$A$ point object emits rays in all directions. Consider a ray emitted from the object that is parallel to the principal axis of the mirror. One of the points required to trace the reflected ray is the point of incidence itself,as shown in the figure. Select a point from the given figure through which the reflected ray passes.

The rays travelling parallel to the principal axis after reflecting from the concave mirror pass or appear to pass through the focus.

$(i)$ Describe an activity to find the approximate value of the focal length of a concave mirror.
$(ii)$ What happens to the size of the image of an object when it is moved gradually away from a convex mirror?
$(iii)$ In an experiment to study refraction through a glass slab,it is observed that a ray of light undergoing refraction emerges parallel to the direction of the incident ray. Why does it happen so? Explain with the help of a diagram.

The focal length of a plane mirror is $.....$

Give one use of a concave lens.

$(a)$ $A$ converging lens forms a real and inverted image of an object at a distance of $100 \, cm$ from it. Where should an object be placed in front of the lens so that the size of the image is twice the size of the object? Also, calculate the power of the lens.
$(b)$ State the laws of refraction.