Physics Class 10 Practical 26 Important Viva Questions with Answers (To find the focal length of a convex lens by parallax method)

                            

Viva Questions and Answers

1. What is the focal length of a lens?

The focal length of a lens is the distance from the lens to the principal focus, where light rays parallel to the optical axis converge or appear to diverge from after passing through the lens.

2. What is the parallax method?

The parallax method involves shifting the position of the lens or object and observing the relative movement of the images formed, to determine the focal length of the lens.

3. Why do we use a convex lens in this experiment?

We use a convex lens because it converges light rays, making it easier to determine the focal length by finding the point where parallel rays converge.

4. How do you set up the apparatus for this experiment?

The apparatus includes a convex lens, a lens holder, a screen, a distant object, and a measuring scale. The lens is placed between the object and the screen, and the distances are adjusted to form a sharp image on the screen.

5. What is the formula to find the focal length using the parallax method?

The formula to find the focal length (f) of the lens is:

f=uv/u+v ​ where u is the distance of the object from the lens and v is the distance of the image from the lens.

6. Why is it important to obtain a sharp image on the screen?

Obtaining a sharp image ensures that the measurements of the distances uu and vv are accurate, leading to a precise calculation of the focal length.

7. What is meant by parallax error and how is it avoided?

Parallax error occurs when the observer's eye is not positioned perpendicularly to the scale or measurement point, causing inaccurate readings. It is avoided by ensuring the observer's eye is in line with the measurement point.

8. How does changing the position of the lens affect the image formed?

Changing the position of the lens affects the distance between the lens and the image, altering the clarity and size of the image. Proper adjustment is required to achieve a sharp image for accurate measurements.

9. Why is it important to use a distant object for this experiment?

Using a distant object ensures that the incoming rays are nearly parallel, simplifying the calculation of the focal length as the object distance u becomes very large.

10. What precautions should be taken during this experiment?

Ensure the lens and screen are clean and free of dust.

Measure the distances accurately using a scale.

Avoid parallax error by aligning the eye properly.

Ensure the setup is stable and not disturbed during measurements.