Light

ಕನ್ನಡದಲ್ಲಿ ನೋಡಿ

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Concept Map

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Additional information

Useful websites

A spectroscope is an optical device that could be used to produce and observe a specific portion of spectrum of light or radiation from a source. A compact disc (CD) can be used to make a simple spectroscope. The following video link (from Arvind Gupta's website) demonstrates how to make it.

C D Spectroscope

The template (in pdf format) shown in the video can be downloaded from:

Template-C D Spectroscope

Reference Books

1. NCERT Textbook Chapter Light
2. NCERT Textbook Chapter Reflection and Refraction
3. NCERT Textbook Chapter Human Eye and the Colourful World
4. Tamil Nadu text book in kannada class 8 chapter 4click here
5. Tamil nadu text book in english class 8 chapter 4click here
6. Tamil nadu text book in tamil class 8 chapter 4click here

An image of a concept map of light

Teaching Outlines

Concept #1 - Light is a form of energy

Learning objectives

1. Light is a form of energy and travels as electro-magnetic radiations. There are other radiations apart from light that travel as electro-magnetic radiation.
2. It is possible look at light and other electro magnetic radiation as a wave or as a particle known as the photon.
3. The different types of photons are seen by us as different colours.
4. There are some kinds of photons that we cannot see – for example heat radiation, X rays, infra-red and ultra-violet rays. We say that light travels as a wave and different coloured lights have different wave-length associated with them.

Notes for teachers

The unit begins by introducing the idea that light is a form of energy. We then look at how this energy is transported from a source to a destination in the form of waves. After a brief description of what waves are and how they actually transport energy the idea that light of different wavelengths are perceived as different colours would be introduced. This would naturally lead to the idea of hues and intensity.

1. Light is a form of energy that is transported as electromagnetic radiation but interacts with material particles as discrete packets of energy that are called photons.
2. The primary source of all energy, including light, is the Sun. All the other kinds of energy that we receive here can be traced back to the sun. Sun provides us with heat and light. This travels through the near vacuum of space. So there is no material to transport energy.
3. When we block light we also block the heat radiation. Any source of heat also radiates some visible light.
   
4. This radiation can be thought of as waves of different wavelengths. The waves are not of material objects but travelling waves changing electric and magnetic fields. Different wavelengths are associated with different energy levels and different kind of radiation.

Click here for more notes on the nature of light, visible light and colours.

Light Fantastic - BBC Documentary - Part 1/4

Light Fantastic - BBC Documentary - Part 2/4

Light Fantastic - BBC Documentary - Part 3/4

Light Fantastic - BBC Documentary - Part 4/4

The following videos are useful to understand the development of theories of light.


Listen to Richard Feynman, a renowned physicist talk about the nature of light.

Activity No # 1 Black is Hot - White is Cool!

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At the end of this experiment children should be able to recognize that black objects absorb more radiant energy than white objects. This is a simple experiment that can be done in the classroom or at home. Children should report their findings in a formal way – this is to help them formalise their understanding:

• Estimated Time - 30 minutes
• Materials/ Resources needed

One piece of black cloth. A similar sized white cloth. Ideally the two materials should be the same and texture. It may not be very critical if they are not. Cloth pieces of other colours but similar material and texture.

• Prerequisites/Instructions, if any - None
• Multimedia resources - None
• Website interactives/ links/ simulations
• Process (How to do the activity)

1. Leave the two pieces of cloth out in direct sunlight for about 15 minutes.
2. At the end of the period feel the clothes.
3. Try this with other coloured clothes
4. Observations written in an organized format - aim of the experiment, method and questions below are to be answered in the observation

• Developmental Questions (What discussion questions)

1. Which seemed warmer the black or the white cloth?
2. If you used other coloured clothes, arrange them in ascending order of warmth.

Which is warmer – the dark cloth or the light cloth?

• Evaluation (Questions for assessment of the child)

1. What happens when you are near a glass window in a closed house?

• Question Corner

Activity No # 2 - Radiation from the Sun

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• Estimated Time - 40 minutes
• Materials/ Resources needed - None
• Prerequisites/Instructions, if any - The Activity #1 should have been done
• Multimedia resources

This is a video that explains radiation as an animation. Use this to initiate the discussion and talk about the questions below.

• Website interactives/ links/ simulations
• Process (How to do the activity)

1. Heat a piece of iron and watch the iron changes colour.
2. Show the video and discuss the questions below, alongwith the observations above

• Developmental Questions (What discussion questions)

From all this can we see the following:

1. What happens to the energy from the sun? - Energy is transported from a source to a destination without a medium
2. Are there different kinds of energy/ forms of energy? - The energy is of different kinds – heat, light of different colours, X ray, eyc.
3. What happened here when you places something in the sun? - One form of the energy can get converted to another. They must therefore have things in common - radiant heat and light are similar in many ways and different in some ways.
   
