Which one of the following materials cannot be used to make a lens?

$A$ concave mirror produces a three times magnified (enlarged) real image of an object placed at $10 \, cm$ in front of it. Where is the image located (in $cm$)?

$A$ doctor has prescribed a corrective lens of power $+1.5\, D.$ Find the focal length of the lens. Is the prescribed lens diverging or converging?

Name the type of mirror used in the following situation.
Support your answer with a reason.
$(a)$ Headlights of a car

Light enters from air to glass having refractive index $1.50$. What is the speed of light in the glass? The speed of light in vacuum is $3 \times 10^8 \, m \, s^{-1}$.

One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.