13.4 Lenses and Image Formation

• What are the two main types of lenses?

  Concave and convex
• What type of image is formed by a concave lens?

  Virtual, upright, smaller
• Steps in the refraction of light through a lens

  1️⃣ Light ray enters the lens

  2️⃣ Light is bent or refracted

  3️⃣ Light exits the lens
• Concave lenses have a negative focal length.


  True
• A concave lens has a positive focal length according to sign conventions.

  False
• For a concave lens, the image distance is always negative.
• In the thin lens equation, the focal length is positive for a convex lens.
• The thin lens equation is \frac{1}{f}
• A concave lens is thinner in the middle
• The principle of refraction states that light is bent or refracted
• The focal length of a convex lens is negative.

  False
• What is the sign of the focal length for a convex lens?

  Positive
• Match the lens type with its image properties:

  Concave Lens ↔️ Virtual, upright, smaller

  Convex Lens ↔️ Real or virtual, inverted or upright, larger or smaller
• For concave lenses, the focal length is negative
• For convex lenses, the focal length is positive
• What does d_o represent in the thin lens equation?

  Object distance
• What does a negative magnification value indicate about the image?

  Inverted
• Which lens type always forms a virtual image?

  Concave
• If the rays converge in ray tracing, the image is real.
• The key to effective ray tracing is identifying and drawing the principal rays
• Steps to determine image properties using principal rays

  1️⃣ Draw the principal rays

  2️⃣ Identify where the rays intersect

  3️⃣ Determine the image location

  4️⃣ Analyze image orientation and magnification
• The parallel ray in a concave lens diverges after passing through the lens
• The central ray in a concave lens continues unrefracted.


  True
• Concave lenses form virtual, upright, and smaller images
• What is the focal length of a lens?

  The distance from the lens to the focal point
• Why is the focal length of a concave lens considered negative?

  Because the focal point is on the same side
• A concave lens has a positive focal length.

  False
• Match the lens property with its correct sign for concave and convex lenses:

  Focal Length ↔️ Negative for concave, positive for convex

  Object Distance ↔️ Positive for both

  Image Distance ↔️ Negative for virtual, positive for real

  Magnification ↔️ Negative for inverted, positive for upright
• Sign conventions are only necessary for calculating focal length.

  False
• What is the sign of the focal length for convex lenses?

  Positive
• The thin lens equation relates the focal length, object distance, and image distance.
• Using the thin lens equation, the image distance for a convex lens with a focal length of 20 cm and an object distance of 30 cm is 60 cm.
• A negative magnification indicates an inverted image.


  True
• A real image forms on the opposite side of the lens from the object.
• What does $d_{i}$ represent in the magnification formula?


  Image distance
• The image distance for a virtual image is always negative.


  True
• Convex lenses converge light rays.
• Concave lenses diverge light rays to form a virtual image.
• The focal point is where parallel light rays converge after passing through a convex lens.
• In sign conventions, a positive object distance indicates the object is on the same side of the lens as the incoming light.