What is the $SI$ unit of the power of a lens?

How many images are formed by two parallel mirrors?

Which of the following always form a virtual image?

Which of the following statements is true?

One half of a convex lens is covered with a black paper.
$(a)$ Show the formation of the image of an object placed at $2F_{1}$ of such a covered lens with the help of a ray diagram. Mention the position and nature of the image.
$(b)$ Draw the ray diagram for the same object at the same position in front of the same lens,but now uncovered. Will there be any difference in the image obtained in the two cases? Give a reason for your answer.

Define the term reflection.