Interpret a light dependent resistor and its illumination
Read an LDR symbol and connect light exposure to resistance without confusing incoming light with LED output.
Section P4.3.1 · Circuit diagrams and circuit components
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recognize resistance adjustment.
- Distinguish a sensing input and an output.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a larger resistance at fixed p.d. do to current?
What you will learn
- Recognize an LDR symbol and incoming-light arrows.
- Predict resistance and simple fixed-p.d. current trends.
Read incoming light
A light-dependent resistor, LDR, has resistance that decreases when illumination increases under its stated conditions. Its usual symbol is a resistor enclosed by a circle with arrows directed toward it. Those arrows show incoming light rather than current direction. A dark LDR generally has a higher resistance than the same LDR in brighter conditions.
The light reaching the sensing surface matters. A brighter room does not guarantee a brighter sensor if it is covered or shaded. Wavelength, temperature and the device calibration also affect exact values. Introductory trend questions assume other relevant conditions are unchanged.
Connect the trend to a circuit
In a simple fixed-p.d. circuit, a lower LDR resistance can lower total path resistance and increase current. A divider output depends on which resistor is the LDR and where the voltage is taken; it does not automatically rise with illumination. The later Extended lessons work through both placements.
An LED symbol, introduced as Supplement content, has outgoing arrows because it emits light. An LDR receives light and changes resistance; it is not a lamp or a generator simply because light arrows appear beside it. Draw and label arrow directions carefully when translating a written circuit description.
Sources for this lesson
Pause and explain
An LDR is covered in an otherwise unchanged illuminated room. What is the expected resistance trend?
Worked example
The same uncovered LDR receives more light while other conditions stay fixed. Predict its resistance.
Show the worked solution
- The arrows into its symbol represent illumination.
- Illumination increases at the sensing surface.
- LDR resistance decreases under the stated conditions.
Answer Resistance decreases.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw an LDR symbol and identify the arrow direction.
Give me a hint
Put the resistor within its sensing circle and show the light input.
Compare my reasoning
- Draw a resistor with two terminal wires.
- Enclose the resistor in a circle.
- Draw light arrows toward it and label the component LDR.
A circled resistor with inward light arrows.
Look for these in your work
- Use incoming arrows.
- Retain electrical terminal wires.
An uncovered LDR becomes more illuminated in a fixed-p.d. path. Predict resistance and current changes if other conditions stay fixed.
Give me a hint
Apply the sensing trend before the circuit trend.
Compare my reasoning
- More illumination decreases LDR resistance.
- The total path resistance decreases.
- At fixed p.d., current increases.
Resistance decreases and current increases under the stated conditions.
Look for these in your work
- State both changes.
- Use the unchanged conditions.
A student labels a circled resistor with inward arrows as a light-emitting output and says room brightness fixes its exact resistance. Correct the description.
Give me a hint
Distinguish a sensor symbol and a calibrated numerical response.
Compare my reasoning
- The inward arrows identify an LDR receiving light.
- Its resistance generally decreases with increased illumination.
- Exact resistance needs calibration and the actual exposure conditions.
It is an LDR sensor, not a light-emitting output; exact resistance requires calibration.
Look for these in your work
- Read arrow direction.
- Separate a qualitative trend and an exact value.
Common mistakes
- LDR arrows point into the sensor.
- Exposure at the component matters.
- The resistance trend is not a universal divider-voltage trend.
What can you explain now?
What do the arrows into an LDR symbol show?
Compare with the explanation
Incoming light.
They describe the sensor input rather than the circuit-current direction.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recognize inward LDR light arrows and apply its illumination–resistance trend under stated conditions.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.