What is the unit of power of lens?

The following diagram shows the use of an optical device to perform an experiment of light.

As per the arrangement shown, the optical device is likely to be a _____.

A converging lens forms a three times magnified image of an object, which can be taken on a screen. If the focal length of the lens is $30 \mathrm{cm}$, then the distance of the object from the lens is _____.

If a lens can converge the sun rays at a point $20 \mathrm{cm}$ away from its optical centre, the power of this lens is _____.

Which of the following statements is true?

An object is placed at a distance $24 \mathrm{cm}$ in front of a convex lens of focal length $8 \mathrm{cm}$.
(i) Calculate the distance of the image from the lens.
(ii) Calculate the magnification of the image.

A student wants to obtain magnified image of an object AB as on a screen. Which one of the following arrangements shows the correct position of AB for him/her to be successful?

Select the correct alternative from those given below each statement and write the completed statement :

The power of a convex lens is $+4 \mathrm{D}$, then its focal length will be ______.

A virtual, diminished image is formed when an object is placed between the optical centre and the principal focus of a lens.
(i) Name the type of lens which forms the above image.
(ii) Draw a ray diagram to show the formation of the image with the above stated characteristics.