can someone please explain how this circuit works. many thanks

 

You need to make YOUR best effort to answer YOUR homework problems.

So describe how you think the circuit will work in the dark and also in bright light.

Is your IC an opamp or a comparator? If it's a comparator, what type (part number)? That's important because many comparators WON'T work in this circuit.

 

the comparator is a ca3140E. When the V ref is more than the Vin (in the dark) then V out will be low. when the Vin Is more than the V ref (in the light)then the V out will be high. If R3 and R1 are reversed then in dark when V ref is more than the Vin then the Vout will be high. in the light, Vin more than V ref the V out will be low.

 

The CA3140E is an opamp, not a comparator (though it can be used as a comparator in circuits that don't need great comparator performance).

Your description needs to be linked to the circuit you are describing. You talk about Vref and Vin, yet look at your diagram -- there is no Vref or Vin indicated on that diagram anywhere. Don't make people guess about what you are referring to -- engineering is not about guessing.

Also, the output of this circuit would seem to be the LED and whether it is ON or OFF. So your description should talk more about what is going on with the LED.

You seem to have the basic understanding of what is going on, just need to polish it up a bit.

 

IC1 is an amplifier, an op-amp. Think in terms if voltage gain.
What happens to the output if the gain is 10?
Since this is an op-amp with no feedback components it is operating with open-loop gain.
What happens if the open-loop gain is 100,000?