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Light Chapter 16 & 17
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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:

Dec 18, 2015

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Page 1: 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

Page 2: 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:

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.

Page 3: 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:

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.

Page 4: 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:

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.

Page 5: 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:

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

Page 6: 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:

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

Page 7: 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:

Electromagnetic spectrum

Page 8: 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:

Visible LIGHT

White light is split into colors (rainbow). ROYGBIV

Longest wavelength is red,lowest f

Page 9: 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:

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.

Page 10: 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:

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

Page 11: 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:

Color Perception

Page 12: 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:
Page 13: 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:

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

Page 14: 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:

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

Page 15: 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:

Because light travels as wavelengths

• If you change what wavelengths are reflected- you change the color.

• Try this with polarized filters.

Page 16: 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:

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)

Page 17: 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:

• 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

Page 18: 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:

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

Page 19: 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:

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

Page 20: 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:

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

Page 21: 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:

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

Page 22: 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:

How to use this

• An equation that represents this

• Illuminance E = P P= lum flux (lm)

• 4πd2 d = distance from surface

Page 23: 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:

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

Page 24: 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:

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