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

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Licence

This content is © Oak National Academy Limited (2023), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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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).

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.

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

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Hello, how are you doing? I hope that you're doing well today.

I'm Mr. Jarvis and I'm gonna be working with you during today's lesson, which comes from the unit photosynthesis, factors affecting the rate.

Today we are looking at light intensity and the inverse square law.

By the end of today's lesson, you should be able to explain why light intensity decreases further from a light source.

There are five key words in today's lesson.

They're on the screen now along with their definitions.

You can read through them now by pausing the video and restarting when you're ready or wait until we get to those words as we go through the lesson and I'll explain them to you then.

The key words today are light intensity, light source, light waves, inversely proportional and inverse square law.

Today's lesson is broken down into two parts.

First, we're going to look at the relationship between light and photosynthesis, and then we're going to move on to sometimes what people find a little bit tricky, the inverse square law.

So if you're ready, let's get started with our first section, which is all about light and photosynthesis.

Light intensity, which is how much light reaches a given surface in a given time, is one factor that limits photosynthesis.

Light transfers the energy that's needed for the reactions of photosynthesis to take place.

Light sources emit light.

These can be natural sources such as the sun or artificial sources such as lumps, and we've got some pictures of those light sources in the image on the screen.

Light can be thought of as a wave and waves occur when something is disturbed or made to oscillate and the disturbance of oscillation travels and you can see the animation on the screen, showing you how the wave or the disturbance is travelling.

Light sources disturb nearby electric and magnetic fields.

The light waves travel outwards, transferring their energy, and you can see that the energy is being transferred in lots of different directions.

Here's a check, what are light waves? Are they oscillations or disturbances in air molecules? Are they oscillations or disturbances in electric and magnetic fields, or are they oscillations or disturbances in water molecules? I'll pause for a few seconds and then we'll check your answer.

The correct answer is B, light waves are oscillations or disturbances in electric and magnetic fields.

Well done if you got that answer correct.

Light travels in straight lines.

We can represent the direction in which a light wave travels using a straight line called a ray, and here's a picture of a light giving out light rays.

Light continues to travel in a straight line until it hits the surface of a material and surfaces, for example leaves of plants, absorb some of the light and reflect the rest, scattering it in many different directions.

When we use a lamp, the room is lit more brightly closer to the lamp than it is further away.

And we can see this from the image on the screen now, right very close to the lamp, it's very bright.

As we move further away, the light becomes much more dim.

This is because light spreads out as it travels and further away from the light source, the light has travelled further and has spread out more so further away from the light source, the light intensity is lower.

Remember, light intensity is the amount of light that reaches a given surface in a given period of time.

Objects that emit light look dimmer from further away.

This is because the light spreads out over a greater area as it travels.

So here we've got a city image and the lights appear much brighter in the foreground of the image because they're much closer to us than they do in the background where they appear much more dim.

Here's a check.

What happens to the light intensity as light travels away from a light source? Is it A, the intensity increases as the light spreads out? Is it B, the intensity increases as the light focuses to a point? Is it C, the intensity decreases as the light spreads out or is it D, the intensity decreases as the light focuses to a point.

I'll pause for a few seconds and then we'll check to see whether you got the answer right.

The correct answer is C.

The light intensity decreases as the light spreads out.

Well done if you got that right.

We're going to look at the effect of light intensity on the rate of photosynthesis.

Light transfers the energy needed for photosynthesis to take place and when light intensity is high, photosynthesis can take place very quickly.

When the light intensity is low, for example, at night, not enough energy is transferred for photosynthesis to occur.

We can investigate the effect of light intensity on photosynthesis by measuring the rate of photosynthesis of a plant at different distances from a light source.

And here's a graphic to illustrate how light intensity will change at different distances from a light source.

Very close to the light, there is highlight intensity.

This is because the light hasn't spread out very much, and so the amount of light that's reaching the surface of the plant is high.

As we move the plant away from the light, the light has spread out more and so there is less light reaching the of the plant.

The light intensity has decreased and further away still, the light spread out even more so the light intensity now has decreased further and is now quite low.

So in summary, light spreads out as it travels, so the light intensity will decrease as the plant is moved further from the light source.

Let's do a check.

Izzy has set up an experiment to investigate the effect of light intensity on the rate of photosynthesis in algae.

At which point will the light intensity be the highest? A, B, or C? I'll pause for a few seconds And then we'll check the answer.

The correct answer is A, the light intensity will be highest at point A.

