Lowering the speed of rotation of wheel of bicycle lowers the rate of change in magnetic flux. So that current will be less and brightness decreases. Consequently, increasing the speed of wheel, magnetic flux changes rapidly and current will be more. Hence , brightness increases.
In the early 1830s, Michael Faraday performed his seminal experimental research on electromagnetic induction, in which he created the first electric dynamo—a machine for continuously converting rotational mechanical energy into electrical energy.A dynamo creates energy. It's short for "dynamoelectric machine," which is a generator that cranks out electric currents. If someone calls you a dynamo, don't get mad, it just means you're energetic. The word dynamo comes from the Greek dynamis for "power." Boom!
Dynamos are divided into two general lands: direct current (D. C.) and alternating-current dynamos (A. C.) or simply alternators.A dynamo is an electric generator that produces direct current using a commutator. ... The dynamo uses wire coils that rotate creating a magnetic field. This action converts the mechanical energy of the rotation to a direct electric current.