Magnification (Lenses) MCQs Quiz | Class 10

This quiz is for Class: X, Subject: Science (Code 086), Unit: Unit III: Natural Phenomena, Topic: Magnification (Lenses) MCQs Quiz. It covers the meaning and sign of magnification, along with basic numerical problems related to lenses. Attempt all questions and click ‘Submit Quiz’ to see your results, then download the detailed answer PDF.

Understanding Magnification in Lenses

Magnification is a crucial concept in optics, especially when studying how lenses form images. It describes how much an image is enlarged or reduced compared to the actual object and also provides information about the image’s orientation (erect or inverted).

Defining Magnification (m)

For lenses, magnification (denoted by ‘m’) is defined as the ratio of the height of the image (h’) to the height of the object (h). It can also be expressed in terms of image distance (v) and object distance (u).

  • Formula: m = h’ / h = v / u
  • Where:
    • h’ = Height of the image
    • h = Height of the object
    • v = Image distance from the optical center
    • u = Object distance from the optical center

It’s important to use the proper Cartesian sign conventions for h’, h, v, and u when applying this formula.

Sign Conventions for Magnification

The sign of magnification tells us about the nature and orientation of the image:

  • Positive Magnification (m > 0):
    • Indicates that the image is erect (upright) with respect to the object.
    • Erect images are always virtual.
    • Commonly formed by concave lenses and sometimes by convex lenses (when the object is placed between the optical center and the principal focus).
  • Negative Magnification (m < 0):
    • Indicates that the image is inverted (upside down) with respect to the object.
    • Inverted images are always real.
    • Typically formed by convex lenses (when the object is placed beyond the principal focus).

Magnitude of Magnification

The absolute value (magnitude) of magnification (|m|) tells us about the size of the image relative to the object:

  • |m| > 1 (e.g., m = +2 or m = -2): The image is magnified (larger than the object).
  • |m| = 1 (e.g., m = +1 or m = -1): The image is of the same size as the object.
  • |m| < 1 (e.g., m = +0.5 or m = -0.5): The image is diminished (smaller than the object).

Summary Table for Magnification

Magnification (m) Image Nature Image Orientation Image Size
m > 0 Virtual Erect Diminished, Same Size, or Magnified
m < 0 Real Inverted Diminished, Same Size, or Magnified
|m| > 1 (Can be Real/Virtual) (Can be Erect/Inverted) Magnified
|m| = 1 (Can be Real/Virtual) (Can be Erect/Inverted) Same Size
|m| < 1 (Can be Real/Virtual) (Can be Erect/Inverted) Diminished

Quick Revision Points

  • Magnification (m) relates image height to object height (h’/h) and image distance to object distance (v/u).
  • Positive ‘m’ means the image is virtual and erect.
  • Negative ‘m’ means the image is real and inverted.
  • ‘|m| > 1’ implies a magnified image.
  • ‘|m| < 1' implies a diminished image.
  • ‘|m| = 1’ implies an image of the same size.
  • Concave lenses always produce virtual, erect, and diminished images (m > 0 and m < 1).
  • Convex lenses can produce various types of images depending on object position.

Extra Practice Questions

  1. If a lens produces an image with a magnification of +2.5, what can you say about the nature, orientation, and size of the image?
  2. An object 4 cm high is placed in front of a lens, and a real, inverted image 8 cm high is formed. Calculate the magnification of the lens.
  3. A convex lens forms a virtual image when an object is placed between F1 and the optical center. What will be the sign of the magnification in this case?
  4. A lens produces an image at 40 cm on the same side as the object, which is placed 20 cm from the lens. What is the magnification of the lens?
  5. If the magnification of a lens is -0.75, describe the image characteristics.