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Year 11
AQA
Higher

Light intensity and the inverse square law

I can explain why light intensity decreases further from a light source.

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New
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Year 11
AQA
Higher

Light intensity and the inverse square law

I can explain why light intensity decreases further from a light source.

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Share activities with pupils
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Lesson details

Key learning points

  1. Light waves spread out as they travel from a light source.
  2. Further away from the light source, the same amount of light is spread out over a greater area.
  3. This means the light intensity decreases with increasing distance from a light source.
  4. Light intensity is inversely proportional to the square of the distance from a light source (inverse square law).

Keywords

  • Light intensity - the amount of light reaching a given surface area in a period of time

  • Light source - an object that emits light

  • Light waves - oscillations/ripples in electric and magnetic fields

  • Inversely proportional - variables are inversely proportional when one increases and the other decreases at the same (proportional) rate, e.g. as one variable doubles, the other variable halves

  • Inverse square law - light intensity is inversely proportional to the square of the distance from the light source

Common misconception

Many pupils will believe that by halving the distance from a light source, the light intensity will double.

As a result of light travelling in all directions, this aspect needs to be thought of in terms of area (two dimensions). The lesson takes pupils through an example which demonstrates how light intensity is proportional to the square of the distance.


To help you plan your year 11 biology lesson on: Light intensity and the inverse square law, download all teaching resources for free and adapt to suit your pupils' needs...

Take pupils through the tricky concept of the inverse square law step by step. Try to provide real world examples of how light intensity decreases over distance. This can be achieved by darkening the room and putting on a lamp in one corner.
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This content is © Oak National Academy Limited (2025), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Starter quiz

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6 Questions

Q1.
What is the role of light in photosynthesis?
Correct answer: it transfers the energy needed for the reactions of photosynthesis
it cools the plant during the photosynthesis process
it releases energy from glucose
it increases the absorption of carbon dioxide
Q2.
What is a light source?
Correct answer: an object that emits light
an object that absorbs light
an object that reflects light
an object that blocks light
Q3.
is a pigment in plant cells that absorbs light energy, primarily from the sun, to drive the process of photosynthesis.
Correct Answer: chlorophyll
Q4.
What does it mean for two variables to be proportional?
both variables increase and decrease independently
there is no relationship between the variables
when one variable increases, the other decreases
Correct answer: when one variable increases, the other increases at a constant rate
Q5.
What is the primary source of light for photosynthesis?
artificial light
moonlight
starlight
Correct answer: sunlight
Q6.
Which factor is most likely to be a limiting factor of photosynthesis at night?
carbon dioxide concentration
Correct answer: light intensity
temperature
water availability

Exit quiz

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6 Questions

Q1.
In what way does light intensity affect the rate of photosynthesis in plants?
higher light intensity decreases the rate of photosynthesis
Correct answer: higher light intensity increases the rate of photosynthesis
light intensity has no effect on photosynthesis
the effect of light intensity varies with plant species
Q2.
What is the relationship between light intensity and the rate of photosynthesis?
Correct answer: the rate increases with light intensity up to a point, then plateaus
the rate decreases as light intensity increases
light intensity has no effect on the rate of photosynthesis
the rate increases indefinitely with increasing light intensity
Q3.
Why does light intensity decrease as the distance from the light source increases?
because light energy is absorbed by the air
because light waves change direction
because the power of the light source decreases
Correct answer: because the same amount of light is spread over a larger area
Q4.
What is the inverse square law?
the intensity of light is directly proportional to the distance from the source
the intensity of light increases with the square of the distance
the intensity of light remains constant regardless of distance
Correct answer: the intensity of light is inversely proportional to the square of the distance
Q5.
What would happen to the light intensity if the distance from the light source is tripled?
it becomes one-third
it becomes one-sixth
Correct answer: it becomes one-ninth
it remains the same
Q6.
What is the relationship between the rate of photosynthesis and the distance from a light source?
rate is directly proportional to the distance from the light source
Correct answer: rate is inversely proportional to the distance from the light source
rate is not related to the distance from the light source