What is the absolute refractive index of water?

Rays from the Sun converge at a point $15 \, cm$ in front of a concave mirror. Where should an object be placed so that the size of its image is equal to the size of the object?

What is the power of a convex lens with a focal length of $20 \text{ cm}$?

An object $3.0 \text{ cm}$ high is placed perpendicular to the principal axis of a concave lens of focal length $7.5 \text{ cm}$. The image is formed at a distance of $5.0 \text{ cm}$ from the lens. Calculate $(i)$ distance at which object is placed,and $(ii)$ size and nature of image formed.

Where should an object be placed in front of a concave mirror to obtain its image at the same position?

$(a)$ State Snell's law of refraction.
$(b)$ When a ray of light travelling in air enters obliquely into a glass slab, it is observed that the light ray emerges parallel to the incident ray but it is shifted sideways slightly. Draw a ray diagram to illustrate it.