Transcript Optical Refraction

Refraction
Optical Density
Inverse measure of speed of light through
transparent medium
Light travels slower in more dense media
Partial reflection occurs at boundary with
more dense medium
If incident angle not 90 degrees, refraction
occurs
Optical Refraction
Bending of light rays as they pass
obliquely from one medium to another of
different optical density
Angle of refraction measured to normal
from refracted ray
Passing from lower to higher density, light
refracted towards normal; high to low,
away from normal
Index of Refraction
Ratio of speed of light in a vacuum (c) to
its speed in a substance
n =c/v
Measured by refractometer, used to test
purity of substance
Snell’s Law
Relates index of refraction to the angle of
refraction
Between any two media ni(sin qi) = nr(sin
qr)
Since nair = 1.00, for light passing from air
into another transparent medium,
n = sin qi / sin qr
Atmospheric Refraction
Causes gradual curve of light from stars
and sun
Creates mirages that look like wet spots on
roads
Makes sun visible 2-3 min. before sunrise
and after sunset
Mirage Formation
Highway Mirage
Laws of Refraction
Incident ray, refracted ray & normal line all
lie in same plane
Index of refraction for homogeneous
medium is constant, independent of
incident angle
Oblique ray passing from low to high
optical density is bent towards normal and
vice versa
Dispersion
Transparent media react differently to
different wavelengths, slowing short waves
more than long waves
Different wavelengths are refracted to a
different degree, violet more than red
Causes spreading of the light according to
wavelength (frequency) - rainbow
Dispersion
Prisms, water drops readily disperse light
due to non-parallel surfaces
Rainbows created by refraction through
many drops
Each color produced by a set of drops at a
certain angle from the eye
Dispersion in Raindrops
Rainbow Physics
Total Reflection
At media boundary, light from denser
medium refracted back into it, rather than
exiting into less dense medium
Critical angle: incident angle that produces
refracted angle of 90 degrees
At critical angle, refracted ray parallel to
media boundary
Total Reflection
o
From Snell’s law: n = sin 90 /sin ic so
sin ic = 1/n
Critical angle for water is 48.5 deg., for
diamond it is 24 deg.
If incident angle > critical angle, total
reflection occurs
Causes diamond’s sparkle, fiber optics
Total Internal Reflection
Fiber Optics
Lenses
Transparent object with nonparallel
surfaces, at least one of which is curved
Usually glass or plastic but can be water,
air, other transparent solid, liquid or gas
Converging: thicker in middle, converges
(focuses) rays
Diverging: thinner in middle, diverges
(spreads) rays
Lens Terms
Each side of lens has center of curvature
and focus
Real focus (converging lens) where light
rays pass through
Real image forms on same side of lens as
real focus, opposite side of object
Lens Terms
Virtual focus (diverging or converging)
where light rays appear to have originated
Virtual image forms on same side of lens
as virtual focus and object
Focal length: distance from center of lens
to focal point; depends on curvature and
index of refraction of lens
Mirrors & Lenses: Differences
Secondary axes pass through center of lens
Principal focus usually near C; use 2F
instead of C in ray diagrams
Real images on opposite side of lens as
object, virtual images on same side
Convex lenses are like concave mirrors,
concave lenses like convex mirrors
Images of Converging Lenses
Object at infinite distance forms point
image at F on opposite side
Object at finite distance > 2F forms real,
reduced image between F and 2F on
opposite side
Object at 2F forms real, same size image at
2F on opposite side
Images of Converging Lenses
Object between F and 2F forms real,
magnified image beyond 2F on opposite
side
Object at F forms no image, rays are
parallel
Object between F and lens forms enlarged,
virtual image on same side (magnifying
glass)
Images of Diverging Lenses
Always virtual, erect, reduced size
Often used to neutralize or minimize effect
of converging lens (glasses)
Lens Equations
1/f = 1/do + 1/di
hi / h o = di / do
For simple magnifier, magnification
M = hi / ho = di / do
for normal vision, di = 25 cm, so
M = 25 cm/f (f - focal length)
f-numbers
Ratio of focal length to aperture (effective
diameter), used to rate camera lenses
Determines light gathering power of lens
“Fast” lenses have low f-numbers, gather
more light, need shorter exposure times
Since area of lens is prop. to square of
diameter, f-2 lens is 4 times faster than f-4,
16 times faster than f-8
The Microscope
Objective lens forms enlarged, real image
Eyepiece magnifies image of objective
producing greatly magnified, inverted,
virtual image
Objective power = tube length/focal length
Total magnification M=25length/fe fo ( all
in cm)
Telescopes
Reflectors have one converging mirror and
a converging eyepiece lens
Refracting telescopes have large objective
lens instead of a mirror
Object at great distance means small, real
image is produced by objective mirror or
lens
Telescopes
Eyepiece lens enlarges objective image
producing magnified, inverted, virtual
image
Large telescopes are reflectors due to size
and expense of large lens
Binoculars, terrestrial telescopes use extra
lens or prism to invert image to upright
position
The Eye
Cornea and lens work together to focus
light on retina producing inverted, small
image
Brain circuitry inverts image so it seems
right side up
Vision Correction
Nearsighted means light focuses in front of
retina—corrected with diverging lens
Farsighted means light would focus behind
retina—corrected with converging lens
Cameras
Cameras focus light on the focal plane
where the film is located
Produce real, inverted, smaller image, like
the eye
Some cameras use a diverging lens for a
viewfinder
Lens Aberrations
Spherical aberration: like mirrors, light
passing through edges not focused at same
point as through center - correct with lens
combination
Chromatic aberration: different colors
refracted differently, focus at different
points - correct with lens coatings, lenses
of different materials