Question

Where a pen is placed in front of a convex lens if the image is equal to the size of the pen? What will be the power of the lens in dioptres?

Answer 1

The object should be placed at a distance twice that of the focal length.

Power is the reciprocal of focal length.

• Convex lenses are converging lenses.
• The middle of the lens is thicker than the endpoints.
• It is used as a magnifying glass, eye glasses, in microscopes and in cameras,
• In a convex lens, if the object is placed at 2F, (where F is focal length), we get an image of the same size as that of the object.
• Also, the image will be real and inverted.
• The power of the lens is given by the inverse of the focal length.
• The focal length of a convex lens is positive, so power will also be positive.
• If F is the focal length in the SI unit, power = (1/F) Diopters.
• #SPJ3

Answer 2

Answer:

Hence the pen is placed at  2F where F is the focal distance and power of the lens is $\frac{1}{F}$

Explanation:

Convex lens also known as converging lens are thicker in the middle and thinner at the edges. In case of a convex lens if an object is placed at a distance twice that of focal length of the lens (2F) then the image of the object which is obtained is real and inverted and same as the size of the object. The power of the lens is determined by the inverse of focal length of the same lens ($\frac{1}{F}$).