Download Chapter 13 Section 1 The Characteristics of light

Chapter 13
The Characteristics of light
Objectives
• Identify the components of the electromagnetic
spectrum.
• Calculate the frequency or wavelength of
electromagnetic radiation.
• Recognize that light has a finite speed .
• Describe how the brightness of a light source is
affected by distance.
Chapter 13 Vocabulary
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Electromagnetic waves – a wave that consists of oscillating electric and
magnetic fields, which radiate outward from the source at the speed of light.
Reflection – the change in direction of an electromagnetic wave at a surface
that causes it to move away from the surface.
Angle of incidence – the angle between a ray that strikes a surface and the
line perpendicular to that surface at the point of contact.
Angle of reflection – the angle formed by the line perpendicular to a surface
and the direction in which a reflected ray moves.
Virtual image – an image that forms at a point from which light rays appear
to come but do not actually come.
Concave spherical mirror – a mirror whose reflecting surface is a segment
of the inside of a sphere.
Real image – an image formed when rays of light actually pass through a
point on the image.
Convex spherical mirror – a mirror whose reflecting surface is an outwardcurved segment of a sphere.
Linear polarization – the alignment of EM waves in such a way that the
vibrations of electric fields in each of the waves are parallel to each other.
Electromagnetic Waves
• Some light cannot be seen by the human eye.
• This is because light is measured in a variety of
forms of radiation.
• These forms are examples of electromagnetic
waves.
• They vary depending on frequency and
wavelength.
• These frequencies are represented on the
electromagnetic spectrum.
The electromagnetic spectrum
All electromagnetic waves move at
the speed of light
• All forms of electromagnetic light travels at light
speed in a vacuum.
• The current accepted value for light speed is
8
2.99792458 X 10 m/s (or 3.0 x 108 m/s)
 The equation to determine wave speed is
•
C=fl
•
Which translates:
• speed of light=frequency X wavelength
Illuminance decreases as the square
of the distance from the source
• The rate at which light is emitted from a source
is called the luminous flux and is measured in
lumens (lm)
• The luminous flux decreases as you move away
from the light source.
To recap:
Mirrors (Sections 2-3)
• Reflection – the change in direction of an electromagnetic wave at a
surface that causes it to move away from the surface
• Angle of incidence – the angle between a ray that strikes a surface
and the line perpendicular to that surface at the point of contact
• Angle of reflection – the angle formed by the line perpendicular to a
surface and the direction in which a reflected ray moves
• Virtual image – an image that forms at a point from which light rays
appear to come but do not actually come
• Concave spherical mirror – a mirror whose reflecting surface is a
segment of the inside of a sphere
• Real image – an image formed when rays of light actually pass
through a point on the image
• Convex spherical mirror – a mirror whose reflecting surface is an
outward-curved segment of a sphere
Mirrors
• Most of us already have a good
understanding of plane mirrors “aka flat
mirrors” and reflection with plane mirrors.
Remember that the angle of incidence
and reflection equate to be the same
degree.
Convex Mirrors
• Although the angle of incidence and reflection
equate to be the same value in a flat mirror, the
angles change in a convex mirror due to the
outward sphere. This makes the light rays go out
into different directions in an organized manner,
thus giving a smaller image, but a wide angle
view. This is why optical engineers use convex
mirrors when a situation calls for a better view of
an area, such as in your passenger side mirrors
on your car.
Concave Mirrors
• A concave mirror is a little harder to understand.
Imagine how when you look at the inside of a
spoon, how your image is upside down. That is
because when the light ray hits the inward
sphere, the rays are reflected to a focus line, a
line parallel to the initial light ray. Once you
move past the focal point, the area where the
light rays come together, the image flips due to
the fact that you are picking up a flipped ray,
where the top ray is now on bottom and the
bottom ray is now on top.
Chapter 14 - Refraction
• Refraction – the bending of a wave front as the wave front passes
between two substances in which the speed of the wave differs.
• Index of refraction – the ratio of the speed of light in a vacuum to the
speed of light in a given transparent medium.
• Lens – a transparent object that refracts light rays such that they
converge or diverge to create an image.
• Total internal reflection – the complete reflection that occurs within a
substance when the angle of incidence of light striking the surface
boundary is greater than the critical angle.
• Critical angle – the angle of incidence at which the refracted light
makes an angle of 90° with the normal (where refraction stops and
reflection begins).
