Astrophysics

• What shape is a converging lens?

  Convex, i.e., thicker in the middle.
• What are axial rays?

  Rays parallel to the principal axis.
• What is the effect of a converging lens on axial rays?

  Rays converge at the principal focus.

  
• What happens to parallel non-axial rays when passing through a converging lens?

  Rays converge at a point on the focal plane that is not the principal focus.

  
• What is the focal length, f?

  Perpendicular distance between lens axis and focal plane.

  
• What is a real image?

  An image formed when light rays from a point on an object actually meet.

  
• What is a virtual image?

  An image formed when light rays from a point on an object appear to have come from another point in space.

  
• What are the characteristics of an image produced by a converging lens from an object beyond 2f?

  • Real
  • Inverted
  • Diminished

  
• What are the characteristics of an image produced by a converging lens from an object between f and 2f?

  • Real
  • Inverted
  • Magnified

  
• What are the characteristics of an image produced by a converging lens from an object inside f?

  • Virtual
  • Upright
  • Magnified
• What is the lens equation?

  f = focal length, u = distance between object and lens axis, v = distance between image and lens axis.

  
• What does a positive v represent in the lens equation?

  Positive v represents a real image.

  
• What does a negative v represent in the lens equation?

  Negative v represents a virtual image.

  
• What are the two lenses in a refracting telescope?

  Objective lens and eye lens.
• What type of lenses are used in a refracting telescope?

  Both are converging (convex) lenses.
• What is the expression for the length of a telescope in normal adjustment?

  Telescope length = $f_o +$$f_e$
• What does normal adjustment mean in terms of the position of the principal foci?

  The principal focus of the objective lens is in the same position as the principal focus of the eye lens.
• What can be assumed about the rays coming from one point on an object in space?

  The rays from one point on the object are parallel.
• What is the role of the objective lens in a refracting telescope?

  Converges the parallel rays from the object to form a real image on its focal plane inside the telescope.

  
• What is the role of the eye lens in a refracting telescope?

  Acts as a magnifying glass on this real image to form a magnified virtual image.

  
• What does normal adjustment mean in terms of the final image produced by a telescope?

  The telescope produces a final magnified virtual image that appears to be at infinity.

  
• What is the shape and function of the primary mirror of a Cassegrain telescope?

  Parabolic and concave; reflects axial rays to converge at its principal focus.

  
• What is the function of the secondary mirror of a Cassegrain telescope?

  Reflects light out through a hole in the primary mirror.
• How is an image seen using a Cassegrain reflecting telescope?

  The light enters the eyepiece lens, allowing a virtual image to be viewed at infinity.

  
• What are two advantages of large diameter telescopes?

  Greater collecting power and greater resolving power.
• Is it easier to build large diameter reflectors or refractors? Explain why.

  Easier to build large diameter reflectors as mirrors can be supported from behind.
• What effect does chromatic aberration have on images?

  It causes multi-coloured edges.
• What causes chromatic aberration?

  Red light is refracted less than blue light, leading to different focal lengths.

  
• Does chromatic aberration affect refractors or reflectors?

  Only affects refractors as they use refraction and lenses.
• What effect does spherical aberration have on images?

  It causes blurred images.
• What causes spherical aberration?

  Rays further from the principal axis are brought to a focus closer to the lens/mirror.
• What types of mirrors and lenses are affected by spherical aberration?

  Spherical mirrors and spherical lenses (including convex lenses).
• How can spherical aberration be overcome when using a concave mirror?

  By using a parabolic mirror, all axial rays converge at the same principal focus.

  
• Describe the two problems associated with the secondary mirror of a reflecting telescope.

  It causes less light to reach the primary mirror and diffracts light as it passes around the secondary mirror.
• Explain why reflectors are lighter than refractors and why this is an advantage.

  Mirrors are lighter than lenses, making reflectors more maneuverable for tracking objects.
• What shape is a converging lens?

  Convex i.e. thicker in the middle
• What are axial rays?

  Rays parallel to the principal axis
• What is the effect of a converging lens on axial rays?

  Rays converge at the principal focus

  
• What is the effect of a converging lens on parallel non-axial rays?

  Rays converge at a point on the focal plane that is not the principal focus

  
• What is the focal length, f?

  Perpendicular distance between lens axis and focal plane