PHYS 182 Lecture Notes - Lecture 8: Light Pollution, Electromagnetic Spectrum, Focal Length
PHYS182: Our Evolving Universe
2017-09-28 LEC 8
3. Telescopes
3.1 The Eye
• Inside the eye there is a lens, and a retina which perceives the image upside down
o Processing in the brain reverses the image
• The image will be produced at a length that is different than the focal length
o Focal length is always measured from the center of the lens
• If the lens diameter does not change when you bring the object closer, the image will not occur
directly on your retina
o You want to change the diameter of the lens with your eye muscles to correct the focal
length
o In a camera, you have some kind of detector where the retina is in your eye, such as a
“negative” or CCD (charge coupled device)
§ A “negative” is a film that is sensitive to light
§ CCD is an electronic solid-state version of a negative
• In a camera (similar to in an eye), you can adjust the focal length to bring the image into focus
Diffraction through a slit
• Light going through a slit is diffracted, emitting waves in all directions
o The smaller the hole, the larger the propagation of light
• This would not happen if light were a particle
• This wave-like property limits the resolution
o Advantage of big telescope: more light focused onto the light
receptors, less diffraction
• Diffraction: a single point will be diffracted into a blob, if you want to
observe single points far away in the sky, you want less diffraction to get
a higher resolution
Angular size: angle between the two stars
• 360 degrees – 1 degree = 60 arcmin – 1 min = 60 arcseconds
• angular size / 360o = physical size / 2πd
o physical size and distance must be measured in the same units
o Angular size measured in degrees
Example: angular separation = 206” * (physical separation / d)
• Note: “ = arcseconds
• We want the angular separation to be sufficiently large that S<a
• Ømin = 2.5x105 “ (λ/D)
o I.e. bigger diameter = better resolution
Example: hubble space space telescope
D = (see photo)
