This document discusses the general properties of light. It defines light as electromagnetic radiation that is visible to the human eye, with wavelengths between 400-700 nanometers, that travels at 299,792 km/s in a vacuum. The key properties of light discussed are its speed, wavelength, frequency, color, and spectrum. Light carries important astronomical information about the location, temperature, composition and distance of stars, planets and other celestial objects.

1. LIGHT General Properties
By: Alex S. Marce BSEd III-B

2. INTRODUCTION
Much of the information we get in astronomy
is carried by “light”.
1. Location of light in the sky- gives position
of a planet, moon or star.
2. Color- gives temperature, speed and
direction of motion, chemical composition.
3. Brightness- gives
distance, composition, periods of motion.
4. Size of light- gives size and distance
information.
*Adopted from Astronomy Chapter 2 handouts, Page 15

3. DEFINITION
• The common term for electromagnetic
radiation, usually referring to that portion visible to
the human eye. However, other bands of the e-m
spectrum are also often referred to as different forms
of light. *(space.about.com)
• Electromagnetic radiation that can be perceived
by the human eye. It is made up of electromagnetic
waves with wavelengths between 4 10-7and
7 10-7 meters. Light, and all other electromagnetic
radiation, travels at a speed of about 299,728 km
(185,831 mi) per second in a vacuum.
*(http://science.yourdictionary.com/light)

4. SPEED OF LIGHT
- The speed of light in empty space is
constant.
- C = 3x108 m/s = 670 billion mph
- This finite speed has a real effect on
information carried by light.
 it takes 11/4 seconds for light to reach us from
the moon.
 it takes 81/3 minutes for light to reach us from
the sun.
 it takes 41/3 years for light to reach us from the
nearest star.
*Adopted from Astronomy Chapter 2 handouts, Page 15

5. PROPERTIES OF LIGHT
Wavelength- the distance between wave
crest. The wavelength is indicated by the Greek
symbol “λ”.

6. PROPERTIES OF LIGHT
Frequency- the number of wave crests per
second passing a point.

7. PROPERTIES OF LIGHT
Color- is determined by wavelength ( or
frequency).

8. SPECTRUM AND MORE
PROPERTIES OF LIGHT
Spectrum- is a
condition that is not limited
to a specific set of values
but can vary infinitely
within a continuum. The
word saw its first scientific
use within the field
of optics to describe
the rainbow of colors in
visible light when
separated using a prism; it
has since been applied by
analogy to many fields
other than optics.
*(www.wikipedia.org)

9. SPECTRUM AND MORE
PROPERTIES OF LIGHT
The study of light at
different wavelengths is
called
“spectroscopy”.
Spectroscopy pertains to the
dispersion of an object's light
into its component colors (i.e.
energies). By performing this
dissection and analysis of an
object's light, astronomers can
infer the physical properties of
that object (such as
temperature, mass, luminosity
and composition).
*(http://loke.as.arizona.edu/~ckulesa/camp/spe
ctroscopy_intro.html)

10. WAVELENTH SPECTRUM

11. QEUSTIONS ABOUT LIGHT
Why do we see “visible” light?
1. Sun emits most strongly at these
wavelengths.
2. Earth’s atmosphere blocks many other
wavelengths.
3. Human eye efficiency detects available
light.

12. QUESTION ABOUT LIGHT
Is light really a wave?
1. Sometimes light acts like a particle “photon”.
2. Sometimes light acts like a wave.
3. It is really both –or neither- we just don’t have
the right words to describe it completely.

13. Energy carried by light depends on
wavelength (color, frequency):
• Short wavelength- have more energy.
• Long wavelength- have less energy.
Example:
1. Blue light carries more energy than red light.
2. X-rays carry more energy than radio waves.
Formula: E=hc/λ
where E is the energy, λ is the wavelength, and
h and c are constant numbers.