Transcript Slide 1

WAVES
• MEDIUM VIBRATES
PERPENDICULARLY TO
THE WAVE DIRECTION
• IF f IS THE WAVE
FREQUENCE AND λ IS THE
WAVELEGTH THEN c, THE
WAVE VELOCITY, IS GIVEN
BY: c = λf
• EXAMPLES
– ELECTROMAGNETIC
WAVES
– WAVES IN A STRING
ELECTROMAGNETIC WAVES
• FOR EVERY ELECTRIC
WAVE THERE IS A
CORRESPONDING
MAGNETIC WAVE AT
RIGHT ANGLES TO IT
(AND VICE VERSA).
LONGITUDINAL WAVES
• PARTICLE MOTION IS
PARALLEL TO THE WAVE
DIRECTION
• EXAMPLE – SOUND
WAVES
ELECTROMAGNETIC (E-M) SPECTRUM
• E-M WAVES IN NATURE RANGE
FROM λ=.01 nm TO λ=1,000
m, A RANGE OF 1013.
• 1 ANGSTROM = 10-8 CM
• FROM SHORT TO LONG
WAVELENGTH:
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–
–
–
GAMMA (γ) RAYS
X-RAYS
ULTRA-VIOLET (UV) RAYS
VISIBLE RAYS (LIGHT)
INFRA-RED (IR) RAYS
RADIO WAVES
• TWO ATMOSPHERIC
WINDOWS (LIGHT AND
RADIO)
REFLECTION
• REACTION OF LIGHT
WAVES WHEN THEY
ENCOUNTER AN OPAQUE
MEDIUM
i=r
WHERE:
i IS THE ANGLE OF INCIDENCE
r IS THE ANGLE OF REFLECTION
REFRACTION
THE REACTION OF LIGHT
WAVES WHEN THEY
ENCOUNTER A
TRANSPARENT INTERFACE
θ1=θ2
WHERE:
θ1 IS THE ANGLE OF INCIDENCE
θ2 IS THE ANGLE OF REFRACTION
THE LIGHT RAY BENDS TOWARD
THE NORMAL LINE IF IT GOES
INTO AN OPTICALLY MORE
DENSE MEDIUM (THE
VELOCITY OF LIGHT IS SLOWER)
TOTAL INTERNAL REFLECTION
• WHEN LIGHT TRAVELS
INTO AN OPTICALLY LESS
DENSE MEDIUM IT WILL
BEND AWAY FROM THE
NORMAL LINE
• θ1 = θC (THE CRITICAL
ANGLE) WHEN θ2 = 90o
• IF θ1>θc THERE WILL BE
NO REFRACTED WAVE
• THE TRANSPARENT
INTERFACE WILL BEHAVE
AS A PERFECT MIRROR
PRISM AS A MIRROR
• THE ADVANTAGE OF
USING A PRISM AS A
MIRROR IS THAT THE
REFLECTING SURFACE
CANNOT GET DIRTY
PRISMS IN BINOCULARS
• PRISMS ARE USED IN
BINOCULARS TO
– INVERT THE IMAGE
– LENGTHEN THE
“TELESCOPE” TO GET
GREATER
MAGNIFICATION
DIFFRACTION
• THE BENDING OF A
WAVE WHEN IT
ENCOUNTERS AN
OBSTACLE
DISPERSION USING VARIABLE
REFRACTION
• DISPERSION IS THE
SPREADING OF LIGHT
INTO ITS COMPONENT
COLORS
• IN A PRISM BLUE LIGHT
IS DISPERSED (BENT)
MORE THAN RED LIGHT
PRISM SPECTROGRAPH
• THE ENTRANCE SLIT
NARROWS THE LIGHT WAVES
BEING CONSIDERED
• ALL THE LIGHT OF A GIVEN
WAVELENGTH (COLOR) IS
FOCUSED AT THE SAME SPOT
ON THE FILM
• BLUE LIGHT IS BENT MORE
THAN RED LIGHT
• THERE WILL BE AN IMAGE OF
THE ENTRANCE SLIT FOR EACH
COLOR THAT IS IN THE
SOURCE.
