Force
Light, Sound and Waves

Seeing with light - Teaching approaches

Classroom Activity for 11-14

A Teaching Approach is both a source of advice and an activity that respects both the physics narrative and the teaching and learning issues for a topic.

The following set of resources is not an exhaustive selection, rather it seeks to exemplify. In general there are already many activities available online; you'll want to select from these wisely, and to assemble and evolve your own repertoire that is matched to the needs of your class and the equipment/resources to hand. We hope that the collection here will enable you to think about your own selection process, considering both the physics narrative and the topic-specific teaching and learning issues.

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Totally in the dark!

Reflection
Light, Sound and Waves

Totally in the dark!

Classroom Activity for 5-11 11-14

What the Activity is for

Experiencing a completely dark space.

This activity allows pupils to enter a completely dark space, something which very few are likely to have previously experienced. Once in this light-free space they will recognise that it is impossible to see anything around them.

What to Prepare

This activity depends on the availability of a light-tight space. If your school has a photography dark room that will be perfect for this activity. If not, a handy store cupboard (partially cleared out for the occasion!) will serve just as well.

What Happens During this Activity

This is a great lesson! The impact of the activity is much enhanced by the discussion between teacher and pupils which leads up to the dark room experience. The starting point is: How are we able to see the things around us? This can lead on to talk about places where the pupils are unable to see anything. Typically pupils will refer to being outside at night or perhaps to the moment when you first switch off your bedroom light and it's completely black. In either case, however, pupils will be happy to tell you that after a bit your eyes get used to the darkness and you can see the things around you in the bedroom.

The discussion can be pushed further by introducing what you might call the: Hand in front of your nose test.

The challenge is: Can you think of a place where it is so dark that you cannot see your hand in front of your nose?

Maybe some pupils will have been potholing or caving and it will be good to listen to their experiences. You might refer to mining accidents where miners have been trapped underground, without light waiting for their rescuers in complete darkness. You might also ask the question of whether Superman would be able to see in the dark, with the special rays coming from his eyes.

To carry out the hand in front of your nose test pupils can go into the dark room in groups of four.

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In straight lines

Reflection
Light, Sound and Waves

In straight lines

Classroom Activity for 5-11 11-14

What the Activity is for

This is a teacher demonstration activity to show that light travels in straight lines.

What to Prepare

  • a bright light source. Use some kind of point source (where the light comes from a relatively small aperture) to produce a well defined shadow on the screen
  • a table tennis ball (or some other smooth-surfaced ball) hung from a piece of cotton (super-glue the cotton to the ball)
  • a screen

What Happens During this Activity

Hang the ball from a metre rule and hold it in the beam of light between the light source and the screen. With a good black-out for the room and a strong point source of light, a striking shadow will be produced on the screen.

Draw the children's attention to the sharpness of the shadow, how clean the line is between light and no light. Also, make the point that the shape of the shadow, although bigger, is exactly the same as the shape of the ball. This leads to the idea that the light must be travelling in straight lines to create such a shadow on the screen. The notion of light travelling in straight lines may seem obvious to many of the children. You might wish to explore an imaginary scenario which involves the light from the source bending around the ball. What would the shadow on the screen look like then?

Ask the children to predict what will happen to the shape and size of the shadow on the screen, if the ball is moved closer to, and further from, the screen.

In developing these ideas, you are likely to find yourself sketching out ray diagrams for the children.

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Seeing with light

Reflection
Light, Sound and Waves

Seeing with light - an activity

Classroom Activity for 5-11 11-14

What the Activity is for

Seeing beams of light.

This is part of a suggested sequence:

A repertoire to develop the idea of seeing


Pupils will very often agree that to be able to see, light must enter the eye. However, it is possible to set up situations where this apparently firmly-fixed belief can be productively challenged, and thereby strengthened. This is the aim for this demonstration.

What to Prepare

  • a source of fine dust – board duster or talc
  • a low power laser that provides an intense, fine beam of light

Safety note: Take care to ensure that light from the laser cannot pass directly into anyone's eyes. A class 2 laser should be used, although a laser pointer can be used if under the careful control of the teacher. You should be aware, however, that some laser pointers are incorrectly assigned a relatively low power rating and are potentially more dangerous than they might appear.

Even if a laser beam does enter the eye, the blink-aversion response is sufficiently fast to avoid any damage to the retina except with the most powerful lasers.

What Happens During this Activity

Set up the laser so that, when switched on, the beam travels across the front of the room, landing on a white screen so that the red spot is clearly visible.

Start by describing the set-up to the pupils, showing the laser and pointing to the screen across the room at which the beam is directed. For the sake of this demonstration, all that the pupils need to know is that the laser provides an intense, or very powerful, beam of light.

Explain that you are going to put the room lights out in a moment and ask the pupils to predict what they will see when the lights are out (with a good black-out) and the laser is switched on. This is where the pupils' ideas about seeing are challenged.

Challenging predictions

Former opinion: You'll see the beam cutting through the darkness.

Latter opinion: You'll just see a spot on the screen where the beam hits.

Experience has shown that the former opinion is very common among pupils in the lower secondary school age range. Make sure to bring it out into the open, so that it gets effectively challenged. It's not enough to simply show the process and assume the learning will be automatic.

Now switch on the laser (with appropriate theatrical build-up). Nothing seems to happen. A red spot appears on the screen at the side of the room, but the laser beam itself can't be seen.

The big question is: Why can't we see the laser beam? (Because none of the laser light is entering our eyes). This can lead to an interesting discussion of situations where it is possible to see beams of light cutting through the air. Pupils may well refer to laser shows at pop concerts, the projection lights in cinemas, car headlamps on a foggy night. In each of these cases there is something which will scatter the light into our eyes.

