Refraction by Spherical Lenses MCQs Quiz | Class 10

Welcome to the Class X Science (Code 086) quiz from Unit III: Natural Phenomena on the topic Refraction by Spherical Lenses. This quiz covers key concepts related to convex and concave lenses, including their properties and ray diagrams. Test your knowledge by attempting all 10 multiple-choice questions, then submit to see your score and download a detailed answer PDF for review.

Understanding Refraction by Spherical Lenses

Refraction, the bending of light as it passes from one medium to another, is a fundamental phenomenon in optics. Spherical lenses are curved pieces of transparent material (like glass or plastic) that use refraction to converge or diverge light rays, forming images.

Types of Spherical Lenses

There are two main types of spherical lenses:

  1. Convex Lens (Converging Lens): These lenses are thicker in the middle and thinner at the edges. They converge parallel rays of light to a single point called the principal focus (F) on the other side. Convex lenses are used in magnifying glasses, cameras, and to correct hypermetropia (farsightedness).
  2. Concave Lens (Diverging Lens): These lenses are thinner in the middle and thicker at the edges. They diverge parallel rays of light such that they appear to come from a single point, the principal focus (F), located on the same side as the object. Concave lenses are used in spectacles to correct myopia (nearsightedness) and in some telescopes.

Key Terms

  • Optical Center (O): The central point of a lens through which light rays pass without deviation.
  • Principal Axis: The imaginary line passing through the optical center and the principal foci of the lens.
  • Principal Focus (F): The point on the principal axis where rays parallel to the principal axis converge (convex) or appear to diverge from (concave) after refraction. Each lens has two principal foci, F1 and F2.
  • Center of Curvature (C or 2F): The center of the spheres from which the lens surfaces are part. Each lens has two centers of curvature, C1 and C2 (or 2F1 and 2F2).
  • Focal Length (f): The distance between the optical center and the principal focus.
  • Aperture: The effective diameter of the circular outline of a spherical lens.

Ray Diagrams for Image Formation

Ray diagrams help visualize the formation of images by spherical lenses. Three common rules for drawing ray diagrams are:

  1. A ray parallel to the principal axis, after refraction, passes through the principal focus (F) for a convex lens, or appears to diverge from the principal focus (F) for a concave lens.
  2. A ray passing through the principal focus (F) for a convex lens, or directed towards the principal focus (F) for a concave lens, becomes parallel to the principal axis after refraction.
  3. A ray passing through the optical center (O) of a spherical lens goes undeviated.

Image Formation by Convex Lens:

Object Position Image Position Nature of Image Size of Image
At infinity At F2 Real, inverted Highly diminished, point-sized
Beyond 2F1 Between F2 and 2F2 Real, inverted Diminished
At 2F1 At 2F2 Real, inverted Same size
Between F1 and 2F1 Beyond 2F2 Real, inverted Magnified
At F1 At infinity Real, inverted Highly magnified
Between F1 and O On the same side as object Virtual, erect Magnified

Image Formation by Concave Lens:

Concave lenses always form virtual, erect, and diminished images for all positions of a real object.

Object Position Image Position Nature of Image Size of Image
At infinity At F1 (on the same side as object) Virtual, erect Highly diminished, point-sized
Between infinity and O Between F1 and O (on the same side as object) Virtual, erect Diminished

Lens Formula and Power of a Lens

  • Lens Formula: 1/v - 1/u = 1/f, where v is image distance, u is object distance, and f is focal length.
  • Magnification (m): m = h'/h = v/u, where h' is image height and h is object height.
  • Power of a Lens (P): P = 1/f (in meters). The unit of power is Dioptre (D). A positive power indicates a convex lens, and a negative power indicates a concave lens.

Quick Revision Points

  • Convex lenses converge light, concave lenses diverge light.
  • Real images can be obtained on a screen; virtual images cannot.
  • Convex lenses can form both real and virtual images, while concave lenses always form virtual images for real objects.
  • Ray diagrams simplify understanding image formation.
  • New Cartesian Sign Convention is crucial for applying lens formulas.

Practice Questions

1. An object is placed at a distance of 10 cm from a convex lens of focal length 15 cm. Find the position and nature of the image.

2. A concave lens has a focal length of 20 cm. At what distance should an object be placed from the lens so that it forms an image at 15 cm from the lens?

3. If the power of a spectacle lens is +2.5 D, what type of lens is it and what is its focal length?

4. Draw a ray diagram for an object placed at 2F of a convex lens. Describe the image formed.

5. Why is a concave lens called a diverging lens?