properties of light. in a perfectly dark room, there is no light to be emitted or reflected. what...
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Properties of lightProperties of light
In a perfectly dark room, there is no light to be emitted or reflected.
What can you see?What can you see?
• luminous (emitting light)
• illuminated (reflecting light)
You can only see things that are either:
Seven properties of lightSeven properties of light
Nothing travels faster than light!
Light travels three hundred MILLION meters each second.
1. Light travels very fast1. Light travels very fast
Light can behave as waves, or as particles called photons.
2. Light has a dual nature2. Light has a dual nature
When unimpeded, light travels along linear paths.
3. Light travels in straight lines3. Light travels in straight lines
Light rays are represented as straight paths.
When they are blocked by solid objects, shadows are formed.
How do shadows form?How do shadows form?
At the same distance, light from a 100 watt bulb is more intense (brighter) than from a 60 watt bulb.
4. Light varies in intensity4. Light varies in intensity
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).
Light varies in intensityLight 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.
Light varies in intensityLight varies in intensity
Light from this bulb spreads out in every direction.
•At radius r, it passes through surface area A1 = 4πr2.
•At radius 2r, it passes through surface area A2 = 4π (2r)2 = 16 πr2
r
Light varies in intensityLight varies in intensity
The same amount of light is spread over 4 times the surface area.
2r
Light intensity is:
Area is proportional to r2, so light intensity decreases as 1/r2.
Light intensity obeys an inverse square law, just like the gravitational and electric forces!
2r
Light varies in intensityLight varies in intensity
r
Light varies in intensityLight varies in intensity
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?
Test your knowledgeTest 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? ¼ as much, so 2.0 W/m2
What is the light intensity 50 cm from the bulb?
Test your knowledgeTest 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? ¼ as much, so 2.0 W/m2
What is the light intensity 50 cm from the bulb? 4 times as much, so 32 W/m2
Test your knowledgeTest your knowledge
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.
5. Light interacts with matter5. Light interacts with matter
White light is composed of many colors. It can be dispersed (separated) using a prism.
6. Light is comprised6. Light is comprisedof many colorsof many colors
Most of the electromagnetic spectrum is not visible to us. We have many uses for these invisible parts of the spectrum.
There is light we can’t seeThere is light we can’t see
We use light to communicate through AM/FM radios, mobile phones, and fiber optic cables.
7. Light carries energy and information
7. Light carries energy and information
1. Light travels very fast.
2. Light has a dual nature.
3. Light travels in straight lines.
4. Light can vary in intensity.
5. Light interacts with matter.
6. Light is comprised of many colors.
7. Light carries energy and information.
Seven properties of lightSeven properties of light
Optical devicesOptical devices
Light moves in straight lines.
An optical device is a piece of equipment that can divert the path of a light ray.
•Mirrors divert light using reflection.
•Lenses divert light using refraction.
•Prisms reflect AND refract light.
Optical devicesOptical devices
Reflection occurs when light strikes a surface and its path is redirected backwards away from the surface.
There are two types of reflection:
• specular reflection
• diffuse reflection
ReflectionReflection
Specular reflection is the organized reflection of light off a very smooth surface, such as:
•a mirror
•a piece of polished metal
•the flat surface of a pond
Specular reflection creates images, such as the image of this wooden figure in the mirror.
Specular reflectionSpecular reflection
Diffuse reflection occurs when light strikes a rough surface.
The reflected light scatters in all directions.
This wooden figure is visible because of diffuse reflection, but no image of the figure is created on the cardboard.
Diffuse reflectionDiffuse reflection
Why does this wrench look bent and shrunken as it emerges from the water?
When light passes from water into air, it bends, distorting the image of the wrench.
This bending is called refraction.Lenses use refraction to create images.
RefractionRefraction
MagnificationMagnificationMirrors and lenses use reflection and refraction to divert light from an object and create an image.
When you hold a magnifying lens close to a page of the textbook . . .
the image of the text appears larger than the original text.
The image is magnified.
• If the image height is the same as the object:
m = 1: No magnification
• If image height is greater than object height:
m > 1: Image is magnified
• If image height is less than object height:
m < 1: Image is reduced in size
Object height
MagnificationMagnificationWhat do magnification values tell you?
If the image is upright, then . . .
• image height is positive ( + )
• the magnification is positive ( + )
If the image is inverted, then . . .
• image height is negative ( - )
• the magnification is negative ( - )
In the investigation, which optical device(s) had a negative magnification?
Image orientationImage orientation
Any material that light can pass through is called a medium.
A medium can be described as transparent, translucent, and/or opaque.
What is a light medium?What is a light medium?
A transparent medium is
one you can see through.
An opaque medium is one
you can’t see through.
Types of light mediaTypes of light media
A translucent medium does let light pass through.
But it scatters the light back and forth as it passes through, blurring the image.
Translucent media can provide natural light and privacy.
Frosted glass is an example of a translucent medium.
Types of light mediaTypes of light mediaA translucent medium
blurs the light.
Test your knowledgeTest your knowledge
Are tinted windows on cars partially translucent or partially opaque?
They are partially opaque. Less light passes through, which reduces glare.
Why would a translucent windshield be a bad idea?
Test your knowledgeTest your knowledge
Are tinted windows on cars partially translucent or partially opaque?
Notes Quiz – Complete and Turn inNotes Quiz – Complete and Turn in
1. A pinhole camera creates an image of a 37-meter-tall tree. If the image is 2.4 cm tall and inverted, what is the magnification of the camera?
2. Mirrors divert light using ______________, lenses divert light using ______________.
3. Describe the difference between specular and diffuse reflection.