Higher The Inverse Square Law and Light Intensity
Part of Photosynthesis — GCSE Biology
This higher tier covers Higher The Inverse Square Law and Light Intensity within Photosynthesis for GCSE Biology. Topic 1: Photosynthesis It is section 10 of 13 in this topic. This section is most useful once the core foundation idea is secure, because it adds the detail that pushes answers higher.
Topic position
Section 10 of 13
Practice
15 questions
Recall
20 flashcards
Higher The Inverse Square Law and Light Intensity
Light intensity decreases with distance according to the inverse square law:
Intensity ∝ 1 / distance²
This means if you double the distance from the light source, the intensity falls to one quarter (not one half). If you treble the distance, intensity falls to one ninth.
Example calculation: A lamp produces an intensity of 400 arbitrary units at 10 cm. What is the intensity at 20 cm?
- Ratio of distances: 20/10 = 2
- Ratio of intensities: 1/2² = 1/4
- Intensity at 20 cm = 400 × 1/4 = 100 units
This is important for interpreting Elodea bubble-counting experiments. A graph of rate of photosynthesis against 1/d² (not against d itself) should give a straight line through the origin when light is the limiting factor, confirming the inverse square relationship.
Graph analysis at higher tier: On a graph of photosynthesis rate vs light intensity, the rate initially increases linearly. The gradient then decreases and the curve levels off — this plateau indicates that another factor (CO2 or temperature) has become limiting. The plateau itself shifts up when CO2 is increased or temperature is raised (up to the enzyme optimum), confirming which factor was limiting.