Voltage Divider Necessity for Analog Sensors

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hi everyone, i'm new here , extremely motivated learn arduino.  i'm aspiring computer engineer (haven't started coursework yet).  before getting myself arduino uno, taught myself c programming language has paid big dividends programming aspect of arduino easier part me comprehend.  what's kind of kicking butt actual electrical theory side.

right now, i'm learning interfacing analog sensors arduino.  such, concept of voltage dividers has arisen.  understand v.d. formula, know how derived, , can calculate resulting output voltage change in r1 or r2.

what don't understand 'why'.  why need 'extra' resistor v.d.?

i know arduino perceive changes sensor through varying voltages.  v.d. provides this.

however, know analog sensor (photoresistor in case) vary resistance depending on how light received.  if the resistance varies, won't resulting voltage vary without need of 'extra' resistor in v.d.?

i've hooked led's arduino , have learned have use voltage in circuit.  led provided 2v drop already, that's necessitated use of resistor not control current, use remaining 3v.

do have conceptually incorrect?

thanks illumination on subject (confused).

-charlius-

the light sensor's resistance varies light therefore current flows through varry light intensity.
an arduino measures voltage, not current.
adding 2nd resistor in series the sensor sample varrying current , convert varrying voltage arduino monitor.



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