How Do You Identify if a Lens Has a Positive or Negative Focal Length?

The focal length of a lens determines how it bends light to form an image. Whether a lens has a positive or negative focal length can be identified based on the nature and position of the image it forms:

• Positive Focal Length (Convex Lens):
  • The focal length is positive when the image is formed on the far side of the lens from the object.
  • In this case, the image is real and can be projected onto a screen.
• Negative Focal Length (Concave Lens):
  • The focal length is negative when the image is formed on the same side of the lens as the object.
  • Here, the image is virtual and cannot be projected.

Key Characteristics to Remember

• Real Image:
  • Formed by a convex lens with a positive focal length.
  • Appears inverted and can be captured on a screen.
• Virtual Image:
  • Produced by a concave lens with a negative focal length.
  • Appears upright and cannot be captured on a screen.

By observing the nature and position of the image formed, you can determine whether a lens has a positive or negative focal length.