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Light
Chapter 16 & 17
What is light?
• Usually we think of only visible light but visible
light is just a part of the electromagnetic
spectrum.
• Light sources: sun, flames, sparks, fireflies, glow
sticks. Lamps. LEDs, lasers, TV etc., If there is
NO light in a room you cannot see anything.
Let’s define light
• Light is the a range of frequencies in the
electromagnetic spectrum that stimulate the
retina of the eye.
• Light waves have a λ of 4.00 x 10-7 m to 7.00 x
10-7 m ( 400-700nm)
• Light travels in a vacuum, does not need
medium. Travels in a straight line.
Ray model of light
• Light travels in a straight-line path.
• That is why you see a shadow. The light in the
straight path is blocked and the light does not go
around.
Speed of Light
• Galileo was the first to guess that light has a
certain speed and he tried to measure it.
• Albert Michelson measured time of light to pass
between 2 mountains. 1st American to receive
Nobel prize in science.
• Speed of light 2.99899 x 108 m/s
• We just say 3.00 x 108 m/s speed of light in a
vacuum. Represented by c
Equation for the speed of light
• c= λf
• EX; What is the frequency of yellow light if
it has a wavelength of 556 nm?
• 3.00 x 108 m/s= 5.56 x 10-7m(f)
• f=5.40 x 1014 Hz
Electromagnetic spectrum
Visible LIGHT
White light is split into colors
(rainbow). ROYGBIV
Longest wavelength is red,lowest f
Color
White light is not a single color; it is made
up of a mixture of the seven colors of
the rainbow. 24 year old Issac Newton
did first experiments on splitting light
(Spectrum)
We can demonstrate this by
splitting white light with a
prism: ROY G BIV
This is how rainbows are
formed: sunlight is “split up”
by raindrops.
Adding colors
White light can be split up to make separate colors.
These colors can be added together again.
The colors of light are red, blue and green:
Adding blue and red
makes magenta
(purple)
Adding red and
green makes
yellow
Adding blue and
green makes cyan
(light blue)
Adding all three
makes white
again
Color Perception
Using colored light
If we look at a colored object in colored
light we see something different. For
example, consider a soccer uniform :
Shirt looks red
White
light
Shorts look blue
In different colors of light this kit would look different:
Red
Shirt looks red
light
Shorts look black
Shirt looks black
Blue
light
Shorts look blue
Because light travels as
wavelengths
• If you change what wavelengths are
reflected- you change the color.
• Try this with polarized filters.
Ready for some confusing
terms?
• Luminous Flux (P) = the rate at which
visible light is emitted from a source.
• Or total amount of light from source
• Units for luminous flux = lumen (lm)
This is over a large (usually circular area)
• Illuminance (E)- how much light emitted
by a luminous object. Ex. How much
light is on the book. (Rate light falls on
a surface)
• Units for Illuminance = lux (lx) = lm/m2
What is the illumination of a spherical area’s surface if a
100 watt bulb is in the middle of the sphere?
• 100 watt bulbs lets off 1750 lm of flux
• Area of the surface is 4πr2
•
At a distance of 1 m from the bulb, comparer amt of light emitted to
area
–
1750 lm/4πr2 m2 = 140 lx
Luminous intensity
• Measured in candela (cd) or candle power
• What is it? It is the luminous flux (rate light
comes from a source) that falls on 1 m2 of a
sphere 1 m in radius.
• So…. Lum flux = 1750 lm = 139 cd
4πd2
4πd2
• candela is SI unit for light intensity
How can you increase light on a
surface?
• 1) Use brighter bulbs (increases lum flux –
lm)
• 2) Move surface closer (decreasing
distance)
• 3) Use twice as much candle power
Summary of terms
Term Used
Definition
variable
units
Luminous flux
Rate light
emitted
P
lm
Illuminance
Illumination of a
surface
E
lux or lx
Luminous
intensity
Rate light is
emitted over a
square meter of
a sphere
cd= candela
Represented by
I
cd
How to use this
• An equation that represents this
• Illuminance E = P
•
4πd2
P= lum flux (lm)
d = distance from surface
Example Problem #1
• What is the illumination on your desktop
if it is lighted by a 1750 lm lamp that is
2.50 m above your desk?
2.50 m
Solution
• Luminous flux, P = 1750 lm
• d = 2.50 m
• Illuminance, E = ?
• E= P
•
4πd2
•
E= 1750 lm
4π(2.50)2
2
22.3 lm/m = 22.3 lx