When a ray of light passes from one medium to another,which quantity remains constant?

$A$ girl standing at point $P$ on a beach wishes to reach a point $Q$ in the sea as quickly as possible. She can run at $6 \, kmh^{-1}$ on the beach and swim at $4 \, kmh^{-1}$ in the sea. She should take the path

$AB$ is a boundary separating two media of different refractive indices. $A$ ray is incident on the boundary and is partially reflected and partially transmitted. Choose the $\text{CORRECT}$ statement.

$A$ ray of monochromatic light is incident on the plane surface of separation between two media $X$ and $Y$ with angle of incidence $i$ in medium $X$ and angle of refraction $r$ in medium $Y$. The given graph shows the relation between $\sin i$ and $\sin r$. If $V_X$ and $V_Y$ are the velocities of the ray in media $X$ and $Y$ respectively,then which of the following is true?

When an object is moved along the principal axis of a concave mirror placed in air,the image coincides with the object if the object is $50 \ cm$ from the mirror. If the mirror is placed at a depth of $20 \ cm$ in a transparent medium,the image coincides with the object when the object is $40 \ cm$ from the mirror. The refractive index of the liquid is

When light enters glass from vacuum,then the wavelength of light