Astrophysics

• What type of lens focuses incident light?

  Convex/converging lens
• What type of lens spreads out incident light?

  Concave/diverging lens
• What is the principal axis of a lens?

  The line through the lens center at 90°
• What is the principal focus (F) in a converging lens?

  Where parallel incident beams converge
• What is the principal focus (F) in a diverging lens?

  Point from which light rays appear to come
• What is the focal length (f) of a lens?

  Distance from lens center to principal focus
• How does focal length relate to lens strength?

  Shorter focal length means stronger lens
• What is a real image?

  Formed when light rays cross after refraction
• Can a real image be formed on a screen?

  Yes, real images can be formed on a screen
• What is a virtual image?

  Formed on the same side of the lens
• Why can't a virtual image be formed on a screen?

  Light rays do not cross for virtual images
• What is the lens formula?

  $1/u +$$1/v =$$1/f$
• What do the variables u, v, and f represent in the lens formula?

  u is object distance, v is image distance, f is focal length
• How is the power of a lens defined?

  Measure of how closely a lens can focus light
• What is the relationship between focal length and lens power?

  Shorter focal length means more powerful lens
• What is the formula for lens power?

  $P =$$1/f$
• What is the unit of lens power?

  Dioptres (D)
• What is the role of the objective lens in a refracting telescope?

  Collects light and creates a real image
• Why should the objective lens have a long focal length?

  To collect as much light as possible
• What is the collecting power of a telescope proportional to?

  Square of the radius of the objective lens
• What does the eyepiece lens do in a refracting telescope?

  Magnifies the image produced by the objective lens
• What is normal adjustment for a refracting telescope?

  Distance between lenses equals sum of their focal lengths
• What is the formula for magnifying power (M) of a telescope?

  $M =$$\frac{\text{angle subtended by image}}{\text{angle subtended by object}}$
• What happens when angles a and b are both less than 10°?

  M can be approximated
• What is the most common configuration of reflecting telescopes?

  Cassegrain reflecting telescope
• What type of mirror does a Cassegrain telescope use?

  Concave primary mirror and convex secondary mirror
• Why is the Cassegrain telescope shorter than other configurations?

  It uses a convex secondary mirror
• What is the purpose of the mirrors in a reflecting telescope?

  To collect and focus light onto an eyepiece lens
• How are the mirrors in a reflecting telescope made?

  Thin coating of aluminum or silver on backing
• What is chromatic aberration?

  Different focal lengths for different colors of light
• How does chromatic aberration affect images?

  Causes colored fringing in images
• Why does chromatic aberration have little effect on reflecting telescopes?

  It occurs mainly in the eyepiece lens
• What is spherical aberration?

  Light rays focused at different positions
• How can spherical aberration be avoided?

  By using parabolic objective mirrors
• What is an achromatic doublet?

  Convex and concave lenses cemented together
• What is the purpose of an achromatic doublet?

  To minimize spherical and chromatic aberration
• What are the disadvantages of refracting telescopes?

  • Glass must be pure and defect-free
  • Large lenses can bend under weight
  • Chromatic and spherical aberration affect lenses
  • Heavy and difficult to maneuver
• What are the advantages of reflecting telescopes?

  • Thin mirrors provide excellent image quality
  • Unaffected by chromatic aberration
  • Spherical aberration can be solved with parabolic mirrors
  • Easier to handle and maneuver
• What is the collecting power of a telescope?

  Ability to collect incident EM radiation
• How does collecting power relate to the size of the objective lens?

  Increases with the size of the objective lens