Focal Length of Convex Lens MCQs Quiz | Class 10

Understanding Focal Length of a Convex Lens

The focal length of a convex lens is a fundamental property that defines its ability to converge light rays. For Class 10 Science, understanding how to determine this value, especially using distant objects, is a key practical skill.

What is a Convex Lens and its Focal Length?

A convex lens is a converging lens, meaning it converges parallel light rays to a single point after refraction. This point is known as the principal focus (F). The distance between the optical center of the lens and its principal focus is called the focal length (f). It’s a crucial parameter in understanding how a lens forms images.

Image of a Distant Object

When an object is placed at a very large distance (effectively at infinity) from a convex lens, the light rays coming from it are considered to be parallel to the principal axis. These parallel rays, after passing through the convex lens, converge at its principal focus. The image formed is:

• Real: It can be obtained on a screen.
• Inverted: Upside down relative to the object.
• Highly Diminished (Point-sized): Much smaller than the object.
• Formed at the principal focus (F) on the opposite side of the lens.

This characteristic property makes distant objects ideal for determining the focal length of a convex lens.

Measurement of Focal Length

The experimental method to determine the focal length of a convex lens using a distant object involves the following steps:

1. Setup: Mount the convex lens on a lens stand and place it on a laboratory table or optical bench.
2. Distant Object: Choose a well-lit distant object (e.g., a tree, a building, or a distant window pane) outside the laboratory. Ensure that the object is far enough so that the rays coming from it can be considered parallel.
3. Screen Adjustment: Place a white screen (e.g., a cardboard sheet) behind the lens on the opposite side of the distant object. Adjust the position of the screen back and forth until a sharp, clear, and well-defined inverted image of the distant object is formed on it.
4. Measurement: Once a sharp image is obtained, measure the distance between the optical center of the convex lens and the screen using a metre scale. This measured distance is the approximate focal length (f) of the convex lens.
5. Repetition: Repeat the experiment a few times by choosing different distant objects and note down the readings. Calculate the mean value to get a more accurate focal length.

Summary of Image Formation by Convex Lens

Quick Revision Points

• A convex lens is a converging lens.
• Focal length is the distance from the optical center to the principal focus.
• Rays from a distant object are considered parallel.
• A real, inverted, and highly diminished image of a distant object is formed at the principal focus of a convex lens.
• This method provides an approximate but effective way to find the focal length.
• A metre scale is used for measurement.

Extra Practice Questions

1. What is the nature of the image formed by a convex lens when a distant object is placed in front of it?
   1. Virtual and erect
   2. Real and erect
   3. Real and inverted
   4. Virtual and inverted
2. If the image of a distant object is formed at 10 cm from a convex lens, what is its power (in dioptres)?
   1. +10 D
   2. -10 D
   3. +5 D
   4. -5 D
3. Why should the distant object be far away in this experiment?
   1. To ensure the image is magnified
   2. To ensure the incident rays are parallel
   3. To ensure the image is virtual
   4. To make the experiment easier to perform
4. Which part of the convex lens is used as the reference point for measuring focal length?
   1. Centre of curvature
   2. Principal focus
   3. Optical centre
   4. Aperture
5. A student is trying to find the focal length of a convex lens using a distant object but gets a blurred image. What is the most likely reason?
   1. The object is too close
   2. The screen is not placed at the focal point
   3. The lens is not clean
   4. All of the above