When small spherical bodies move through a viscous medium, the bodies drag the layers of the medium that are in contact with them. This dragging results in relative motion between different layers, which are away from the body. Therefore, a viscous drag comes into play, opposing the motion of the body. It is found that this backward force or viscous drag, increases with increase in velocity of the body.

According to Stoke, the viscous drag 'f'' depends on-
 1. the coefficient of viscosity 'h' of the medium,
2. the velocity (v) of the body and
3. radius (r) of the spherical body

By methods of dimensions, the values of a, b and c are 1,1,1 respectively. Therefore,    
            F drctly prtnl to v.n.r            ie,
                              F= 6 pie .v.n.r
At low velocities, the frictional force on a spherical body moving through a fluid at constant velocity is equal to 6π times the product of the velocity, the fluid viscosity, and the radius of the sphere. 
 The wavelength of luminescence excited by radiation is always greater than that of the exciting radiation.