Transcript L32

L 32 Light and Optics [2]
• Measurements of the speed of light 
• The bending of light – refraction 
• Total internal reflection 
• Dispersion
• Dispersion
• Rainbows
• Atmospheric scattering
• Blue sky and red sunsets
• Mirrors and lenses
n
The index of refraction (n) depends
of the color (wavelength) of the light
color
Wavelength (nm)
n
Red
660
1.520
orange
610
1.522
yellow
580
1.523
green
550
1.526
blue
470
1.531
violet
410
1.538
1 nm = 0.000000001 m
Different colors are refracted (bent)
by different amounts
White
light
contains all
wavelengths
(colors)
red
Glass prism
violet
The rainbow
• Rainbows are caused by dispersion of sunlight
from water droplets which act as tiny prisms
Why is it a rain BOW ?
The drops must be
At just the right
Angle (42 degrees)
Between your eyes
And the sun to see
The rainbow. This
Angle is maintained
Along the arc of a
Circle.
Atmospheric scattering
• Why is the sky blue and sunsets red?
• It is due to the way that sunlight is
scattered by the atmosphere (N2 and O2)
• Scattering atoms absorb light energy
and re-emit it but not at the same
wavelength
• Sunlight contains a full range of
wavelengths in the visible region
Atmospheric scattering: blue sky
• Short wavelengths are
scattered more than long
wavelengths
• Blue light (short) is
scattered 10 times more
than red light
• The light that we see in
the sky when not looking
directly at the sun is
scattered blue light
Atmospheric scattering: red sunset
• At sunset, the sun
is low on the
horizon
• When looking at the
sun it appears red
because much of
the blue light is
scattered out
leaving only the red
Mirrors  reflection
• Light does not pass thru metals – it is reflected
at the surface
• Two types of reflection: diffuse and specular
Diffuse reflection:
Fuzzy or no image
specular reflection:
Sharp image
The law of reflection
• The angle of reflection = angle of incidence
• Incident ray, reflected ray and normal all lie
in the same plane
normal
Incident
ray
mirror
i
r
reflected
ray
image formation by plane mirrors
The rays appear to originate
from the image behind the
mirror. Of course, there is no
light behind the mirror  this
is called a virtual image
Mirrors appear
to make rooms
look larger.
You only need a mirror half as tall
as you are to see your whole self
Homer’s image
Homer
The image of your right
hand is your left hand
AMBULANCE is painted
backward so that you see
it correctly in your real-view mirror
Spherical or curved mirrors
Concave
mirror
Focus
parallel light rays are focused to one point
convex mirror
focus
parallel rays diverge from a focus behind
the mirror
Dish antennas
signal from
satellite
detector at
the focal
point of the dish
Magnifying mirrors
Homer
• when something placed
within the focus of a concave
mirror, an enlarged, upright
image is formed.
• this principle is used in a
shaving or makeup mirror
Homer’s
image
Convex mirrors: wide angle view
Object
Image
A convex lens provides a wide
angle view. Since it sees more,
the images are reduced in size.
Passenger side mirrors are often of
this type with the warning: “objects
appear further than they actually
are". Because they appear smaller
they look further away.
Image formation with lenses
• converging lens
(positive lens)
• diverging lens
(negative lens)
• the human eye
– correcting for
nearsightedness
– correcting for
farsightedness
• optical instruments
• lenses are relatively
simple optical devices
• the principle behind
the operation of a
lens is refraction
the bending of light as
it passes from air into
glass (or plastic)
converging lens
focal
point F
a converging lens focuses parallel rays to
a point called the focal point.
 a thicker lens has a shorter focal length
Diverging lens
F
A diverging lens causes
parallel rays to diverge
as if they came from a
focal point F
Image formation by a
converging lens
image
object
2F
F
If the object is located at a distance ofat least 2F from the
lens, the image is inverted and smaller than the object.
The image is called a REAL image since light rays
actually converge at the image location
A converging lens is used to
focus rays from the sun to a point
since the sun is very
far from the lens, the
rays are nearly
parallel
converging lens is used in a
camera to focus light onto the film
when you focus a camera,
you adjust the distance
between the lens and the
film depending on the
object location.
Image formation by a
diverging lens
Object
image
The diverging lens produces an image that is upright
and diminished in size.
It is a VIRTUAL image, since light rays do not
actually pass through the image point
a magnifying lens
F
F
Object
virtual image
By placing the lens close to the object
we get a magnified virtual image.
Sight – the human eye
• Physics of the human eye
• Corrections for abnormal vision
• Nearsightedness
• Farsightedness
• light enters through
the cornea
• the iris controls the
amount of light that
gets in, a muscle can
close it or open it, the
iris is the colored part
• the lens is filled with a
jelly-like substance;
the ciliary muscle can
change the shape of
the lens and thus
change its focal
length
The Eye
by changing the focal
length, (accommodation) the lens
is able to focus light onto the
retina for objects located at
various distances
the physics of the human eye
The relaxed eye can easily focus on distant
objects. To focus on close objects the lens is squeezed to
shorten it’s focal length, making it possible to converge the
rays onto the retina. The near point is the distance at which
the closest object can be seen clearly. It recedes with age.
When a nearsighted person views a distant object, the lens
cannot relax enough to focus at the retina. The rays converge
too quickly. The remedy is to place a diverging lens in front
of the eye to first diverge the
When a farsighted person tries to focus on a close object
the lens cannot be squeezed enough to focus on the retina.
W point is behind the retina. The remedy is to place
The focus
e
a converging
lens in front of the eye to converge the rays
h
before they enter the eye.
Pencil in lucite block
 the top half of the pencil is glued exactly at the position
where the image of the bottom half is formed in the block
due to refraction at the front surface the bottom of the pencil
(its image ) appears closer to the front surface of the block
the bottom half of the pencil cannot be seen from the sides
of the block because any ray from the bottom of the pencil
suffers total internal reflection on the sides of the block.
top view