The refractive index of the material of crystal is 1.68 and that of castor oil is 1.2. When a ray of light passes from oil to glass, its velocity will change by a factor

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

A convex lens of focal length ' $f$ ' is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is

The critical angle for light going from medium ' $x^{\prime}$ to medium ' $y$ ' is ' $\theta$ '. The speed of light in medium ' $x$ ' is ' $v_x$ '. The speed of light in medium ' $y$ ' is