Download Lecture 12 - Empyrean Quest Publishers

NOTES: Reflection and Refraction
Principle of reflection:
the angle of incidence equals the angle of reflection--for all mirrors.
A parabolic mirror creates an image with no distortion.
Spherical mirrors (cheaper to make) work only close to the axis.
Principle of refraction:
light bends when it passes at an angle between two media with
different speeds of light like air and glass--lenses utilize this
principle to focalize images.
Lenses
The Focal length of a lens is the distance parallel rays
entering the lens focus to a point.
Rays from a distant object may be considered to be parallel rays.
A convex or converging lens is curved outward on both sides.
A concave or diverging lens is curved inward on both sides.
Eyeglasses are often convex-concave.
Principle of reflection:
the angle of incidence equals the angle of reflection
--for all mirrors, flat or curved.
The Focal length of a mirror is the distance from the mirror
parallel rays hitting it focus to a point. (Note: each ray
Is reflected at an angle = to it’s angle of incidence.)
A parabolic mirror focuses rays to a point…
…but a spherical mirror does NOT!
Most cheap telescope mirrors are spherical,
but the distortion is minimal.
Principle of refraction:
light bends when it passes at an angle between two media
with different speeds of light like air and glass.
This is similar to waves on the ocean. They change their path
when coming in at an angle.
This is because the speed of the wave is smaller in shallow water.
The speed of light is smaller in glass or water than in air.
Thus the light changes its direction.
This principle is used in lenses.
Converging or convex lens
Diverging or concave lens
Note: rays from a distant object
come in nearly parallel.
Problem:
lenses suffer from
chromatic aberration.
Solution: add an achromat.
Different colors (wavelengths) have different
speeds of light,
and diffract over different angles.
Lenses make images:
We can use ray diagrams with three principal rays to
determine where they are.
The images are real or virtual:
Real images are formed by light rays
virtual images are not—they are
optical illusions.
NOTES: Telescopes
Reflecting or Newtonian telescope (reflector):
A large curved objective mirror focuses an image,
a small eyepiece lens magnifies it. The image is inverted.
Refracting telescope (refractor): A large objective lens
(usually converging) focuses an image and a small eyepiece lens magnifies it.
The image is also inverted.
Seeing--the following conditions interfere with clear viewing:
1. Atmospheric turbulence--why stars twinkle.
2. Atmospheric absorption--some wavelengths.
3. Weather--cloud cover.
4. Light pollution--city lights.
Powers of Telescopes:
1. Light gathering power--proportional to the area of objective.
2. Magnification--m = F/f (F focal length of objective, f focal length of eyepiece)
3. Resolution--the angle of separation at which two objects merge and look like one.
Interferometry: using two radio telescopes or more and time delays
to simulate on large telescope. Usually a radio telescope.
Satellites: necessary for most infrared, x-ray, gamma ray, micro-wave, UV.
Uhuru--xray satellite discovered first black hole.
Telescopes
Reflecting or
Newtonian
telescope
(reflector):
Equatorial
Mount.
A large curved objective mirror
focuses an image,
a small eyepiece lens magnifies
it. The image is inverted.
Refracting telescope (refractor): A large objective lens
(usually converging) focuses an image and a small
eyepiece lens magnifies it. The image is also inverted.
Distance between lenses is a little more than the sum of the two focal lengths.
Seeing--the following conditions interfere with clear
viewing:
1. Atmospheric turbulence--why stars twinkle.
The Keck Twin Telescopes in Hawaii compensate for turbulence
by mechanically bending component mirrors.
This is called adaptive optics.
2. Atmospheric absorption--some wavelengths.
3. Weather--cloud cover.
4. Light pollution--city lights.
Powers of Telescopes:
1. Light gathering power--proportional to the area of objective.
2. Magnification--m = F/f
(F focal length of objective, f focal length of eyepiece)
3. Resolution--the angle of separation at which two objects merge
and look like one.
Interferometry: using two telescopes
or more and time delays on computer
to simulate on large telescope.
Usually a radio telescope.
Example:
Very Large Array
In New Mexico
Very
Long
Baseline
Array:
World’s
largest
aperture
Satellites: necessary for most infrared, x-ray,
gamma ray, micro-wave, UV.
The Chandra gamma (and x-ray) observatory:
Named after 1983 Nobel laureate Subrahmanyan Chandrasekhar. (NASA.)
Uhuru—x-ray satellite discovered x-rays from first black hole,
Cygnus X-1. Uhuru means freedom in Swahili.
Uhuru NOT Uhura