The refractive index is maximum for violet colour of light and minimum for red colour of light. i. e., μ v > μ R therefore critical angle is maximum for red colour of light and minimum for violet colour of light, i. e., C v < C R
Total internal reflection occurs if angle of incidence in denser medium exceeds critical angle.
Mirage is an optical illusion observed in deserts and roads on a hot day when the air near the ground is holler and hence rarer than the air above.
Optical Fibres
Optical fibres are also based on the phenomenon of total internal reflection. Optical fibres consist of several thousands of very long fine quality fibres of glass or quartz. The diameter of each fibre is of the order of 10-4 cm with refractive index of material being of the order of 1.5. Optical fibres are used in transmission and reception of electrical signals by converting them first into light signals. For Refraction at a Convex or Concave Spherical Surface
where, μ = refractive index, u = distance of object, v = distance of image and R = radius of curvature of the spherical surface
Lens
A lens is a uniform transparent medium bounded between two spherical or one spherical and one plane surface.
Convex Lens
A lens which is thinner at edges and thicker at middle is called a convex or converging lens.
Concave Lens
A lens which is thicker at edges and thinner at middle, is called a concave or diverging lens.