The conditions given below are in the context of observing Tyndall effect in colloidal solutions:
(a) The diameter of the colloidal particles is comparable to the wavelength of light used.
(b) The diameter of the colloidal particles in much smaller than the wavelength of light used.
(c) The diameter of the colloidal particles is much larger than the wavelength of light used.
(d) The refractive indices of the dispersed phase and the dispersion medium are comparable.
(e) The dispersed phase has a very different refractive index from the dispersion medium.
Choose the most appropriate conditions from the options given below.
(a) The diameter of the colloidal particles is comparable to the wavelength of light used.
(b) The diameter of the colloidal particles in much smaller than the wavelength of light used.
(c) The diameter of the colloidal particles is much larger than the wavelength of light used.
(d) The refractive indices of the dispersed phase and the dispersion medium are comparable.
(e) The dispersed phase has a very different refractive index from the dispersion medium.
Choose the most appropriate conditions from the options given below.
- (A)(a) and (d) only
- (B)(a) and (e) only✓ Correct
- (C)(c) and (d) only
- (D)(b) and (e) only
Answer: (B)
Solution
Light is scattered strongly only when the scattering centres are of about the same size as its wavelength: particles much smaller than scatter it feebly, and particles much larger reflect and refract the beam instead of scattering it, so no luminous cone is seen. Scattering also needs an optical discontinuity, which exists only when the dispersed phase and the dispersion medium differ appreciably in refractive index; with matched refractive indices the colloid is optically uniform and the beam passes through unseen. The Tyndall effect therefore requires that the diameter of the colloidal particles be comparable to the wavelength of light used and that the dispersed phase have a very different refractive index from the dispersion medium.
JEE Main · 2021 · 20 Jul 2021 · Shift 1 · Single correct option