Why, can you think of an answer to that question? I'll pause again for a few seconds and then we'll run through the answer.

So you might have thought that at point A, there will be more light reaching the surface of the algae in a given period of time.

This is because the light spreads out as it travels further.

So as we reach B and C, the light spread out more, so less light reaches the surface of the algae.

Well done if you thought of that answer.

Let's move to our first task.

Light provides the energy required for the reactions of photosynthesis to take place.

I'd like you first of all to state what happens to light as it travels.

And secondly, I'd like you to explain what that means in terms of light intensity as the distance from a light source increases.

You'll need to pause the video to write down your answer and then when you're ready, press play and we'll carry on and check to see how well you've done.

Good luck.

So I asked you two questions.

First of all, I asked you to state what happens as light travels.

Well, light waves travel in straight lines and as light travels, it spreads out.

Then I asked you to explain what that means in terms of light intensity as distance from a light source increases, you should have written down something like this.

The further from the light source, the more spread out the light becomes.

Light intensity is the amount of light that reaches a surface in a given time, and as the light becomes more spread out, the further it travels from a light source, there's less light reaching a surface in a given time.

So the light intensity decreases.

Well done if you've got something like that as your answer.

That brings us to the second part of the lesson today, which is all about the inverse square law.

So if you're ready, let's carry on.

The relationship between light intensity, that's how much light reaches a given surface in a given time period and the rate of photosynthesis, how quickly photosynthesis happens, is not simple.

This is because as the distance from a light source increases, the light intensity decreases, the distance and the light intensity are inversely proportional to each other.

Variables are inversely proportional when one increases and one decreases at the same rate.

Here's a question, which of these examples have variables that are inversely proportional to each other? Remember, one of the variables needs to increase and one needs to decrease at the same rate.

So A is the faster the speed of a train, the less time a journey takes, and B, the higher the temperature, the faster an enzyme reaction takes place.

Which of those two are inversely proportional? They may be both.

I'll pause the video while you work out which of the answers are correct and then we'll check to see whether you got them right.

Good luck.

So A was the faster the speed of a train, the less time the journey takes.

That is inversely proportional.

I can work that out because travelling 100 miles at 100 miles per hour, it takes one hour.

At 50 miles per hour, it takes two hours.

So as the speed doubles the time halves, the two variables are inversely proportional to each other.

The second example B, the higher the temperature, the faster and the enzyme reaction takes place.

That's not inversely proportional because the variables are both increasing together.

Well done if you've got those answers correct, light intensity is inversely proportional to the square of the distance from the light source.

And this is called the inverse square law.

But what does that actually mean? Well, let's investigate this a little bit further and explain what the inverse square law is.

Let's think about a square torch.

Here it is, when we switch it on, the torch produces a certain amount of light and the light spreads out as it travels.

We can see this if we look at the distance and look at right close to the torch, the the light doesn't spread out very far at all.

As we get to the end of my diagram, you can see the light has spread out a long distance.

As the light spreads out, the light intensity decreases.

Remember that light intensity is the amount of light that reaches a surface in a given period of time.

If a screen is placed in front of the torch at a certain distance, then a square of light would be projected onto that screen.

We'll the distance that we place the screen one unit.

If we look at the area of projection, the area will be one unit wide by one unit tall.

So the area will be one multiplied by one.

So one square unit, if we cut a hole in that first screen, the light will pass through and a screen is placed at double the distance and the light continues to spread out as it travels.

So here's our screen at distance of two units.

Now the square on the second screen at two units is two units wide and two units high.

So the area will be two units multiplied by two units, which is four square units.

Putting a hole in that second screen will allow the light to pass through that and another screen is now placed at double the distance again.

So at four units.

Here, we've now got an area on the screen of four units multiplied by four units, which is an area of 16 units squared.

So the light has continued to spread out.

The inverse square law states that as the distance D, that the light travels doubles.

The light intensity is four times less.

This can be represented by an equation.

Light intensity is proportional to one divided by the distance squared, and sometimes that's abbreviated to one divided by D squared.

So what does that mean about light intensity? We have our equation that light intensity is proportional to one divided by distance squared or D squared.

Let's look up the example of the square torch projecting onto our screens.

So at distance one, the area is one multiplied by one, which is one unit squared.

The intensity is proportional to one divided by the distance squared.

So one squared is one.

So one divided by one gives us one for our light intensity.

If we then look at distance two, the area is two by two, which is four units squared.

The intensity is one divided by the distance squared, two squared is four.

