What will be the angle of refraction for a light ray incident normally at the surface (in $^{\circ}$)?

Give two uses of a convex lens.

$A$ convex mirror always forms a virtual and erect image.

When is the magnitude of the power of a lens equal to its focal length?

$(a)$ $A$ ray of light falls normally on a face of a glass slab. What are the values of angle of incidence and angle of refraction of this ray?
$(b)$ Light enters from air to a medium $X$. Its speed in medium $X$ becomes $1.5 \times 10^{8} \ m \ s^{-1}$. Speed of light in air is $3 \times 10^{8} \ m \ s^{-1}$. Find the refractive index of medium $X$.

An object $3 \, cm$ high is placed at a distance of $20 \, cm$ in front of a convex lens of focal length $12 \, cm$. Find the position,nature,and size of the image formed.