Refracting Telescopes

• What is a refracting telescope also known as?

  Refractor
• How does a refracting telescope project images?

  It uses two converging lenses
• What is the function of the objective lens in a refracting telescope?

  It collects light from stars
• Where does the objective lens bring light to focus?

  At its focal length $f_o$
• What is the role of the eyepiece lens in a refracting telescope?

  It produces parallel rays of light

  
• What is normal adjustment in a refracting telescope?

  The final image is at infinity
• What must be true for a refractor to be in normal adjustment?

  Both lenses' focal points must meet
• What is the relationship between the focal lengths of the objective and eyepiece lenses?

  $f_o > f_e$

  
• What are the steps to draw a ray diagram for a refracting telescope?

  1. Draw and label the two lenses
  2. Mark and label the common principal foci
  3. Draw an off-axis ray through the objective
  4. Draw an arrow for the intermediate image
  5. Draw a construction line through the eyepiece
  6. Draw two rays to the eyepiece
  7. Draw the continuation of the rays parallel to the construction line
  8. Check the final image for completeness

  
• What is the angle subtended by an object at a distance?

  $\theta =$$\frac{h}{d}$
• What is angular magnification defined as?

  $M =$$\frac{\beta}{\alpha}$
• How do telescopes affect the angular size of distant objects?

  They magnify the angular size
• What is the relationship between the angles subtended by the object and image?

  $\alpha < \beta$
• What is the significance of similar triangles in a refracting telescope?

  They help determine angular magnification
• What is the equation for angular magnification in a refractor?

  $M =$$\frac{f_o}{f_e}$
• What is required for greater magnifications in telescopes?

  Longer objective and shorter eyepiece focal lengths

  
• What is the lens equation used to calculate focal length?

  $\frac{1}{f} =$$\frac{1}{u} +$$\frac{1}{v}$
• How do you calculate the focal length of a lens given $u =$$207 \text{ cm}$ and $v =$$93 \text{ cm}$?


  $f =$$64 \text{ cm}$
• What type of image does the eyepiece form?

  A virtual, inverted, magnified image

  
• What does the objective lens form?

  A real, inverted, diminished image

  
• What is the total length of a refracting telescope?

  $f_o +$$f_e$
• How do you calculate the focal lengths of the objective and eyepiece lenses given the telescope length and angular magnification?

  Use $f_o +$$f_e =$$1.23 \text{ m}$ and $M =$$\frac{f_o}{f_e} =$$200$

  
• How do you determine the angle subtended by Neptune at the unaided eye?

  $\theta =$$\frac{h}{d}$ where $h =$$4.9 \times 10^4 \text{ km}$ and $d =$$4.3 \times 10^9 \text{ km}$
• How do you calculate the angle subtended by the image of Neptune at the eyepiece?

  $\beta =$$\alpha M$ where $M =$$200$
• What is the angle subtended by the image of Neptune at the eyepiece?

  $\beta =$$2.28 \times 10^{-3} \text{ rad}$
• What is the definition of angular magnification?

  Ratio of angles subtended by image and object
• Why is the principal axis important in ray diagrams?

  It serves as a reference for ray paths
• What common mistakes should be avoided in ray diagrams?

  Drawing axial rays instead of non-axial rays
• What is the relationship between the angles subtended by the object and image?

  The image subtends a larger angle than the object
• How do you determine the diameter of an object from its angular size?

  Use $h =$$d \cdot \theta$