april 3, 2014. what we call “light” is merely a small fraction of the total electromagnetic...
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Light
April 3, 2014
EM SpectrumWhat we call “light” is merely a small fraction of the total electromagnetic spectrum.
The electromagnetic spectrum Consists of transverse waves of varying frequenciesOccur due to oscillation of electric and magnetic fieldsDo not require a mediumTravel at the speed of light (c = 3.00 X 108 m/s) in a vacuum
Like any wave, an EM wave carries what?
Energy!
VisionAncients believe that we could see because of light beams that left our eyes and hit objects. What day-to-day experiences tell you that isn’t so?
VisionWe see when light when visible light enters our eyes and focuses on our retinas. The light may come from a luminous source (something that emits light, such as the sun or a light bulb) or may bounce (reflect) off an illuminated object.
Light
Light
Light
Luminous source
Illluminated object
VisionWe have photoreceptor cells in our retinas that send a signal to our brain whenever light hits them. • Rods are sensitive to all wavelengths of visible light
provide black and white vision
• Cones are sensitive to only certain wavelengths provide color vision
We have three cones types: blue, green, red.
Our brain interprets color based on the combinations of cones that are triggered by certain light. … if only the red cone is triggered, we see
… if red and green are triggered equally, we see
red
yellow
VisionDifferent materials absorb, reflect, and/or transmit different wavelengths of light.
• Absorption – light energy that has the same natural frequency as the electrons in the object will be absorbed – the energy will be converted into vibrational motion of the electrons.
• Reflection – the light ‘bounces off’ the surface of the object• Transmission – the light goes through the object and out the
opposite side
Which type of light hits our eyes? Reflected or transmitted light!
Transparent objects transmit most of the light that hits them. Translucent objects transmit some of the light that hits them.Opaque objects reflect or absorb most of the light that hits them.
VisionHow would the following objects appear?1. An object that reflects blue and green light; absorbing other
wavelengths.
2. An object that reflects all visible light waves.
3. An object that absorbs all visible light waves.
4. An object that reflects pink light but transmits all other wavelengths.
5. An object that transmits most of the light hitting it.
•
Opaque blue-green
Opaque white
Opaque black
Translucent pink
transparent
Barrier Behavior of WavesWaves of any sort – light, sound, water, etc. – exhibit different behaviors when they encounter a barrier.
• Reflection – bouncing back of a wave as it encounters a new medium
• Refraction – the bending of a wave as it transmits through a different medium
• Dispersion – The separation of a wave into its component frequencies
• Diffraction – the spreading of a wave behind an obstruction
Law of ReflectionAngle of reflection is
equal to angle of incidence.
All waves, including light, sound, water obey this relationship, the law of reflection.
(the angles are measured to the normal to the barrier).
Reflection
i = r
When a wave passes from one medium to another, its velocity changes. The change in speed results in a change
in direction of propagation of the refracted wave.
Refraction
As a toy car rolls from a hardwood floor onto carpet, it changes direction because the wheel that hits the carpet first is slowed down first.
Visualization of refraction
When a wave passes from one medium to another, its velocity changes. The change in speed results in a change in direction of propagation of the refracted wave.
Light – faster in air than water
Sound – Faster in water than air
Frequency is determined by the source so it doesn’t change. Only wavelength changes. Wavelength of the same wave is smaller in the medium with smaller speed.
1 2
1 2
v vf = =
λ λ
Refraction
θ1
θ2
is constant for the given frequency. The Snell’s law is of course valid for all types of waves.
Greater speed – greater angleSlower speed – slower angle
Snell’s LawWe can predict the degree of refraction using Snell’s Law.
Snell’s Law states that the ratio
Turn & Talk
1) Why is there no bending of light in the first animation?
2) Is the block a slow medium or a fast medium? Justify your response.
www.le.ac.uk/ua/mjm33/wave2/images/Snell.gif More Refraction animations!
Exit Ticket
1) You see a translucent purple ruler. What frequencies of light are absorbed? What are reflected? What are transmitted?
2) Draw a reflected wave, labeling the angle of incidence and angle of reflection. Also compare the size of the two angles.
3) Draw a light wave passing through two different mediums, first fast then slow. Label the angles of incidence and angles of refraction.
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