So one divided by four gives us our light intensity of a quarter.

What we can see here is the distance has doubled, but the area that the light is spread over has quadrupled.

The intensity is one quarter of what it was.

Does that rule still apply if we double the distance again? Well, let's look at distance four.

The area here is four multiplied by four, which is 16 units squared.

The intensity is proportional to one divided by the distance squared, so one divided by four squared, one divided by 16.

So what we can see is we've doubled the distance, we've quadrupled the area, and the light intensity is a quarter of what it was at the distance of two units.

Here's a check.

The diagram shows the light intensity at distances of one, two, and four units from a square torch.

What's the intensity of light at three units from the torch? You need to use that equation of the light intensity is proportional to one divided by D squared.

I'll pause for a few seconds and then we'll check your answer.

The correct answer is C, 1/9.

Why is it 1/9? Because the light intensity is equal to one divided by the distance squared.

The distance is three, three squared is nine, and so the light intensity is one divided by nine, 1/9.

Well done if you got that right.

We can investigate the effect of light intensity on the rate of photosynthesis in pondweed.

Light intensity is varied by moving the pondweed away from the light source.

The lamp is placed at different distances, D, from the light, the number of bubbles produced in 60 seconds is counted.

And here is our table of our results.

What is the light intensity at each distance? You may need to pause the video at this point and grab yourself a calculator, but I'd like you to try and work out the light intensity at each of those distances from the light source.

Work out your answers, scribble them down on a piece of paper, and then what we'll do is we'll check to see whether you got them right.

So how did you do? Let's work them out.

Remember, our light intensity is proportional to one divided by the distance squared.

So in the first example, one over the distance squared is one divided by 10 squared, one divided by 10 squared is the same as one divided by 100.

And if we then convert that into a decimal, it's 0.

01, well done if you got that one.

At 20 centimetres from the light source, it's one divided by 20 squared.

So one divided by 20 squared is 0.

025.

At 30 centimetres, it's one divided by 30 squared, and that gives us an answer of 0.

0011.

And finally at 40 centimetres it's one divided by 40 squared and that's 0.

000625.

Well done if you got that right.

So what does that tell us? Well, as we double the distance from the light source from 10 to 20 centimetres, the light intensity quarters from 0.

01 to 0.

025.

The same pattern exists between 20 and 40 centimetres.

As the light intensity decreases, the number of bubbles produced in 60 seconds decreases too, and that tells us that photosynthesis is slowing down.

Here's a check.

Lucas will measure the rate of photosynthesis in algae.

The algae will be moved to different distances from the light.

He says, I predict the results will show that the rate of photosynthesis is, I'd like you to complete Lucas's sentence.

Is it A, directly proportional to the distance from the light source, B, inversely proportional to light intensity, C, inversely proportional to the distance from the light source or D, directly proportional to temperature? I'll pause for a few seconds and then we'll check your answer.

The correct ending to Lucas' sentence, I predict the results will show that the rate of photosynthesis is inversely proportional to the distance from the light source.

That's answer C, well done if you got that.

Let's move to our final task of today.

The number of bubbles produced by a pondweed, Bacopa, at different distances from a lamp was used to determine the rate of photosynthesis.

You can see some bubbles being produced by Bacopa in the animation on the screen.

Light intensity can be calculated using the inverse square law, and that states that the light intensity is proportional to one divided by D squared.

The Bacopa was placed at five, 10 and 20 centimetres from the light source.

I'd like you to explain how light intensity changes as the distance of the Bacopa from the light source is doubled.

You should include calculations in your answer.

You'll need to pause the video at this point, write down your answers and do some calculations to support your answer.

And then when you're ready, press play and we'll check to see how well you've done.

Good luck.

So how did you get on with that? I asked you to explain how light intensity changes as the distance of the Bacopa from the light source is doubled.

You should include calculations in your answer, so you might have included that light intensity is quartered when the distance is doubled, and this can be demonstrated by the following calculations.

At distance five centimetres, the light intensity is proportional to one divided by the distance squared.

That's one divided by five squared, which is 1/25, and that equals 0.

04.

At distance 10 then, light intensity is proportional to one over D squared.

That's one over 10 squared, which equals to 1/100, and that's 0.

01.

And then at distance 20 centimetres, the light intensity is proportional to one over D squared, which is one over 20 squared.

That's one over 400 or 0.

025.

And what we can see here is as we double the distance from five to 10 centimetres, then the light intensity is quartered from 0.

04 to 0.

