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 m to 7.00 x m ( nm) 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. 1 st American to receive Nobel prize in science. Speed of light x 10 8 m/s We just say 3.00 x 10 8 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 10 8 m/s= 5.56 x m(f) f=5.40 x 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 blue and green makes cyan (light blue) Adding all three makes white again Adding red and green makes yellow

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 : White light Shorts look blue Shirt looks red

In different colors of light this kit would look different: Red light Shirt 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/m 2

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 4 πr 2 At a distance of 1 m from the bulb, comparer amt of light emitted to area – 1750 lm/4πr 2 m 2 = 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 m 2 of a sphere 1 m in radius. So…. Lum flux = 1750 lm = 139 cd 4 πd 2 4 πd 2 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 UsedDefinitionvariableunits Luminous fluxRate light emitted Plm IlluminanceIllumination 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 P= lum flux (lm) 4 πd 2 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= PE= 1750 lm 4 πd 2 4π(2.50) lm/m 2 = 22.3 lx