$A$ ray of light enters from a medium with refractive index $n_1$ into a medium with refractive index $n_2$. If the angle of incidence is $\theta_1$ and the angle of refraction is $\theta_2$,then $\sin \theta_1 / \sin \theta_2 = \dots$

Which type of mirror can produce a virtual image?

What does negative magnification mean in the context of spherical mirrors?

$A$ concave mirror can be used to obtain a parallel beam of light from a small lamp.

Under what condition in an arrangement of two plane mirrors,will the incident ray and the final reflected ray always be parallel to each other,regardless of the angle of incidence? Show the same with the help of a diagram.

$(i)$ 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.