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Light in a Newtonian view
Chapter 16
Introducing: light
• Light is the most important source of
information for humans
• Concept of light rays
- there are received and NOT emitted by eyes
(seeing is a passive and not an active process)
- traveling in straight line
- does not necessitate a medium to propagate
- it dissipates when traveling in a medium
- there are mediums in which they cannot propagate
(opaque objects)
- it was believed to be tinny particles that travel with very
high speed
- different in many aspects from sound
• Light sources: high temperature objects or gases,
excited matter
point-like (a tiny light-bulb), (a)
extended (a neon tube) (b) (each point of it a point-like
source)
directed sources (reflectors, lasers)
Shadows
• Consequence of
- straight light rays
- opaque objects
• shadows produced by different type of sources
point like and directed sources produce clear shadows!
extended objects: shadows with diffuse edges
• concept of umbra and penumbra for shadows of extended objects
(penumbra is not due to diffraction, but due to extended sources)
• eclipses: the most spectacular shadows
The simplest camera: the pinhole camera
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A camera without lenses
a closed box + a hole on one wall + film
based on the light rays straight propagation
principle very simple (figure)
- big hole: each part of the film receives light from many parts of the vase and
flowers --> no image recorded
- smaller hole: the screen restricts the light so that each part of the film receives
light from a small portion of the scene
- pinhole: results a sharper but dimmer image (light intensity strongly reduced)
Speed of light
• extremely large, early attempts to measure the speed of light fail
• light considered to have infinite speed
• to measure speed of light easily we need:
- very strong sources
- long distances
- measuring small time intervals
• wiser approach: Hippolyte Fizeau (1849)
• speed of light in vacuum: 299,792,458 m/s (~ 3x108 m/s)
• speed of light smaller in air or fluids
• universal constant in vacuum: does not depend on the relative
speed of the source and observer--> special relativity
• Color adds beauty to our lives
• understanding colors involves: physics, chemistry,
physiology and psychology
• Visible light is an electromagnetic wave with
frequencies between: 1014-1016 Hz
• The color of the light is determined by the
frequencies of the light rays entering our eyes
• the colors we perceive from objects are determined
by two factors: the color present in the illuminating
light and the colors reflected by the object
• Different frequency light rays are interpreted in
different ways by our brain, creating the sensation of
colors
• brain has an additive effect: different frequency light
rays combine in our eyes, and create sensation of
new colors
• a combination of light beams that produces most of
the colors: red, green and blue
• complimentary colors: combined produce white
• why the Sun is yellow and the sky blue?
Colors
REFLECTION OF LIGHT
• Some things are visible because they are light sources
• things can be visible if they reflect light
• reflection from rough surfaces: diffuse reflection
• reflection from smooth surfaces: clear reflection
• laws of reflection:
1.The angle of reflection is equal to the angle of incidence
2. The reflected ray lies in the same plane as the normal
and incident ray
Flat Mirrors
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A smooth reflecting plane surface
image of objects produced behind the mirror
same size, and same distance from the mirror
image has a definite location in space
determining the position of the image
- seeing is a passive activity
- our eye-brain system records only the direction from which the light arrives
main method: determining where the reflected light rays coming from different
points of the object intersect
if the reflected light rays really intersect --> real image
if only the inverse direction continuation of light rays intersect
-->virtual image
Multiple reflections
• Reflection from two or more mirrors
• Used in many devices
- Optical illusions
- Kaleidoscopes
- Periscopes
- Retroreflectors
Curved mirrors
• Concave or Convex mirrors
(slice of the surface of a sphere)
• produce: distorsion,
magnification, wide view ….
• Characteristic points, lines and
distances:
- center of the sphere (C)
- optic axis (the line passing
through C and center of the
mirror)
- focal point (halfway between
the mirror and C), light rays
coming parallel with the optical
axis intersect here
- focal distance: length of F-M
Images Produced by Curved Mirrors
• Concave mirror: two different type of image
- for an object at large distance: inverted and reduced in size (real
image)
- if the object is between C and the mirror: magnified, erect
image (virtual image)
• essential difference between real and virtual images: whether the
light actually comes from the image location or only appears to come
from there
- real image can be projected on a screen
- virtual images cannot be projected on a screen
• Convex mirror: always form reduced size and erect virtual image
Principles for Locating the Image
• We can predict the position and size of the image by geometrical
construction
• We look at a few special rays, and use the principle:
All light rays starting from a given point of the object, are
converging in one point (the image point)
Consequence: Following two rays are sufficient to construct the image
• The path of three light rays are easy to construct (we use these)
1. along radius (coming back on itself) 2.parallel to optical axis (through focal point)
3. through focal point (parallel to optic axis)
• Additional principle: the image of a point on the optic axis is on the optic axis
Constructing the Image for Curved Mirrors
• We use the previous principles, and special light rays
m
i
o
Concave
mirror, virtual
image
Concave
mirror, real
image
Convex
mirror,
virtual image
Formula
f: focal distance
i: distance of image
o: distance of objects
(distances measured
from center of mirror)
f>0: for concave
f<0: for convex
i>0 for real image
i<0 for virtual image
(same for : o)
m: magnification
m>0 image erect
m<0 image reversed
1 1 1
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i o f
i
m
o
Summary
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Light travels in straight lines
formation of shadows and images understood by straight light rays
light travels through vacuum at 299,792,458 m/s
we detect different frequency visible electromagnetic radiation as
light with different colors, additive effect of colors, complimentary
colors
light reflects from surfaces
the laws of reflection: (1) the angles the incident and reflected rays
make with the normal to the surface are equal (2) the reflected ray
lies in the same plane as the normal and incident ray
mirrors produce real and virtual images
light converges to from real images which can be projected
light only appears to come from virtual images, which cannot be
projected
constructing the image for flat and curved mirrors
Home-work assignment
• Part I.
412/1-2, 5-13 ; 414/46-49 ; 415/50-53, 56-60
415/1-2; 416/19-24;
• Part II.
412/15; 413/16-23,25; 414/29,31-33,35-37,39;
414/43-44; 415/5; 416/6-17