Concave Lens and Principal Focus Quiz

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9 Questions

What is the nature of the image formed by a convex lens when the object is placed 15 cm from the lens?

Real and inverted

If a concave lens is used instead of a convex lens with the same object distance of 15 cm, what would be the nature of the image?

Real and inverted

What is the magnification of the image formed by the convex lens when the object distance is 15 cm?


Based on the given information, where is the image formed by the convex lens located with respect to the optical center?

30 cm after the optical center

What does a positive image distance indicate in the context of this problem?

Real image formed

If a concave lens of short focal length were used instead, what would be the position of the image compared to a convex lens?

Farther from the optical center

What factor determines whether a lens converges or diverges light rays?

Focal length

The degree of convergence or divergence of light rays achieved by a lens is expressed in terms of its ___.


What happens to light rays when passing through a concave lens with very short focal length?

They diverge strongly

Study Notes

Action of a Concave Lens

  • A concave lens refracts parallel rays of light, making them appear to diverge from a point on the principal axis, called the principal focus (F).
  • A concave lens has two principal foci, F1 and F2, on either side of the lens.
  • The distance between the principal focus and the optical center of a lens is its focal length (f).

Image Formation by Lenses

  • Lenses form images by refracting light.
  • Image formation by a convex lens can be demonstrated using five parallel straight lines, a convex lens, and a lens stand.
  • The mirror formula, 1/u + 1/v = 1/f, relates the object distance (u), image distance (v), and focal length (f).


  • Magnification is the relative extent to which the image of an object is magnified with respect to the object size.
  • Magnification (m) is expressed as the ratio of the height of the image (h') to the height of the object (h), m = h'/h.
  • Magnification can also be expressed as m = -v/u, where v is the image distance and u is the object distance.

Image Characteristics

  • The height of the object is taken to be positive when placed above the principal axis.
  • The height of the image is positive for virtual images and negative for real images.
  • The nature, position, and size of an image can be determined by the object distance, image distance, and focal length.

Power of a Lens

  • The power of a lens is a measure of its ability to converge or diverge light rays.
  • The power of a lens depends on its focal length, with shorter focal lengths resulting in larger convergence or divergence angles.

Test your knowledge on the action of a concave lens, including the concept of principal focus. Learn how parallel rays of light behave when passing through a concave lens and understand the significance of the principal focus point.

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