What type of lens behaviour will an air bubble inside water show?

$(a)$ State the relation between object distance,image distance and the focal length of a spherical mirror.
$(b)$ Draw a ray diagram to show the image formed by a concave mirror when an object is placed between the pole and focus of the mirror.
$(c)$ $A$ concave mirror of focal length $15\, cm$ forms an image of an object kept at a distance of $10\, cm$ from the mirror. Find the position,nature and size of the image formed by it.

$(a)$ What happens to a ray of light when it travels from one medium to another having equal refractive indices?
$(b)$ State the cause of refraction of light.

What is the lens placed near the eye in a compound microscope called?

If the absolute refractive indices of water and glass are $\frac{4}{3}$ and $\frac{3}{2}$ respectively,then what will be the ratio of the velocity of light in water to that of glass?

Draw a ray diagram showing the path of rays of light when it enters with oblique incidence: $(i)$ from air into water; $(ii)$ from water into air.