scatter red
light. When we look towards the sun at sunset, we see red and
orange colors because the blue light has been scattered out and
away from the line of sight.
• The molecules of oxygen and nitrogen in the air are sufficient to
account for the scattering.
Questions Chapter 7
• p. 201-202 Applying the Concepts, # 1, 2,
3, 4, 9, 14, 17
• New Book: p. 217-220 # 1, 2, 4, 6, 11, 13,
22, 23, 24, 27, 31, 34, 35, 36, 37, 38, 40,
41, 42, 49.
Review Chapter 7
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Luminous and incandescent.
Blackbody radiation-given off at any temperature by objects.
Electromagnetic waves.
Speed of light, frequency, wavelength, amplitude, troughs, crests.
Components of the electromagnetic spectrum: gamma rays, X rays, UV,
visible, microwave, infrared (heat), radio and TV. This is the order of
increasing energy and frequency, decreasing wavelength.
Relationship between frequency (f), λ
wavelength (λ), and the speed of light ( c ).
Relationship between frequency and energy. (Planck’s equation)
Variation of energy of electromagnetic radiation with temperature.
Transmission, absorption, reflection of light, transparent and opaque
objects.
Color is the visible portion of the electromagnetic spectrum. White is the
mixture of all colors. Black is the absence of colors.
Color of the sky-Blue light is reflected more by the O2 and N2.