Lenses and Ray Diagrams for Telescopes

• What is a lens used for?

  To form an image by refracting light

  
• What are the two types of lenses?

  Convex and concave lenses

  
• Which type of lens will be discussed in the Astrophysics module?

  Only converging lenses
• What happens to parallel rays of light in a converging lens?

  They are brought to a focus along the principal axis

  
• What is the focal point of a lens?

  The point where light converges

  
• What is the focal length of a lens?

  The distance from the lens center to the focal point
• What does the focal length depend on?

  How curved or thick the lens is
• How does the curvature of a lens affect its focal length?

  More curved lenses have shorter focal lengths
• What is the relationship between focal length and lens power?

  Shorter focal lengths mean more powerful lenses

  
• What are the two types of images produced by lenses?

  Real and virtual images
• How does light behave in a real image?

  Light converges towards a focal point

  
• What is the orientation of a real image?

  Always inverted
• Can a real image be projected onto a screen?

  Yes, it can be projected onto a screen
• How does light behave in a virtual image?

  Light diverges away from a focal point
• What is the orientation of a virtual image?

  Always upright
• Can a virtual image be projected onto a screen?

  No, it cannot be projected onto a screen
• What indicates a real image in a ray diagram?

  Intersection of two solid lines
• What indicates a virtual image in a ray diagram?

  Intersection of two dashed lines
• What are the three main rules for constructing ray diagrams?

  1. Rays through the principal axis pass undeviated through the optical center.
  2. Rays parallel to the principal axis refract through the focal point.
  3. Rays through the focal point emerge parallel to the principal axis.

  
• What properties describe images formed by lenses?

  • Nature: Real or virtual
  • Orientation: Inverted or upright
  • Size: Magnified, diminished, or same size

  
• What happens to the image when an object is placed at a distance greater than 2 focal lengths?

  The image is real, inverted, and diminished

  
• What happens to the image when an object is placed at a distance equal to 2 focal lengths?

  The image is real, inverted, and the same size

  
• What happens to the image when an object is placed between 1 and 2 focal lengths?

  The image is real, inverted, and magnified

  
• What happens to the image when an object is placed at a distance less than the focal length?

  The image is virtual, upright, and magnified

  
• What are the steps to draw a ray diagram for a converging lens forming a diminished image?

  1. Draw and label the principal axis and lens.
  2. Mark and label the focal points.
  3. Draw the object at a distance greater than the focal length.
  4. Draw a ray through the optical center.
  5. Draw a second ray parallel to the principal axis.
  6. Draw the continuation of the ray through the focal point.
  7. Draw and label the image where rays meet.
  8. Ensure all elements are included for marks.

  
• What is the lens equation?

  \($\frac{1}{f} =$$\frac{1}{u} +$ \frac{1}{v}\)
• What does the lens equation relate?

  Focal length to distances of image and object

  
• What is the sign of \(v\) for a real image?

  Positive
• What is the sign of \(v\) for a virtual image?

  Negative
• What is magnification, \(M\), defined as?

  The ratio of image height to object height
• How can magnification also be expressed using distances?

  \(M = \frac{v}{u}\)
• What does a negative value of \(v\) indicate?

  A virtual image is formed
• What is the magnification of a magnifying glass with a focal length of 15 cm held 5 cm away?


  1.5
• What are the examiner tips for drawing ray diagrams?

  • Draw rays through the optical center.
  • Ensure rays produce the correct image size.
  • Label both object and image correctly.