Properties of Light

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Properties of Light
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Objectives

• Describe the seven properties of light.
• Describe the characteristics of the
  electromagnetic spectrum.
• Describe how the intensity of light changes with
  distance from the source.

3
Physics terms

• light rays
• reflection
• emission
• refraction
• dispersion
• electromagnetic radiation

4
Equations
NOTES
Intensity is power per unit area.
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Think, pair, share
If you were in a perfectly dark room with your
best friends, could you see them? What if
you stayed in there for a really long time?
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What can you see?
NOTES
You can only see things that are either

• luminous (emitting light)
• illuminated (reflecting light)

In a perfectly dark room, there is no light to be
emitted or reflected.
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Seven Properties of Light

• Light travels very fast.
• Light has a dual nature.
• Light travels in straight lines.
• Light can vary in intensity.
• Light interacts with matter.
• Light is comprised of many colors.
• Light carries energy and information.

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1. Light Travels Very Fast
NOTES
Nothing travels faster than light! Light
travels three hundred MILLION meters each second.
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2. Light Has a Dual Nature
NOTES
Light can behave as waves, or as particles called
photons.
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3. Light Travels in Straight Lines
NOTES
When unimpeded, light travels along linear paths.
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How do shadows form?
Light rays are represented as straight paths.
When they are blocked by solid objects, shadows
are formed.
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Shadows vary
Objects close to the screen form sharp shadows.
Object far from the screen form blurry shadows.
Objects in front of large light sources form
blurry shadows.
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4. Light Varies in Intensity
NOTES
The intensity of light is a measure of its
brightness. Intensity is defined as power P
per unit area A. Its units are watts per square
meter (W/m2).
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4. Light Varies in Intensity

• The intensity of light from a source depends on
  how close you are to it.
• When you are close to the source, the light is
  more intense.
• When you are far from the source, the light is
  less intense.

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4. Light Varies in Intensity
NOTES

• Light from this bulb spreads out in every
  direction.
• At radius r, it passes through surface area A1
  4pr2.
• At radius 2r, it passes through surface area A2
  4p (2r)2 16 pr2

r
2r
The same amount of light is spread over 4 times
the surface area.
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4. Light Varies in Intensity
NOTES
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Test your knowledge
The light intensity 1.0 meter from a 100 Watt
light bulb is 8.0 W/m2. What is the light
intensity 2.0 meters from the bulb? What
is the light intensity 50 cm from the bulb?

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5. Light Interacts with Matter
NOTES
Light can change speed and direction when it
interacts with matter. Light reflects off a
mirror and refracts as it passes through from air
to glass to water.
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6. Light is Comprised
NOTES
of Many Colors
White light is composed of many colors. It can
be dispersed (separated) using a prism.
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What kind of light can we see?
There are sounds that are too high or low
frequency for our ears to hear. There is also
light that is too high or low frequency for our
eyes to see.
The light we can see is called visible light. It
includes all the colors of the rainbow. Can you
name any of the types of light that we cant see?
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There is light we cant see
Most of the electromagnetic spectrum is not
visible to us. We have many uses for these
invisible parts of the spectrum.
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1. Light Carries Energy and Information

NOTES
We use light to communicate through AM/FM radios,
mobile phones, and fiber optic cables.
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Seven Properties of Light

• Light travels very fast.
• Light has a dual nature.
• Light travels in straight lines.
• Light can vary in intensity.
• Light interacts with matter.
• Light is comprised of many colors.
• Light carries energy and information.

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Assessment

• 1. Which of these is NOT a property of light?
• Light travels very fast.
• Light has constant intensity.
• Light carries energy and information.
• Light exhibits both wave and particle behavior.

• Light travels in straight lines. Can you name
  two ways to get a light beam to change direction?

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Assessment

• Name at least three types of radiation that are
  part of the electromagnetic spectrum but are NOT
  visible.
• 4. Why is it that we can see the Moon?
• It is luminous (emits light).
• It is illuminated (reflects light from another
  source).

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Assessment

• Assume that the light intensity 1.0 meter from a
  light bulb is 9.0 W/m2.

• What is the light intensity 2.0 meters from the
  bulb?
• b) What is the light intensity 3.0 meters from
  the bulb?