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### Chapter 13 Light and Reflection Ms. Hanan Anabusi 13-1 Characteristics of Light Objectives: Identify the components of the electromagnetic spectrum. – PowerPoint PPT presentation

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Title: Chapter%2013%20Light%20and%20Reflection

1
Chapter 13 Light and Reflection
• Ms. Hanan Anabusi

2
13-1 Characteristics of Light
• Objectives
• Identify the components of the electromagnetic
spectrum.
• Calculate the frequency or wavelength of the
• Recognize that light has a finite speed.
• Describe how the brightness of a light source is
affected by distance.

3
Vocabulary
• Electromagnetic wave
• Spectrum
• Wavelength
• Frequency
• Speed of light
• Rays
• Luminous flux
• lumens
• Illuminance

4
Nature of Electromagnetic Waves
• They are Transverse waves without a medium. (They
can travel through empty space)
• They travel as vibrations in electrical and
magnetic fields.
• Have some magnetic and some electrical
properties to them.
• Speed of electromagnetic waves 300,000,000
meters/second (Takes light 8 minutes to move from
the sun to earth 150 million miles at this
speed.)

5
• When an electric field changes, so does the
magnetic field. The changing magnetic field
causes the electric field to change. When one
field vibratesso does the other.
• RESULT-An electromagnetic wave.
• http//www.walter-fendt.de/ph14e/emwave.htm

6
• Waves or Particles?
• Electromagnetic radiation has properties of waves
but also can be thought of as a stream of
particles.
• Example Light
• Light as a wave Light behaves as a transverse
wave which we can filter using polarized lenses.
• Light as particles (photons)
• When directed at a substance light can knock
electrons off of a substance (Photoelectric
effect)

7
Waves of the Electromagnetic Spectrum
• Electromagnetic Spectrumname for the range of
electromagnetic waves when placed in order of
increasing frequency

GAMMA RAYS
ULTRAVIOLET RAYS
INFRARED RAYS
X-RAYS
MICROWAVES
VISIBLE LIGHT
8
Examples include (textbook page 447 Table
13-1)
Picture source http//imagine.gsfc.nasa.gov/docs/
science/know_l1/emspectrum.html
9
The Electromagnetic Spectrum
Picture from the New York Physical Setting/
Physics reference tables
10
The Electromagnetic Spectrum
• More than meets the eye!

11
Examples from Space!
12
Wavelength
• The distance from one wave crest to the next
• Radio waves have longest wavelength and Gamma
rays have shortest!

13
Listed below are the approximate wavelength, and
frequency limits of the various regions of the
electromagnetic spectrum.
Wavelength (?) Frequency (f)
Radio waves ? gt 30 cm f lt 1.0 x 109 Hz
Microwaves 30 cm gt ? gt 1 mm 1.0 x 109 Hz lt f lt 3.0 x 1011 Hz
Infrared (IR) waves 1 mm gt ? gt 700 nm 3.0 x 1011 Hz lt f lt 4.3 x 1014 Hz
Visible light 700 nm (red) gt ? gt 400 nm (violet) 4.3 x 1014 Hz lt f lt 7.5 x 1014 Hz
Ultraviolet (UV) light 400 nm gt ? gt 60 nm 7.5 x 1014 Hz lt f lt 5.0 x 1015 Hz
X-rays 60 nm gt ? gt 10-4 nm 5.0 x 1015 Hz lt f lt 3.0 x 1021 Hz
Gamma-rays 0.1 nm gt ? gt 10-5 nm 3.0 x 1018 Hz lt f lt 3.0 x 1022 Hz
14
Questions
• Which visible light has the shortest
wavelength?___________________
• Which visible light has the longest wavelength?
__________________
• Which electromagnetic spectrum item has the
smallest frequency? ______________
• Which electromagnetic spectrum item has the
shortest wavelength? _____________

Violet
Red
Gamma Rays
15
• All electromagnetic waves move at the speed of
light.
• In vacuum light travels at 2.99792458 x 10 8 m/s.
• In air light travels at 2.99709 x 10 8 m/s,
slightly slower.
• For our purposes we will use 3.0 x 10 8 m/s.

16
•

17
Sample Problem
• The AM radio band extends from 5.4 x 105 Hz to
1.7 x106 Hz. What are the longest and shortest
wavelengths in this frequency range?
• Given
• Unknown

18
• Use the wave speed equation

19
Pop Question!!!
the following question
• Which of the following electromagnetic waves has
the highest frequency?
• Blue light

20
Huygens Principle
Huygens principle Every point on a wave front
acts as a point source the wave front as it
develops is tangent to their envelope. Huygens
principle is used to derive the properties of any
wave that interacts with matter. The straight
line perpendicular to the wave front is called a
ray. This simplification is called ray
approximation.
21
Wave Front and Rays
22
Illuminance or Brightness
• Intensity of light depends on
• Amount of light energy emitted (watts)
• Distance from the source (m)
• Light bulbs are rated by their input measured in
watts (W) and their light output.
• The rate at which light is emitted from a source
is called the luminous flux and is measured in
lumens (lm)
• Luminous flux is a measure of power output, but
is weighted to take into account the response of
the human eye to light.
• Illuminance is the luminous flux divided by the
area of the surface and measured in lm/m2

23
Light from a source spreads out in space. The
further from the source the less light per unit
area there will be (the source is not as bright).
The brightness drops off as one over the distance
squared. This is called the inverse square law.
24
Inverse Square Law of Brightness
Expressed mathematically
• In words
• Brightness of a source is inversely proportional
to the square of its distance from you.
• When the distance doubles, the brightness goes
down by a factor of four.

25
Brightness and Distance
The Inverse Square Law Scientists have
calculated a theoretical relationship between
brightness and distance. This predicted
relationship between brightness and distance is
called the inverse square law. It says that when
the distance from a light doubles, its brightness
should decrease by a factor of four. The equation
for the brightness, written as B in the equation
below, and the distance from the light, written
as d, is B C/d2 In this equation, C is a
constant that depends on how luminous the light
is (in other words, what "wattage" the light bulb
is). The equation for C is C B d2. You do
not need to understand this equation in detail.
The important point is that the brightness
depends on distance, and that when the distance
doubles, the brightness goes down by a factor of
four.
26
Assignments
• Class-work
• Practice A , page 449, even questions.
• Homework
• Section review on page 450 odd questions.
• Additional practice A, odd questions.