1. Light and Illumination
Presented to: Waverly Honors Physics
May 11, 2018

2. Objectives: After completing this lesson, you will be able to:
Define light, discuss its properties, and give the range of wavelengths for the visible spectrum.
Apply the relationship between frequencies and wavelengths for optical waves.
Define the concepts of light rays & shadows, sensitivity curves, and luminous flux.
Solve Light Intensity problems.

3. 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.

4. 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 (to be transmitted).
3 x 108 m/s

5. The speed of light
The speed at which light travels through air is approximately 300 million meters per second.
Light travels almost a million times faster than sound.

6. Example #1: Calculate time
1) You are asked for time.
2) You are given distance and you may find the speed of sound and light.
3) t = d ÷ v
4) For sound:
t = (1,609 m) ÷ (340 m/sec) = 4.73 seconds
For light:
t = (1,609 m) ÷ (3 x 108 m/sec) = seconds (5.4 x 10-6 sec)
Calculate the time it takes light and sound to travel the distance of one mile, which is 1,609 meters.

7. 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

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

9. Example 2: Light from a Helium-Neon laser has a wavelength of 632 nm
Example 2: 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

10. Properties of Light
Any study of the nature of light must explain the following observed properties:
Rectilinear propagation: Light travels in straight lines (Huygens’ Principle) and can be represented as rays.
Reflection: Light striking a smooth surface turns back into the original medium.
Refraction: Light bends when entering a transparent medium.

11. 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.

12. 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

13. 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.

14. Example 3: 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

15. 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.

16. 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.

17. Luminous Flux
Luminous flux (P) is the portion of total radiant power that is capable of affecting the sense of sight. It is the rate at which light is emitted form a source.
Typically only about 10% of the power (flux) emitted from a light bulb falls in the visible region.
The unit for luminous flux is the lumen.

18. The Lumen as a Unit of Flux
One lumen (lm) is the luminous flux emitted from a 1/60 cm2 opening in a standard, spheroid light source (think of a light-bulb).
In practice, sources of light are usually rated by comparison to a commercially prepared standard light source.
A typical 100-W incandescent light bulb emits a total radiant power of about 1750 lm. This is for light emitted in all directions.

19. The Lumen in Power Units
Recalling that luminous flux is really radiant power allows us to define the lumen as follows:
One lumen is equal to 1/680 W of yellow-green light of wavelength 555 nm.
A disadvantage of this approach is the need to refer to sensitivity curves to determine the flux for different colors of light.
Wavelength l
Sensitivity curve

20. Light Intensity (A.K.A. illuminance)
The Light Intensity I of a surface A is defined as the luminous flux per unit area (P/A). The units for Intensity is lumens per square meter which is also named a lux (lx) or in units of Watts per square meter.
For the purposes of today’s lesson, we will use the units Watts per square meter.

21. Light Intensity obeys the Inverse Square Law

22. Example #4
What is the illumination (light intensity) on a surface 3.0 meters below a 150 watt light source that emits a luminous flux of 2275 lumens? Calculate the answer in lumens per square meter (lux) and in watts per square meter.

23. Example #5
At the surface of the Sun the intensity of the solar radiation is about 6.33×107 W/m2! What is the average yearly intensity of solar radiation if the Earth is an Astronomical Unit (1.5x1011 m) away and the Sun has a luminous flux of 6.808x1027 Lumens?
1 Watt = Lumens