Transcript Chapter 14 Light and Reflection

Chapter 13
Light and Reflection
Ms. Hanan Anabusi
13-1 Characteristics of Light
Objectives:
•
Identify the components of the electromagnetic
spectrum.
•
Calculate the frequency or wavelength of the
electromagnetic radiation.
•
Recognize that light has a finite speed.
•
Describe how the brightness of a light source is
affected by distance.
Vocabulary
Electromagnetic wave
Spectrum
Wavelength
Frequency
Speed of light
Rays
Luminous
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.)
• When an electric field changes, so does the
magnetic field. The changing magnetic field
causes the electric field to change. When one
field vibrates—so does the other.
• RESULT-An electromagnetic wave.
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)
Waves of the Electromagnetic Spectrum
• Electromagnetic Spectrum—name for the range of
electromagnetic waves when placed in order of increasing
frequency
RADIO
WAVES
INFRARED
RAYS
MICROWAVES
ULTRAVIOLET RAYS
VISIBLE LIGHT
X-RAYS
GAMMA
RAYS
Examples include:
(textbook page 447 – Table 13-1)
Picture source:
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html
The Electromagnetic Spectrum
Picture from the New
York Physical Setting/
Physics reference
tables
The Electromagnetic Spectrum
More than meets the eye!
Examples from Space!
Wavelength
• The distance from one wave crest to the next
• Radio waves have longest wavelength and Gamma
rays have shortest!
Listed below are the approximate wavelength, and
frequency limits of the various regions of the
electromagnetic spectrum.
Wavelength (λ)
Frequency (f)
Radio waves
λ > 30 cm
f < 1.0 x 109 Hz
Microwaves
30 cm > λ > 1 mm
1.0 x 109 Hz < f < 3.0 x 1011 Hz
Infrared (IR) waves
1 mm > λ > 700 nm
3.0 x 1011 Hz < f < 4.3 x 1014 Hz
Visible light
700 nm (red) > λ > 400 nm (violet)
4.3 x 1014 Hz < f < 7.5 x 1014 Hz
Ultraviolet (UV) light
400 nm > λ > 60 nm
7.5 x 1014 Hz < f < 5.0 x 1015 Hz
X-rays
60 nm > λ > 10-4 nm
5.0 x 1015 Hz < f < 3.0 x 1021 Hz
Gamma-rays
0.1 nm > λ > 10-5 nm
3.0 x 1018 Hz < f < 3.0 x 1022 Hz
Questions
• Which visible light has the shortest
Violet
wavelength?___________________
• Which visible light has the longest
wavelength? __________________
Red
• Which electromagnetic spectrum item has
Radio Waves
the smallest frequency? ______________
• Which electromagnetic spectrum item has
Gamma Rays
the shortest wavelength? _____________
• 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.
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:
Use the wave speed equation:
Pop Question!!!
Using Table 1 page 447 in your text book
answer the following question:
Which of the following electromagnetic waves has
the highest frequency?
a) Radio
b) Ultraviolet radiation
c) Blue light
d) Infrared radiation
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.
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
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.
Inverse Square Law of Brightness
Expressed mathematically:
1
B  2
d
In words:
• Brightness of a source is inversely
proportional to the square of its distance
from you.
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 d 2.
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.
Assignments
• Class-work:
Practice A , page 449, even questions.
• Homework:
Section review on page 450 odd
questions.
Additional practice A, odd questions.