Transcript light - Churchill High School

Physics
(Ch. 16 -18)
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Light travels almost unimaginably fast and far.
Light carries energy and information.
Light travels in straight lines.
Light bounces and bends when it comes in contact with
objects.
Light has color.
Light has different intensities, it can be bright or dim.
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The process of making light with heat is called
incandescence.
Incandescent bulbs generate light when electricity
passes through a thin piece of metal wire called a
filament.
 The filament heats up and
gives off light.
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The other common kind of electric light is the
fluorescent bulb.
 Fluorescent bulbs
convert electricity
directly to light
without generating a
lot of heat.
 Fluorescent bulbs
use high-voltage
electricity to
energize atoms of
gas that fill the bulb.
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Light is a form of energy that travels.
The intensity of light is the amount of energy per
second falling on a surface.
Most light sources distribute their light equally in all
directions, making a spherical pattern.
Because light spreads out in a sphere, the intensity
decreases the farther you get from the source.
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The intensity of light from a small source follows an
inverse square law because its intensity diminishes as
the square of the distance.
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The speed at which light travels through air is approximately 300
million m/s.
Light travels almost a million times faster than sound.
The speed of light is so important in physics that it is given its own
symbol, a lower case c.
c = 3 × 108 m/sec.
Calculate the time it takes light and sound to travel
the distance of 1 mile, which is 1,609 meters.
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You are asked for time.
You are given distance.
Use v = d ÷ t , rearrange to solve for t = d ÷ v
Solve for sound: t = (1,609 m) ÷ (340 m/s) = 4.73 s
Solve for light: t= (1,609 m) ÷ (3 x 108 m/s) = 5.4 x 10-6 s
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Objects that are in front of a mirror appear as if
they are behind the mirror.
This is because light rays are reflected by the
mirror.
Your brain perceives the light as if it always
traveled in a straight line.
The light that bends as it crosses a surface
into a material refracts and is shown as a
refracted ray.
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An example of refraction of
light is the twinkling of a star
in the night sky
As starlight travels from space
into the Earth’s atmosphere,
the rays are refracted.
Since the atmosphere is
constantly changing, the
amount of refraction also
changes.
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When all the colors of the rainbow are combined, we do
not see any particular color.
We see light without any color.
We call this combination of all the colors of light "white
light ".
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The retina in the back of the eye contains
photoreceptors. (rods and cones)
These receptors release chemical signals.
Chemical signals travel to the brain along the optic
nerve.
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Which chemical
signal gets sent
depends on how
much energy the light
has.
If the brain gets a
signal from ONLY
green cones, we see
green.
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The three color receptors in the
eye allow us to see millions of
different colors.
The additive primary colors are
red, green, and blue.
We don’t see everything white
because the strength of the
signal matters.
All the different shades of color
we can see are made by
changing the proportions of red,
green, and blue.
The light that reaches our
eyes can come from two
different processes:
1. The light can be emitted
directly from the object, like
a light bulb or glow stick.
2. The light can come from
somewhere else, like when
we see objects by reflected
light from the sun.
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Plants absorb light energy and
convert it to chemical energy in
the form of sugar (food for the
plant).
Chlorophyll is an important
molecule that absorbs blue and
red light.
Plants must reflect some light to
avoid absorbing too much
energy. The color green is
reflected by Chlorophyll.
A plant will die if placed
under ONLY green light!
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A lens is an optical device
that is used to bend light in a
specific way.
A converging lens bends light
so that the light rays come
together to a point.
A diverging lens bends light
so it spreads light apart
instead of coming together.
The overall study of how
light behaves is called
optics.
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Light rays that enter a converging lens parallel to its axis
bend to meet at a point called the focal point.
The distance from the center of the lens to the focal
point is called the focal length.
The optical axis usually goes through the center of the
lens.
We see a world of images created on the retina of
the eye by the lens in the front of the eye.
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Mirrors reflect light and allow us to see ourselves.
A prism is another optical device that can cause light to
change directions.
A prism is a solid piece of glass with flat polished surfaces.
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Images appear in mirrors
because of how light is
reflected by mirrors.
The incident ray follows the
light falling onto the mirror.
The reflected ray follows the
light bouncing off the mirror.
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In specular reflection each incident ray bounces off a
smooth/shiny surface in a single direction.
A surface that is not shiny creates diffuse reflection.
In diffuse reflection, a single ray of light scatters into
many directions.
The incident ray
strikes the mirror.
The reflected ray
bounces off.
The angle of
incidence equals
the angle of
reflection.
A light ray is incident on a plane mirror with a
30 degree angle of incidence. Sketch the
incident and reflected rays and determine the
angle of reflection.
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You are asked for a ray diagram
and the angle of reflection.
You are given the angle of incidence.
Use the law of reflection which states the angle of
reflection equals the angle of incidence.
Sketch a ray diagram showing the angle of reflection is
30o.
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Light rays may bend as they
cross a boundary from one
material to another, like from
air to water.
This bending of light rays is
known as refraction.
The light rays from the straw
are refracted (or bent) when
they cross from water back
into air before reaching your
eyes.
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When a ray of light crosses from one material to another,
the amount it bends depends on the difference in index of
refraction between the two materials.
The ability of a material to bend rays of light is
described by the index of refraction (n).
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When white light passes through a glass prism,
blue is bent more than red.
Colors between blue and red are bent proportional
to their position in the spectrum.
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The variation in refractive
index with color is called
dispersion.
A rainbow is an example of
dispersion in nature.
Tiny rain droplets act as
prisms separating the
colors in the white light rays
from the sun.
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If you could shake the
magnet up and down 450
trillion times per second,
you would make waves
of red light with a
frequency of about 450
THz.
These waves are called
electromagnetic waves.
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Electromagnetic waves have
both an electric part and a
magnetic part. The two parts
exchange energy back and
forth.
A 3-D view of an
electromagnetic wave shows
the electric and magnetic
portions.
Light is a transverse wave.
 The wavelength and amplitude of the
waves are labeled λ and A, respectively.
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The higher the frequency of the light, the higher the energy
of the wave.
Since color is related to energy, there is also a direct
relation between color, frequency, and wavelength.
 The speed of light is incredibly fast (3 × 108
m/s) and is represented by its own symbol, c.
Speed of light
3 x 108 m/sec
c = f l
Wavelength (m)
Frequency (Hz)
Calculate the wavelength in air of blue-green light that has
a frequency of 600 × 1012 Hz.
You are asked for wavelength.
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You are given frequency.
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Use speed of light, c = ƒ l
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Solve l = c ÷ƒ
◦ l = (3 x 108 m/s) ÷ ( 600 x 1012 Hz)
◦ l = 5 x 10 -7 m
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The red color in the sky at sunrise & sunset is due to
an effect called Rayleigh scattering.
The sky appears to be blue in the day-time (when the
sun is closest to us) because the Oxygen and
Nitrogen in the atmosphere scatter violet and blue
light due to their small size.
This blue light is received by the observer.
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The light has to travels through more atmosphere during
sunrise and sunset because the distance that the light has to
travel from the Sun to an observer is at its greatest.
Blue/violet light is scattered the most & red light is
scattered the least.
This means the large amount of blue and violet light has
been scattered so the light that is received by an observer is
mostly of a longer wavelength and therefore appears to be
red.
Visible light is a small part of the energy range of electromagnetic
waves.
The whole range is called the electromagnetic spectrum and visible
light is in the middle of it.
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Light speed and equation
Electromagnetic spectrum sections, energy,
wavelength, and frequency
Colors of light in order
Basic principles of optics (reflection and refraction)
Structure of the eye
Absorption of light to see colors
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