4. When we heated the iron what changes did we see? Why did the colour change? (From a dull grey to red to yellow to white when light of all wavelengths are radiated in addition to the heat.)
5. On a cold day the temperature inside a glass house is much higher – why? Because light of a shorter wavelength can travel through glass and is absorbed by the ground and plants. The ground reflects back radiation of a higher wavelength – infra-red and heat radiation. This cannot pass through glass and so the glass house is much warmer.
6. Explain that studies done on how light travels has helped us visualise it now as changing electrical magnetic fields that interact with other material objects as packets of energy. This can be extended into the discussion on Nature of Light.

• Evaluation (Questions for assessment of the child)

1. What are the various forms of energy?
2. Where does all energy come from?

• Question Corner

Concept #2 Light and Colours

Learning objectives

1. The different types of photons are seen by us as different colours.
2. There are some kinds of photons that we cannot see – for example heat radiation, X rays, infra-red and ultra-violet rays. We say that light travels as a wave and different coloured lights have different wave-length associated with them.
3. White light is a combination of all colours. Different colour lights can be produced from this by absorption and by reflection. The colours of the rainbow spectrum – Violet, Indigo, Blue, Green, Yellow, Orange & Red – ‘VIBGYOR’ or ‘Roy G BIV.’ Black colour is the absence of colour. Objects that are black do not reflect any light but absorb all the energy.

iv. Understand the idea of primary additive colours and primary subtractive colours and how differe

Notes for teachers

For some helpful tips on primary colours and the pigment mixing activity below, watch the following videos.
Colour mixing in Paint

Primary Colours

Activity No #1A - Mixing pigments

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The objective is to observe and understand the constituents of light.

• Estimated Time 30 minutes
• Materials/ Resources needed

Poster Colours in Red, Blue and Yellow.

• Prerequisites/Instructions, if any
• Multimedia resources
• Website interactives/ links/ simulations
• Process (How to do the activity)

1. We can use drops of poster colour or take small quantities of any water soluble paint. Be sure to use Red, Blue and Yellow – the primary colours for mixing pigments.
2. Observations written in an organized format - aim of the experiment, method and questions below are to be answered in the observation

• Developmental Questions (What discussion questions)

What did you observe when

1. Red Pigment is mixed with Blue
2. Blue is mixed with Yellow
3. Yellow is mixed with Red
4. All mixed together

• Evaluation (Questions for assessment of the child)

1. How have the children recorded their observations?
2. What is their working method?

• Question Corner

Activity No # 1B - Recombination of Colours

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To study recombination of colours. This experiment allows children to try and look at what happens when they mix coloured light.

• Estimated Time - 45 minutes
• Materials/ Resources needed

Three Torches. Any kind of torch light torch will do. Make sure all of them have beams of the same intensity Coloured transparent cellophane Sheets – Colours required Red, Blue and Green. Cellotape

• Prerequisites/Instructions, if any

It is preferable if they have done Activity 1A before.

• Multimedia resources
• Website interactives/ links/ simulations
• Process (How to do the activity)

Part-1

1. Cut small sheets of red, blue and green cellophane sheets. These pieces should be big enough to fold over and still be big enough to cover the torch beam.
2. Wrap each of the coloured sheets over separate torches making sure that the sheet is flat over the transparent glass in the front of the torch.
3. Tape it securely from the sides. We should now get torches which give a clear beam of red, green and blue light respectively.
4. If we shine the torch on a white paper so that we get a clear circular image of each of the colours.
5. Observe what happens when

• The red beam and green beam are mixed
  The red and the blue are mixed
  The blue and the green are mixed
  When all three of them fall together
  

1. Repeat the experiment but this time use only one torch. Now keep placing different colour sheets tightly over the torch beam adding them one by one. And find out the colour of the beam that comes through. We should find that we get similar combinations as when the beams are mixed – only the light will be weaker.
2. Observations written in an organized format - aim of the experiment, method and questions below are to be answered in the observation

• Developmental Questions (What discussion questions)

What do you observe when:

1. The red beam and green beam are mixed
2. The red and the blue are mixed
3. The blue and the green are mixed
4. When all three of them fall together
5. When we used more than one sheet with the same sheet
6. Red and blue sheet together
7. Blue and green sheet together
8. Green and red sheet together
9. All three sheets together

• Evaluation (Questions for assessment of the child)

1. In the second part of the experiment uses 2 or 3 colour filters with one torch. How does this work? (The cellophane filter only enriches the red colour. The other colours are still there.)
2. Why does the light become dimmer? When we add one more sheet the colour will be the addition of the other filters. More and more light will get absorbed by the filters.
3. How are we able to see? (We see objects because they reflect light).
4. What is white light? When do we see white and black? (If they reflect all the incoming white light then we see it as white. If it does not reflect any light but absorbs all of it then we see it as black. If it reflects some colours and absorbs others then we see it as the colour of the light that it reflects. A tomato is red because it absorbs colours other than red and reflects red light. A green leaf is green because it absorbs all light other than green).
5. How are pigments different from light?

• Question Corner