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 electromagnetic radiation.
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
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.

6. Waves of the Electromagnetic Spectrum
Electromagnetic Spectrum—name for the range of electromagnetic waves when placed in order of increasing frequency
GAMMA RAYS
ULTRAVIOLET RAYS
RADIO WAVES
INFRARED RAYS
X-RAYS
MICROWAVES
VISIBLE LIGHT

7. The Electromagnetic Spectrum
Picture from the New York Physical Setting/ Physics reference tables

8. The Electromagnetic Spectrum
More than meets the eye!

9. Wavelength The distance from one wave crest to the next
Radio waves have longest wavelength and Gamma rays have shortest!

10. 700 nm (red) > λ > 400 nm (violet) Ultraviolet (UV) light
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

11. 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
Radio Waves
Gamma Rays

12. All electromagnetic waves move at the speed of light.
In vacuum light travels at x 10 8 m/s.
In air light travels at x 10 8 m/s, slightly slower.
For our purposes we will use 3.0 x 10 8 m/s.

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

15. Use the wave speed equation:

16. 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?
Radio
Ultraviolet radiation
Blue light
Infrared radiation

17. 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.

18. Wave Front and Rays

19. 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

20. Light from a source spreads out in space
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.

21. 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.

22. Assignments Class-work: Practice A , page 449, even questions.
Homework:
Section review on page 450 odd questions.
Additional practice A, odd questions.