1. Introduction - Light

2. A Beginning Definition
All objects are emitting and absorbing EM radia-tion. Consider a poker placed in a fire.
As heating occurs, the emitted EM waves have higher energy and eventually become visible. First red then white. 1 2 3 4
Light may be defined as electromagnetic radiation that is capable of affecting the sense of sight.

3. Electromagnetic Waves
Wave Properties: c E B
Electric E
Magnetic B
Waves travel at the speed of light c.
Perpendicular electric and magnetic fields.
Require no medium for propagation. That means it can travel in a vacuum like space!
3 x 108 m/s

4. Electromagnetic waves

5. The Wavelengths of Light
The electromagnetic spectrum spreads over a tremendous range of frequencies or wavelengths. The wavelength l is related to the frequency f:
c = fl c = 3 x 108 m/s
Those EM waves that are visible (light) have wave-lengths that range from to cm.
Red, l cm
Violet, l cm

6. Will continue tomorrow!!!

7. Continue from yesterday

8. The EM Spectrum A wavelength of one nanometer 1 nm is:
Frequency wavelength
f (Hz) l ( nm)
Gamma rays
X-rays
Infrared rays
Short Radio waves
Broadcast Radio
Long Radio waves
Ultraviolet
A wavelength of one nanometer 1 nm is:
1 nm = 1 x 10-9 m
400 nm  700 nm
Visible Spectrum
Red 700 nm  Violet 400 nm
c = fl c = 3 x 108 m/s
As wavelength increases, frequency decreases. Also the opposite happens when wavelength decreases frequency increases.

9. dispersion
Isaac Newton showed that light shining through a prism will be separated into its different wavelengths and will thus show the various colors that visible light is comprised of. The separation of visible light into its different colors is known as dispersion. 

10. dispersion
Dispersion of visible light produces the colors red (R), orange (O), yellow (Y), green (G), blue (B), and violet (V). It is because of this that visible light is sometimes referred to as ROY G. BIV.

11. Red light Wavelength l = 632 nm f = 4.75 x 1014 Hz
Example 1. Light from a Helium-Neon laser has a wavelength of 632 nm. What is the frequency of this wave?
The Helium Neon Laser
Wavelength
l = 632 nm
Laser
f = 4.75 x 1014 Hz
Red light

12. Properties of Light
Any study of the nature of light must explain the following observed properties:
Rectilinear propagation: Light travels in straight lines.
Reflection: Light striking a smooth surface turns back into the original medium.
Refraction: Light bends when entering a transparent medium.

13. Dispersion of white light into colors.
The Nature of Light
Physicists have studied light for centuries, finding that it sometimes behaves as a particle and sometimes as a wave. Actually, both are correct!
Reflection and rectilinear propagation (straight line path)
Dispersion of white light into colors.

14. Photons and Light Rays
Light may be thought of as little bundles of waves emitted in discrete packets called photons.
photons
The wave treatment uses rays to show the direction of advancing wave fronts.
Light rays are convenient for describing how light behaves.
Light ray

15. Light Rays and Shadows
A geometric analysis may be made of shadows by tracing light rays from a point light source:
shadow
screen
Point source
The dimensions of the shadow can be found by using geometry and known distances.

16. Example 2: The diameter of the ball is 4 cm and it is located 20 cm from the point light source. If the screen is 80 cm from the source, what is the diameter of the shadow?
4 cm
20 cm
80 cm h
The ratio of shadow to the source is same as that of ball to source. Therefore:
h = 16 cm

17. Shadows of Extended Objects
Extended source
penumbra
umbra
The umbra is the region where no light reaches the screen.
The umbra is the region where no light reaches the screen.
The penumbra is the outer area where only part of the light reaches the screen.

18. The Sensitivity Curve
Human eyes are not equally sensitive to all colors.
Sensitivity curve
Wavelength l
Sensitivity
555 nm
Eyes are most sensi- tive in the mid-range near l = 555 nm.
400 nm
700 nm
40 W
40 W
Yellow light appears brighter to the eye than does red light.

19. What do you know?
Consider the electromagnetic spectrum as you answer these three questions.
Which region of the electromagnetic spectrum has the highest frequency?
Which region of the electromagnetic spectrum has the longest wavelength?
Which region of the electromagnetic spectrum will travel with the fastest speed?
Gamma Rays
Radio Waves
All regions have the same speed

20. What do you know?
Consider the visible light spectrum as you answer these two questions.
Which color of the visible light spectrum has the greatest frequency?
Which color of the visible light spectrum has the greatest wavelength?
Violet
RED