Which of the following statements is correct?

$A$ camera lens is set at $f/2$ with an exposure time of $1/100 \ s$. If the lens is set at $f/4$,what will be the correct exposure time?

During a cloudy day,a primary and a secondary rainbow may be created,then the:

The curvature radii of a concavo-convex glass lens are $20\, cm$ and $60\, cm$. The convex surface of the lens is silvered. With the lens horizontal,the concave surface is filled with water. The focal length of the effective mirror is $(\mu_{\text{glass}} = 1.5, \mu_{\text{water}} = 4/3)$.

In an experiment for the determination of focal length of a convex mirror,a convex lens of focal length $20 \ cm$ is placed on an optical bench and an object pin is placed at a distance $30 \ cm$ from the lens. When a convex mirror is introduced in between the lens and the real and inverted image of the object,the final image of the object $O$ is formed at $O$ itself. If the distance between the lens and the mirror is $10 \ cm$,then the focal length of the mirror is.......$cm$

In an experiment to determine the focal length of a concave mirror,the object distance is $x$ and the image distance is $y$,both measured from the pole of the mirror. Which of the following graphs correctly represents the relationship between $y$ and $x$?