01.

If we look at the distance 10 to 20, again, we are doubling the distance and we're quartering the light intensity.

There are four 0.

025 in 0.

01.

Well done if you've got something like that in your answer.

That brings us to the summary of today's lesson.

We've seen that light waves travel from a light source in straight lines, and as though that light travels, it spreads out.

This spreading outta the light means that light intensity, that's the amount of light reaching a surface in a given time, decreases with increasing distance from the light source.

When one variable increases and another decreases at the same rate, we say that they're inversely proportional.

The inverse square law states that light intensity is inversely proportional to the square of the distance.

I hope that you've enjoyed that lesson today, and I look forward to seeing you again sometime soon.

Bye-bye for now and have a good day.

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no effect on photosynthesis" }, { "text": "the effect of light intensity varies with plant species" } ], "id": "f883d6ac-dd1c-132d-be60-8762389ad542", "lessonId": "f7f19932-dd96-e306-82e8-e40216d81e6e" }, { "type": "Question", "question": { "text": "What is the relationship between light intensity and the rate of photosynthesis?" }, "questionType": "Multiple choice", "correct": [ { "text": "the rate increases with light intensity up to a point, then plateaus" } ], "choices": [ { "text": "the rate increases with light intensity up to a point, then plateaus" }, { "text": "the rate decreases as light intensity increases" }, { "text": "light intensity has no effect on the rate of photosynthesis" }, { "text": "the rate increases indefinitely with increasing light intensity" } ], "id": "9a91f6cd-00c1-b8e8-b3b4-e9c5380337ae", "lessonId": "f7f19932-dd96-e306-82e8-e40216d81e6e" }, { "type": "Question", "question": { "text": "Why does light intensity decrease as the distance from the light source increases?" }, "questionType": "Multiple choice", "correct": [ { "text": "because the same amount of light is spread over a larger area" } ], "choices": [ { "text": "because light energy is absorbed by the air" }, { "text": "because light waves change direction" }, { "text": "because the power of the light source decreases" }, { "text": "because the same amount of light is spread over a larger area" } ], "id": "632c0047-f044-9630-39ea-c75a6acce33d", "lessonId": "f7f19932-dd96-e306-82e8-e40216d81e6e" }, { "type": "Question", "question": { "text": "What is the inverse square law?" }, "questionType": "Multiple choice", "correct": [ { "text": "the intensity of light is inversely proportional to the square of the distance" } ], "choices": [ { "text": "the intensity of light is directly proportional to the distance from the source" }, { "text": "the intensity of light increases with the square of the distance" }, { "text": "the intensity of light remains constant regardless of distance" }, { "text": "the intensity of light is inversely proportional to the square of the distance" } ], "id": "99c81487-c715-18b5-593e-1f9474f29eb8", "lessonId": "f7f19932-dd96-e306-82e8-e40216d81e6e" }, { "type": "Question", "question": { "text": "What would happen to the light intensity if the distance from the light source is tripled?" }, "questionType": "Multiple choice", "correct": [ { "text": "it becomes one-ninth" } ], "choices": [ { "text": "it becomes one-third" }, { "text": "it becomes one-sixth" }, { "text": "it becomes one-ninth" }, { "text": "it remains the same" } ], "id": "34f2b7b0-19df-182e-ee2b-6e1d5b458f5e", "lessonId": "f7f19932-dd96-e306-82e8-e40216d81e6e" }, { "type": "Question", "question": { "text": "What is the relationship between the rate of photosynthesis and the distance from a light source?" }, "questionType": "Multiple choice", "correct": [ { "text": "rate is inversely proportional to the distance from the light source" } ], "choices": [ { "text": "rate is directly proportional to the distance from the light source" }, { "text": "rate is inversely proportional to the distance from the light source" }, { "text": "rate is not related to the distance from the light source" } ], "id": "9c249227-63ed-91c2-06e8-8e99d1a1419d", "lessonId": "f7f19932-dd96-e306-82e8-e40216d81e6e" } ], "lessonId": "f7f19932-dd96-e306-82e8-e40216d81e6e", "meta": { "gradeStageId": "ks4", "subject": "Biology", "tier": "Foundation", "examBoard": "AQA", "grade": "Year 11", "topic": null, "unit": "Photosynthesis: factors affecting the rate", "lessonCount": "9", "age": [ 15, 16 ], "gradeId": "age_15_16", "gradeStage": "GCSE", "lesson": "Light intensity and the inverse square law" } } ] }