• Dispersion – the process of separating polychromatic light into its
component wavelengths
• Chromatic aberration – the focusing of different colors of light at
different distances behind a lens.
Refraction
• Refraction is the bending of a wave as it enters a new medium at
an angle.
• When a wave enters a medium at an angle, refraction occurs
because one side of the wave moves more slowly than the other
side.
• Refraction only occurs when the two sides of a wave travel at
different speeds.
If light travels from one transparent medium to another
at any angle other than straight on, the light ray changes
direction when it meets the boundary. In case of
reflection, the angles of incoming and refracted rays are
measured with respect to the normal. When studying
refraction, the normal line is extended into the refracting
medium. When light moves from one medium to
another, part of it is reflected and part is refracted.
An important property of transparent substances is the
index of refraction. Index of refraction is the ratio of the
speed of light in a vacuum to the speed of light in a
given transparent medium.
Index of refraction = Speed of Light (in vacuum)
Speed of Light (in medium)
Index of
Refraction
Snell’s Law
Snell’s law determines the angle of refraction
The index of refraction of a material can be used to
figure out how much a ray of light will be refracted as
it passes from one medium to another. The greater
the index of refraction, the more refraction occurs.
The angle of refraction was first found in 1621 by
Willebrord Snell, who experimented with light
passing through different media. He then developed
a relationship called Snell’s law, which can be used
to find the angle of refraction for light travelling
between any two media
Snell’s Law
• Snell’s Law = nisinθi = nrsinθr
• (Index of refraction of incident material)
times the sin(incident angle in degrees) is
equal to (index of refraction of refractive
material) times the sin(refracted angle in
degrees)
Sample Problem
• A light ray of wavelength 589 nm (produced by a sodium
lamp) traveling through air strikes a smooth, flat slab of
crown glass at an angle of 30.0° to the normal. Find the
angle of refraction, Θr.
• Given:
Θi = 30.0
ni = 1.00
nr = 1.52
• Unknown: Θr
• Solve:
nisinθi = nrsinθr
nisinθi / nr = sinθr
θr = sin-1(nisinθi / nr)
θr = sin-1[(1.00)(sin30.0°)/1.52]
θr = 19.2°
Lenses
Lenses, in a sense, are the complete opposite of mirrors, they use
refraction, which is the bending of light as it goes from one
medium to the next, to make a focal point, or spread out a view.
Convex Lenses
• Convex Lenses are the complete opposite, in
theory, of convex mirrors. They too have an
outward sphere appearance, but instead use
refraction to create a focal point, compared
to a convex mirror which spreads out the light
rays to make a larger image. A good example
of this would be positive lens glasses, or close
up glasses. They center the light into a
common focal point and make things easier to
distinguish close up.
Concave Lenses
• Concave Lenses are the complete opposite, in
theory, of concave mirrors. They too have an
inward sphere appearance, but instead use
refraction, again the bending of light as it goes
from one medium to the next, to spread out the
light rays and create a larger image. A good
example of this would be a tube television, the
screen is concave, which makes the image
spread out throughout the whole room, making
things easier to see.
Ch. 15 – Interference &
Diffraction
• Coherence – the correlation between the phases of two or more waves
• Path difference – the difference in the distance traveled by two beams
when they are scattered in the same direction from different points
• Order number – the number assigned to interference fringes with respect
to the central bright fringe
• Diffraction – a change in the direction of a wave when the wave
encounters an obstacle, an opening, or an edge
• Resolving power – the ability of an optical instruments to form separate
images of two objects that are close together
• Laser – a device that produces coherent light at a single wavelength
Interference
• There are two types of wave interference: constructive
and destructive
– In constructive interference, waves combine to form a resultant
wave with the same wavelength but a greater amplitude than
either of the original waves.
– In destructive interference, waves combine to form a resultant
wave whose resulting amplitude is smaller than the original and
whose wavelength is not the same as it was
• There are formulas for calculating constructive and
destructive interference: d sin Θ = ±mλ (constructive)
and d sin Θ = ±(m+½)λ (destructive).
Diffraction
• Diffraction is a
change in the direction
of a wave when the
wave encounters an
obstacle, an opening,
or an edge.
• A wave diffracts more
if its wavelength is
large compared to the
size of an opening or
obstacle.