With the help of a suitable diagram explain the meaning of the following term.

Principal focus

We are given a convex lens of focal length $20 \mathrm{cm}$. Draw a ray diagram to show the nature, size, and position of the image formed when the object is kept at a distance of $55 \mathrm{cm}$. (Note: For drawing the ray diagram, use an appropriate scale.)

We are given a convex lens of focal length $20 \mathrm{cm}$. Draw a ray diagram to show the nature, size, and position of the image formed when the object is kept at a distance of $35 \mathrm{cm}$. (Note: For drawing the ray diagram, use an appropriate scale.)

We are given a convex lens of focal length $20 \mathrm{cm}$. Draw a ray diagram to show the nature, size, and position of the image formed when the object is kept at a distance of $15 \mathrm{cm}$. (Note: For drawing the ray diagram, use an appropriate scale.)

We are given a concave lens of focal length $15 \mathrm{cm}$. Draw a ray diagram to show the nature, size, and position of the image formed when the object is kept at a distance of $30 \mathrm{cm}$. (Note: For drawing ray diagrams, use an appropriate scale)

We are given a concave lens of focal length $15 \mathrm{cm}$. Draw a ray diagram to show the nature, size, and position of the image formed when the object is kept at a distance of $15 \mathrm{cm}$. (Note: For drawing ray diagrams, use an appropriate scale)

We are given a concave lens of focal length $15 \mathrm{cm}$. Draw a ray diagram to show the nature, size, and position of the image formed when the object is kept at a distance of $10 \mathrm{cm}$. (Note: For drawing ray diagrams, use an appropriate scale)

What type of lens is used as a 'magnifying glass'? How is the object positioned with respect to this lens? Draw the appropriate ray diagram.