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LIGHT!!!!
Properties of Light
•Light travels in straight lines:
Laser
•Light travels VERY FAST – around
300,000 kilometres per second.
At this speed it can
go around the world 8
times in one second.
•We see things because they reflect light
into our eyes:
Homework
Nature of Light
• Particle?
• Wave?
• Both!!!!!!
• Photon
Wavelength
Electromagnetic Spectrum
Increasing
Wavelength
Visible Light
The Visible Spectrum
400nm
500nm
600nm
700nm
Wavelength means COLOR
WAVE PROPERTIES
1. Reflection
2. Refraction
3. Diffraction
4. Interference
Part 1 - Reflection
• Reflection from a mirror:
Normal
Reflected ray
Incident ray
Angle of
incidence
Angle of
reflection
Mirror
•The Law of Reflection
Angle of incidence = Angle of reflection
The
same !!!
2. REFRACTION -Bending of light as it travels from one
medium into another due to a change in velocity
3. DIFFRACTION –
Bending of wave through openings
4. Wave Interference
Basically
-
Light is a form of electromagnetic radiation (like radio waves, infrared, xrays)
It travels like a wave (no medium required)
It is the fastest thing we know
It is made up of photons (which also transmit electromagnetic forces)
The photon is considered massless (yet it has momentum)
Photons are created when energy is released, energy is absorbed when
photons are absorbed
Calculations – Speed of light in a vacuum (C)
•d (distance) = C x t (seconds)
•d = 300,000 km/s x t
•Or = 300,000,000 m/s x t
•How far will light travel in 1 minute?
•d = 300,000 km/s x 1 min x 60 s/min
•d = 18,000,000 km
Practice Questions
1.
2.
3.
4.
How far will light travel in 1 year?
If the earth is 150,000,000 km away how long will it take the light to
reach us from the sun?
If it take 2 hours for a radio signal to travel to a far away planet, how far
is the planet?
Andromeda is 2.6 million light years away, how far is that?
More on Refraction
•The bending of light is due to the change in speed of light as it travels into a
new medium.
•The index of refraction is known as the fraction of speed of light in a vacuum
over the speed of light in another medium
• n=C/V
•Ex. What is the index of refraction of glass if the speed of light is 210,000
km/s?
• n = 300,000 km/s = 1.4
•
210,000 km/s
Practice Questions
1. What is the speed of light in diamond if n= 2.4?
2. If the speed of light is 225,000,000 m/s what is the index of refraction?
•3. If glass has an n value of 1.5, how long would it take the light to travel
through a hypothetical 1 km, piece of glass?
What happens……..
•When a laser is fired through a narrowing slit?
Refraction
• The change in direction of a wave as is crosses the boundary
between two media in which the wave travels at different speeds.
Snell’s Law
n1 sin 1  n2 sin 2
Index of Refraction
Material
Vacuum
Air
Ice
Water
Ethyl Alcohol
Plexiglas
Crown Glass
Light Flint Glass
Dense Flint Glass
Zircon
Diamond
Rutile
Gallium phosphide
Index of Refraction
1.0000
1.0003
1.3100
1.3330
1.3600
1.5100
1.5200
1.5800
1.6600
1.9230
2.4170
2.9070
3.5000
Dispersion
• The separation of light into colors arranged according to their
frequency, by interaction with a prism or diffraction grating.
Rainbows
• White light separates into different colors
(wavelengths) on entering the raindrop because
red light is refracted by a lesser angle than blue
light. On leaving the raindrop, the red rays have
turned through a smaller angle than the blue rays,
producing a rainbow.
Sample Questions
•1. What is the angle of refraction of light
entering water from air at an angle of 45
degrees?
n1 sin 1  n2 sin 2
Calculate the refracted angle for
• 1. Light leaving water at 10 degrees into air.
• 2. Light entering a diamond from air with an angle
of 20 degrees.
• 3. Light entering a diamond from water with an
angle of 40 degrees.
Total Internal Reflection
• When traveling from more dense to less dense,
there’s a critical angle whereby light doesn’t enter
the second medium
Sin (crit) =
n
refracted
n
incident
•Determine the critical angle for
diamond to water
•Sin (crit) = 1.33
2.47
Inv Sin = 32.6 degrees
Calculate the critical angle for
•1. Diamond to Air
•2. Water to Air
•3. Zircon to plexi-glass