Question

Describe a method to find the focal length of a convex lens.

Hint:

While using the Sun for the experiment, avoid looking directly through the lens as it may harm the eyes.

Answer 1

Step 1: Background
The focal length of a convex lens is the distance between its optical centre and the principal focus (the point where parallel rays of light converge after refraction). Finding the focal length is important for optical instruments such as cameras, microscopes, and telescopes.
Step 2: Apparatus Required
Convex lens.
Lens holder.
Screen (white paper or cardboard).
A distant object (like the Sun or a tree).
Scale or ruler.
Step 3: Procedure
Fix the convex lens in the lens holder.
Place the screen behind the lens.
Focus the lens on a distant object (for better accuracy, the Sun can be used).
Adjust the position of the screen until a sharp and inverted image of the object is obtained.
Measure the distance between the lens and the screen using a scale.
Step 4: Observation
When the sharp image is formed, the distance between the lens and the screen gives the focal length of the convex lens.
Step 5: Formula
In general, the lens formula can also be used: \[ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \] where $f =$ focal length, $v =$ image distance, $u =$ object distance.
Step 6: Real-Life Applications
Used in the design of eyeglasses for correcting vision.
Essential in optical devices like projectors, cameras, and microscopes.
Helps determine magnification power of the lens.
Final Answer:
The focal length of a convex lens can be found by focusing a distant object onto a screen and measuring the distance between the lens and the screen. This distance is equal to the focal length.