Inverse Square Law and Light Intensity Flashcards

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In photosynthesis experiments, what does light intensity mean?

Light intensity means how bright the light is on the plant or pondweed.

What does the inverse square law say about light intensity and distance from a lamp?

As the distance from a lamp increases, the light intensity decreases with the square of the distance.

What is a simple formula that shows how light intensity depends on distance?

Light intensity is proportional to 1 divided by distance squared: intensity ∝ 1 ÷ distance², often written as intensity = 1 ÷ d² in relative units.

How do you use the inverse square law to work out relative light intensity from a distance?

Measure the distance from the lamp to the plant, square this distance, then do 1 ÷ (distance²) to get the relative light intensity.

If you move a lamp from 10 cm to 20 cm away from pondweed, how does the light intensity change?

The distance has doubled, so the light intensity becomes one quarter of what it was.

If you move a lamp from 30 cm to 15 cm away from pondweed, how does the light intensity change?

The distance is halved, so the light intensity becomes four times greater.

Why does light intensity follow an inverse square law as you move away from a small lamp?

The same light energy spreads out over a larger area as distance increases, so each square metre gets less light.

Why do we often use 1 ÷ distance² instead of a light meter in school experiments?

Because it is easy to calculate from the distance to the lamp and follows the inverse square law, so it matches how light intensity really changes.

At low light levels, how does the rate of photosynthesis change when light intensity increases?

At low light levels, the rate of photosynthesis increases in direct proportion to light intensity.

In a pondweed experiment on light intensity, what do you change and what do you measure?

You change the distance from the lamp (and use it to calculate 1 ÷ distance² for light intensity) and you measure the rate of photosynthesis, for example by counting bubbles per minute.

On a graph of rate of photosynthesis against 1 ÷ distance², what shape do you expect at low light levels?

You expect a straight line going up, showing that the rate of photosynthesis is directly proportional to light intensity.

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