A light-emitting diode (LED) emits red light when it is positively biased and has a potential difference (p.d.) greater than about 1.8 V.
The energy of a photon of red light is about 1.8 eV.
Calculate the wavelength λ of this red light.
λ = .......................................... m
The LED is connected into a circuit, as shown below.
The battery has electromotive force (e.m.f.) 4.5 V and negligible internal resistance.
The resistor R has resistance 150 Ω.
Assume the p.d. across the LED is 1.8 V.
Calculate the ratio 
ratio = .......................................
The diagram below shows a circuit designed by a student.
The LED is very close to, and facing the light dependent resistor (LDR).
The circuit is taken into a dark room.
i) The student thought that the LED would switch on.
Instead, the LED was found to repeatedly switch on and off.
Explain this behaviour of the LED in this potential divider circuit.
[2]
ii) Suggest a possible refinement so that the LED switches on permanently when taken into the dark room.
[1]
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