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:

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

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

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

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

Solution

• Luminous flux, P = 1750 lm

• d = 2.50 m

• Illuminance, E = ?

• E= P E= 1750 lm

• 4πd2

4π(2.50)2

• 22.3 lm/m

2 = 22.3 lx