Light Chapter 16 & 17
Dec 18, 2015
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
Visible LIGHT
White light is split into colors (rainbow). ROYGBIV
Longest wavelength is red,lowest f
ColorWhite 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 colorsWhite 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 blue and green makes cyan
(light blue)
Adding all three makes white
again
Adding red and green makes yellow
Using colored light
If we look at a colored object in colored light we see something different. For example, consider a soccer uniform :
White
light
Shorts look blue
Shirt looks red
In different colors of light this kit would look different:
Red
lightShirt looks red
Shorts look black
Blue
light
Shirt looks black
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 termsTerm 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= candelaRepresented byI
cd
How to use this
• An equation that represents this
• Illuminance E = P P= lum flux (lm)
• 4πd2 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