Light intensity and the inverse square law
Lesson details
Learning outcome
I can explain why light intensity decreases further from a light source.
Key learning points
- Light waves spread out as they travel from a light source.
- Further away from the light source, the same amount of light is spread out over a greater area.
- This means the light intensity decreases with increasing distance from a light source.
- 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.
Teacher tip
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.
Licence
except where otherwise stated. See Oak's terms & conditions (Collection 2).Lesson video
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Prior knowledge starter quiz
6 Questions
Q1.What is the role of light in photosynthesis?
Q2.What is a light source?
Q3. is a pigment in plant cells that absorbs light energy, primarily from the sun, to drive the process of photosynthesis.
Q4.What does it mean for two variables to be proportional?
Q5.What is the primary source of light for photosynthesis?
Q6.Which factor is most likely to be a limiting factor of photosynthesis at night?
Assessment exit quiz
6 Questions
Q1.In what way does light intensity affect the rate of photosynthesis in plants?
Q2.What is the relationship between light intensity and the rate of photosynthesis?
Q3.Why does light intensity decrease as the distance from the light source increases?
Q4.What is the inverse square law?
Q5.What would happen to the light intensity if the distance from the light source is tripled?
Q6.What is the relationship between the rate of photosynthesis and the distance from a light source?
To help you plan your 11 biology lesson on: Light intensity and the inverse square law, download all teaching resources for free and adapt to suit your pupils' needs...
To help you plan your 11 biology lesson on: Light intensity and the inverse square law, download all teaching resources for free and adapt to suit your pupils' needs.
The starter quiz will activate and check your pupils' prior knowledge, with versions available both with and without answers in PDF format.
We use learning cycles to break down learning into key concepts or ideas linked to the learning outcome. Each learning cycle features explanations with checks for understanding and practice tasks with feedback. All of this is found in our slide decks, ready for you to download and edit. The practice tasks are also available as printable worksheets and some lessons have additional materials with extra material you might need for teaching the lesson.
The assessment exit quiz will test your pupils' understanding of the key learning points.
Our video is a tool for planning, showing how other teachers might teach the lesson, offering helpful tips, modelled explanations and inspiration for your own delivery in the classroom. Plus, you can set it as homework or revision for pupils and keep their learning on track by sharing an online pupil version of this lesson.
Explore more key stage 4 biology lessons from the Photosynthesis: factors affecting the rate unit, dive into the full secondary biology curriculum, or learn more about lesson planning.