The cause of great shine in a diamond is :

The correct answer is total internal reflection.

Key Points

• Total internal reflection (TIR): Total internal reflection (TIR) occurs at the interface of two transparent mediums when the ray of light travels from a denser medium to a rarer medium.
  • It is the reflection of light within the same medium when a ray of light is incident at the interface of two medium at an angle of incidence which is greater than the critical angle for the pair of media i.e. i > ic
• The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90°.

EXPLANATION:

• Diamond is the hardest substance in nature and diamond has a high refractive index.
• The sparkling/shining of a diamond is because of total internal reflection followed by the trapping of light rays. So option 2 is correct.

EXTRA POINTS:

• Diffraction: In diffraction, when a light wave encounters an obstacle or a slit, it bends around the corners of the obstacle or aperture. 
• Diffusion: When the light rays are reflected from a piece of paper and the reflected beam is scattered. Then this phenomenon is called diffusion of light.
• Reflection of light: When the light rays fall on a non-transparent material/object then it gets reflected back in the same medium. This phenomenon is called a reflection of light.