Light waves travel from an optically rarer medium to an optically denser medium. Its velocity decreases because of a change in . . . . . . .

$A$ circular disc of radius $R$ is placed co-axially and horizontally inside an opaque hemispherical bowl of radius $a$ (See figure). The far edge of the disc is just visible when viewed from the edge of the bowl. The bowl is filled with a transparent liquid of refractive index $\mu$ and the near edge of the disc becomes just visible. How far below the top of the bowl is the disc placed?

$A$ rectangular block is composed of three different glass prisms (with refractive indices $\mu_1, \mu_2$ and $\mu_3$) as shown in the figure below. $A$ ray of light incident normal to the left face emerges normal to the right face. Then,the refractive indices are related by

The apparent depth of water in a cylindrical water tank of diameter $2R \, cm$ is reducing at the rate of $x \, cm/minute$ when water is being drained out at a constant rate. The amount of water drained in $c.c./minute$ is ($n_1 =$ refractive index of air,$n_2 =$ refractive index of water).

$A$ 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 light is refracted from a surface,which of its following physical parameters does not change?