• HENCE THE TERM “SPECTRAL
LINE”
GRATING SPECTROGRAPH
• DISPERSION IS
ACCOMPLISHED BY
DIFFRACTION AND
INTERFERENCE
• RED LIGHT IS BENT MORE
• THE ZERO ORDER IS
CALLED THE “WHITE”
FRINGE
• HIGHER ORDERS ARE
DISPERSED MORE (THE
SPECTRAL LINES ARE
FARTHER APART)
RESOLUTION (RESOLVING POWER)
• RESOLUTION (α), THE MINIMUM ANGLE BETWEEN TWO OBJECTS
SUCH THAT THEY CAN JUST BE DISTINGUISHED
• SINCE THE MINIMUM ANGLE IS SUBJECTIVE, LORD RAYLEIGH
DEFINED IT TO BE WHERE THE AIRY DISKS OF ADJACENT STAR
IMAGES OVERLAPPED AT “HALF POWER”
• THEN THE EXPRESSION FOR THE RESOLUTION BECAME:
α(arcsec) = 250,000 λ/a
• NOTE: HIGH RESOLUTION CORRESPONDS TO SMALL α
• TO MAKE α SMALL EITHER λ MUST BE SMALL OR a MUST BE LARGE
• THAT’S WHY LARGER TELESCOPES HAVE HIGHER RESOLUTION
THE AIRY DISK
SPHERICAL LENS
CHROMATIC ABERRATION
SPHERICAL ABERRATION
COMPOUND LENSES
•
•
•
•
•
ACHROMATIC DOUBLET – TWO LENSES
MADE OF DIFFERENT TYPES OF GLASS
(HAVING DIFFERENT INDICES OF
REFRACTION)
THE INDEX OF REFRACTION IS THE
VELOCITY OF LIGHT IN FREE SPACE
DIVIDED BY THE VELOCITY OF LIGHT IN
THE MEDIUM.
YOU CAN CHOOSE TWO WAVELENGTHS
(COLORS) WHICH FOCUS AT THE SAME
PLACE
IF YOU USE THREE LENSES YOU CAN
CHOOSE THREE WAVELENGTHS THAT
FOCUS AT THE SAME PLACE
IN ANY MULTIPLE LENSE ARRANGEMENT
YOU CAN CHOOSE AS MANY
WAVELENGTHS WHICH FOCUS AT THE
SAME PLACE AS LENSES THAT YOU USE.
TELESCOPE PROPERTIES
• MAGNIFICATION (M): M = f0/fe, where f0 is the
objective focal length and fe is the eyepiece
focal length
• SPEED (f#, f stop, focal ratio): f# = f0/a, where a
is the aperture size
• RESOLUTION (α), the minimum angle between
two objects such that they can just be
distinguished: α(arcsec) = 250,000 λ/a
GALILEAN TELESCOPE
VIRTUAL ERECT IMAGE – CURRENT DAY OPERA GLASSES
REFRACTING TELESCOPE
INVERTED IMAGE
REFLECTING TELESCOPE
PRIME FOCUSING SYSTEM
REFLECTING TELESCOPE
NEWTONIAN FOCUSING SYSTEM
REFLECTING TELESCOPE
CASSAGRAIN FOCUSING SYSTEM
REFLECTING TELESCOPE
COUDE’ FOCUSING SYSTEM
REFLECTING TELESCOPES
ADVANTAGES
• CAN BE TRULY
PARABOLOIDAL
• CAN BE MADE LARGER
• ONLY ONE SURFACE TO
GRIND
• EASIER TO SUPPORT
• FASTER (SHORTER FOCAL
LENGTH)
DISADVANTAGES
• SMALL FIELD OF VIEW
SCHMIDT CATADIOPTRIC TELESCOPE
WIDE FIELD OF VIEW
PRIMARY MIRROR IS SPHERICAL
MAKSUTOV CATADIOTRIC TELESCOPE
WIDE FIELD OF VIEW
PRIMARY MIRROR IS SPHERICAL
ABERRATION OF STARLIGHT
• THE APPARENT CHANGE
IN A STAR’S LOCATION
CAUSED BY THE
EARTH’S MOTION
• DISCOVERED BY
BRADLEY 1N 1729