An interesting follow-up question is: How come we can see the red spot on the screen? (Here laser light is being reflected in all directions from the wall and some travels to our eyes).

Use chalk dust from the board rubber (if you still use one) or a fine powder such as talc, or alternatively a fine water spray to scatter the beam. The room light is off and the effect is stunning as the beam becomes visible as light is scattered away from it.

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Euclid and his visual rays

The Electromagnetic Spectrum
Light, Sound and Waves

Euclid and his visual rays

Classroom Activity for 11-14

What the Activity is for

The case of Euclid and his visual rays allows pupils to see that some ideas from the history of science are similar to existing common sense ways of thinking about natural phenomena. In addition the pupils are asked to challenge a scientific idea or theory (in this case the idea of visual rays) by presenting new pieces of evidence.

What to Prepare

  • printed copies of the support sheet (see below)

What Happens During this Activity

There are various ways in which you might draw on this story from the ancient history of science. You might simply tell it as a story and engage the pupils in discussion around the kinds of questions listed below. Alternatively you might use the text for a class reading and comprehension exercise. First of all the class read through the passage, then the pupils work in pairs to talk through the questions and make notes about their responses. Pairs of pupils then report back in whole-class discussion.

Euclid's two main arguments in favour of emission by the eyes were as follows.

In searching for a small object like a needle, or in looking at a page of a book, you do not immediately see the needle, or all of the letters on the page. Euclid argued that this could not happen if the images of these things were reaching the eye and making an impression. It must therefore be the eye which is the active emitting organ such that eventually one's eye rays might land on the missing needle or particular words on the page.

The second argument relates to the shape of the eye. Here, Euclid argued that the ear is hollow and is obviously a receiving organ, such that sound enters the ear. Conversely, since the eye has a protruding shape, it must be an emitting organ, thereby giving out light.

Questions for pupils to consider:

  • What idea did Euclid have to explain how we are able to see things around us?
  • What two arguments did Euclid propose to support this idea? Outline each argument in your own words.
  • Supposing you met Euclid. What evidence would you offer to try to persuade him that we see things when light enters the eye, rather than when light is given out by the eye?
  • How would you counter (argue against) the first of Euclid's arguments relating to problems experienced in searching for a small object such as a pin?

Resources

Download the support sheet / student worksheet for this activity.

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Modelling the working of the eye with a webcam

The Electromagnetic Spectrum
Light, Sound and Waves

Modelling the working of the eye with a webcam

Classroom Activity for 11-14

What the Activity is for

This demonstration is to present parallels between the structure and function of a webcam and the structure and function of an eye. Small webcams are about the size of the eye and provide a live shareable image of how the world appears to them. This can be manipulated and used to promote discussion about the working of the eye. At this early stage we aim to keep the discussions simple. There is much that can be done, but probably only some that should be done.

What to Prepare

  • a webcam connected to a computer, and a means of sharing a large view of a computer screen
  • some objects to place in the field of view of the webcam, both luminous and not – a bright lamp is very useful

You'll need to practice with the webcam beforehand, as each comes with its own particular brand of driving software, some of which may need a tweak before you get the results that you want. In general, for the uses suggested here, it is worth having a webcam with a small number of pixels (say 640 by 480), but a reasonable colour depth (at least 8 bits/pixel).

What Happens During this Activity

Start with the webcam seeing the class. You'll see some delay in the refreshing of the picture – just like the eye. Pick up the webcam, talking your way around it, from the hole at the front, to the light sensitive bit at the back. Draw parallels with the simplest possible model of the eye, used in this episode.

Look at some non-luminous objects, emphasising the reflecting of the light off the object and then the trip from the object to the webcam. Put some objects out of the field of view of the webcam and ask why they cannot be seen.

Choose a bright, luminous object and point it straight at the webcam. For a while the image will be burnt out, but then the webcam will adjust, allowing less light in. The eye does this by making the hole at the front smaller, the webcam probably does not.

Choose a fair image and blow it up, until you can see the individual blocks of colour that make up the image. Smooth shadings are only apparent – like our eye the webcam has discrete sensing elements. Seeing may be one of the most natural things in the world for almost all of us, but that doesn't make it simple. One outcome from this brief exploration of the similarities is that pupils should feel that they have a simple model of the eye, but that there are plenty of phenomena left to explore and model.

Long sightedness, short sightedness and depth of field can all be modelled with the webcam. But we suggest leaving these until later and keeping it both short and light, for now.

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Galileo and the speed of light

The Electromagnetic Spectrum
Light, Sound and Waves

Galileo and the speed of light

Classroom Activity for 11-14

What the Activity is for

This activity involves telling a story, which you can use as a means of starting a discussion about the role of measurement in science, here relating ideas to evidence.

What to Prepare

  • a copy of the story and possibly a portrait of Galileo projected onto a white board (see below)

What Happens During this Activity

There are various ways in which you might draw on this story. You might simply tell it as a story and engage the pupils in discussion around the kinds of questions listed below. Alternatively you might use the text for a class reading and comprehension exercise. First of all the class read through the passage, then the pupils work in pairs to talk through the questions and make notes about their responses. Pairs of pupils then report back in whole-class discussion.

Questions for talking through with your class:

  • What do you think Galileo wrote down as his conclusion for the experiment?
  • What do you think the pattern (the more recordings taken, the shorter the measured time) in Galileo's results suggested to him?
  • What does the pattern in Galileo's results suggest to you? How could you support your suggestion?
  • What was the big problem which Galileo faced with his experiment?
  • Would it be possible to use this kind of direct measurement approach to find the speed of sound?

The speed of light is huge, but was measured before we had clocks to time very short intervals. Further research could focus on the questions: So how could the measurement be made? and How was it done?

Resources

Download the support sheet / student worksheet for this activity.

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