Physics teacher
Mr. Umesh Singh Rana
Classroom Rules
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Arrive on time.
Raise your hand before speaking.
Do your assignments.
Bring materials and have them ready.
Agree to the seating plan or talk to your teacher
privately.
Go to the toilet during break and lunchtime. You will
not be allowed during a lesson unless you have a
medical pass.
Listen to directions.
Always be polite and considerate to everybody.
Obey all school rules.
Be safe!
Fundamentals of Light
SECTION 1
Illumination
• the light travels in a straight line.
Ray Model of Light
After interacting at a boundary, the ray still moves in a straight
line, but its direction is changed. These basic principles—that light
travels in straight lines and that its direction can be changed by
encountering a boundary—constitute the ray model of light.
Sources of light
Worksheet
• Objects such as the Sun that emit their own light
are luminous sources, while those that you see
due to light reflecting from them are illuminated
sources.
Light and matter
Objects can absorb, reflect, or transmit light.
Worksheet
Quantity of Light
• Three main factors determine the brightness:
1. the quantity of light the flashlight produces.(depends on
luminous flux)
2.the distance between the light bulb and your eye.(illuminance
decreases as distance increases)
3.and the angle at which the light rays hit your eye.(depends on
luminous intensity )
luminous flux
• The rate at which the bulb, a luminous source,
produces light energy is called the luminous
flux (P) and is measured in lumens (lm).
Illuminance
• the rate at which the light strikes the unit area of the
surface called illuminance (E).
• It is measured in lux (lx), which is equivalent to lumens
per square meter (lm/ m 2 ).
Figure shows that the
illuminance produced by a
point source is proportional
to 1/r2 , an inverse- square
relationship.
Luminous intensity
• Luminous intensity:
The luminous flux per unit solid angle as measured in a
given direction relative to the emitting source.
• so luminous intensity is luminous flux divided by 4π.
• Luminous intensity is measured in candelas (cd).
• Example: A bulb with 1750 lm of flux has an intensity of 1750
lm/4π = 139 cd
If P is constant, then E is inversely
proportional to r 2
2
• When r increases, E decreases.
• When r 2 decreases, E increases.
If both sources in Figure had the same luminous intensity,
the source at 2r would only provide one-quarter the
illuminance to the screen. This is consistent with the inversesquare relationship we just developed.
Home work: page 24 – Questions 30,33,34
Surface Illumination
• If an object is illuminated by a point source of light, then
the illuminance at the object is equal to the luminous flux
of the light source divided by the surface area of the
sphere whose radius is equal to the distance the object
is from the light source.
P
E
2
4 r
The Speed of Light
• Michelson’s measurements
• Michelson used a set of rotating mirrors to measure such
small time intervals. Michelson’s best result was
(2.99796 ± 0.00004)×10^8 m/s.
• For this work, he became the first American to receive a
Nobel Prize in science.
• the speed of light in a vacuum to be c = 299,792,458 m/s.
• light travels 9.46×1 0^12 km in a year. This distance is called a
light-year.
SECTION 2
The Wave Nature of Light
• Diffraction and the Wave Model: the bending of light as it
passes the edge of a barrier diffraction.
• Huygens’ principle: According to Huygens’ principle, all the
points of a wavefront of light can be thought of as new sources of
smaller waves. These smaller waves, or wavelets, expand in
every direction and are in step with one another.
Color
• In 1666 Newton performed experiments on the colors produced
when a narrow beam of sunlight passed through a glass prism, as
shown in Figure.
• Different wavelengths: The work of Grimaldi, Huygens, Newton, and
others suggested that the color of light is related to wavelength.
• Visible light falls within the range of wavelengths from about 400 nm
(4.00×10^-7 m) to 700 nm (7.00×1 0 ^-7 m). The longest visible
wavelengths are seen as red light and the shortest as violet.
Color by addition of light
Red, green and blue are called primary colors
Complementary colors are two
colors of light that can be
combined to produce white light.
Thus, yellow is a complementary
color of blue, and vice versa.
Color by subtraction of light
Dyes
Pigments
Dyes can be made from plant or
insect extracts.
The difference between a dye and a
pigment is that pigments usually
are made of crushed minerals
rather than plant or insect extracts.
For example, purple dye can be
extracted from the berries of a
black mulberry tree.
For example, hematite produces a
red pigment.
primary pigment
secondary pigment
A pigment that absorbs only one primary
color and
reflects two from white light is called a
primary pigment.
A pigment that absorbs two primary colors
and reflects one color is called a secondary
pigment.
Yellow pigment absorbs blue light and
reflects red and green light. Yellow, cyan,
and magenta are the colors of primary
pigments
The colors of secondary pigments are red
(which absorbs green and blue light), green
(which absorbs red and blue light), and blue
(which absorbs red and green
light).
Polarization of Light
• Polarization is the production of light with a specific pattern
of oscillation.
Polarization by filtering
•
•
•
The lines in the polarizer in Figure represent a polarizing axis. The light with the
portion of the electric field that oscillates parallel to these lines passes through.
The light with the portion of the electric field that oscillates perpendicular to
these lines is absorbed.
If a polarizer is placed in a beam of non-polarized light, only the components of
the waves in the same direction as the polarizing axis can pass through. As a
result, half of the total light passes through, reducing the intensity of the light by
half.
Polarization by reflection
• Photographers can use polarizing filters over
camera lenses to block reflected light. This
result is shown in Figure.
MALUS’S LAW
• The intensity of light coming out of a second
polarizing filter is equal to the intensity of polarized
light coming out of a first polarizing filter multiplied
by the cosine, squared, of the angle between the
polarizing axes of the two filters.
I 2  I1 cos 
2
Speed, Wavelength, and Frequency of
Light
• For light of a given frequency traveling through a
vacuum, wavelength is a function of the speed of light
(c), which can be written as λ = c/f.
• The Doppler effect:
• Observed Light Frequency
• The observed frequency of light from a source is equal to the
actual frequency of the light generated by the source, times
the quantity 1 plus the relative speed along the axis, divided
by the speed of light, between the source and the observer if
they are moving toward each other, or 1 minus the relative
speed, divided by the speed of light, if they are moving away
from each other.
f obs. = f (1 ± v/c)
•
Doppler Shift
• The difference between the observed wavelength of light and
the actual wavelength of light generated by a source is equal
to the actual wavelength of light generated by the source,
times the relative speed of the source and observer, divided
by the speed of light.
• ( λ obs. - λ) = ∆λ= ± (v/c)λ
CONNECTION TO ASTRONOMY