Lens Calculations

• What is the formula for calculating the power of a lens?

  P = $\frac{1}{f}$
• What does the variable P represent in lens calculations?

  Power of the lens in dioptres
• How is the focal length of a lens measured?

  From the centre of the lens
• What is the relationship between power and focal length?

  Power is inversely proportional to focal length
• What is the power of a diverging lens?

  Negative value
• If a diverging lens has a focal length of $-1.5 \text{ cm}$, what is its power?


  P = $-66.6 \text{ D}$
• What are the steps to calculate the power of a lens?

  1. Write known values
  2. Convert focal length to meters
  3. Use the equation P = $\frac{1}{f}$
  4. Calculate power
• What does the lens equation relate to?

  Focal length, image distance, and object distance

  
• What is the lens equation?

  $\frac{1}{f} =$$\frac{1}{u} +$$\frac{1}{v}$
• What must be true about the units of f, u, and v in the lens equation?

  They must be in the same units
• What is the significance of positive values for image distance?

  Indicates a real image
• What is the significance of negative values for image distance?

  Indicates a virtual image
• What is the significance of negative values for focal length?

  Indicates a diverging lens
• What is magnification in optics?

  • How much larger the image is than the object
  • Ratio of image height to object height
• What is the formula for linear magnification as a ratio of heights?

  $m =$$\frac{h_i}{h_o}$
• How can magnification also be represented?

  As a ratio of distances from the lens
• What is the formula for magnification as a ratio of distances?

  $m =$$\frac{v}{u}$
• If a magnifying glass has a focal length of $15 \text{ cm}$ and is held $5 \text{ cm}$ away from an object, what is the magnification?


  $1.5$
• If an object height is $10 \text{ cm}$ and the image height is $20 \text{ cm}$, what is the magnification?


  $2$
• What are common mistakes in magnification calculations?

  • Getting the formula upside down
  • Not checking if magnified objects are larger (m > 1)
• What should be checked regarding units in magnification calculations?

  • Ensure all distances are in the same units
  • SI units are not strictly necessary for ratios