ICSE Class 7 Physics

Colours of Light

Discover the science behind colours, white light, rainbows and the colourful screens we use every day!

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Observe • Explore • Think • Learn

1. Can You Imagine a World Without Colours?

Imagine waking up one morning and seeing everything in shades of grey. Your red school bag, green trees, blue sky and colourful flowers would all look different!

Colours make our world beautiful. But have you ever wondered where colours come from? Why does grass look green? Why does a tomato look red? And how can a television screen display millions of colours?

Let us explore these fascinating questions through the science of light.

2. Learning Objectives

By the end of this lesson, you will be able to:

• Explain that white light is made up of different colours.
• Name the seven colours of the visible spectrum.
• Explain why objects appear to have different colours.
• Identify the primary and secondary colours of light.
• Explain the formation of secondary colours by mixing coloured lights.
• Differentiate between additive colour mixing and colour subtraction.
• Apply the concepts of light and colour to everyday situations.

3. White Light and the Spectrum

Natural light from the Sun appears white. However, white light is actually a mixture of different colours.

When white light is separated into its constituent colours, a band of colours called the spectrum is formed.

Meet VIBGYOR!

The seven colours of the visible spectrum, in order, are:

Violet Indigo Blue Green Yellow Orange Red

Memory trick: VIBGYOR stands for Violet, Indigo, Blue, Green, Yellow, Orange and Red.

How is a rainbow formed?

A rainbow is a natural display of colours. It forms when sunlight enters raindrops, bends, separates into different colours and undergoes internal reflection before emerging from the drops.

We generally see a rainbow when the Sun is behind us and raindrops are in front of us.

Think about it: Why do we see a rainbow after rain when sunlight is present?

4. Why Do Objects Have Different Colours?

Objects appear coloured because they reflect some colours of light and absorb others.

Example 1: Green Grass

Grass appears green in white light because it reflects green light towards our eyes and absorbs much of the other visible light.

Example 2: A Red Tomato

A red tomato appears red in white light because it reflects red light and absorbs much of the other visible light.

Example 3: A Black Object

A black object absorbs most of the visible light falling on it and reflects very little light.

Example 4: A White Object

A white object reflects most of the visible light falling on it, without strongly favouring one colour.

Object	What happens to light?	Appearance
Green leaf	Reflects green light	Green
Red tomato	Reflects red light	Red
Black cloth	Absorbs most visible light	Black
White paper	Reflects most visible light	White

Important: The colour we see depends on the light illuminating an object and the colours the object reflects. A red object cannot reflect red light if no red light falls on it.

5. Primary Colours of Light

When coloured lights are mixed, they can produce new colours. The three primary colours of light are:

RED

GREEN

BLUE

Red, green and blue are called the primary colours of light because they are used in different proportions to produce a wide range of colours by additive mixing.

These three colours are commonly represented by the letters RGB.

Where do we use RGB?

Television screens, mobile phones, computer monitors and digital projectors use combinations of red, green and blue light to display colours.

6. Secondary Colours of Light

When any two primary colours of light are mixed in equal proportions, they form a secondary colour.

Primary Colour 1	Primary Colour 2	Secondary Colour
Red	Green	Yellow
Green	Blue	Cyan
Blue	Red	Magenta

Try to Remember

Red + Green = Yellow

Green + Blue = Cyan

Blue + Red = Magenta

Red + Green + Blue, in suitable proportions = White light.

Note: Cyan, magenta and yellow are the secondary colours of light in the additive RGB system.

These combinations are different from mixing paints, crayons or coloured pigments. Mixing coloured lights and mixing pigments are not the same process.

7. Formation of Colours by Addition

When two or more coloured lights overlap, their light combines. This process is called additive colour mixing.

Imagine shining red, green and blue lights onto a white screen.

• Red and green light combine to produce yellow light.
• Green and blue light combine to produce cyan light.
• Blue and red light combine to produce magenta light.
• Red, green and blue light together can produce white light when combined in suitable proportions.

Science in Your Pocket!

Your mobile phone screen contains tiny light-emitting elements that produce red, green and blue light. By varying their brightness, the screen creates the colours of photographs, cartoons and videos.

8. Formation of Colours by Subtraction

Colours can also be produced when some colours are absorbed or removed from white light. This is called colour subtraction.

For example, a red tomato reflects red light and absorbs much of the other visible light. When illuminated by white light, it looks red.

Now imagine illuminating the same tomato with blue light only.

The blue light contains no red component. The tomato absorbs much of the blue light and has no red light available to reflect. Therefore, it may appear very dark or nearly black.

Remember the Difference

Additive mixing: Different coloured lights are combined.

Colour subtraction: Some colours are absorbed or removed from the light reaching our eyes.

Coloured filters and pigments can selectively absorb some wavelengths and transmit or reflect others. This changes the colour of the light that finally reaches our eyes.

9. A Short Story: The Mystery of the Red Tomato

Meet Aarav, the Young Scientist!

One afternoon, Aarav was helping his mother prepare a salad. He picked up a bright red tomato and asked, "Mum, why is this tomato red?"

His mother smiled and said, "What do you think?"

Aarav remembered his Physics lesson. "The tomato reflects red light and absorbs much of the other visible light!"

His mother then switched on a blue lamp. Under the blue light, the tomato looked very dark.

Aarav was surprised. He explained, "There is no red light in the blue light falling on the tomato. So the tomato cannot reflect red light into my eyes!"

The next day, Aarav used a red, a green and a blue light source to investigate how coloured lights combine. He discovered that red and green light could produce yellow light on a screen.

He wrote in his science notebook:

"Colours are not just beautiful. They are clues that help us understand how light interacts with matter."

Story-Based Thinking Questions

1. Why did the tomato look red in white light?
2. Why did it appear dark under blue light?
3. Which two primary colours of light can be mixed to produce yellow?

10. Extraordinary Science Facts

Fact 1: Newton and White Light

In the 1660s, Isaac Newton used a prism to investigate white light and demonstrated that it could be separated into different colours.

Fact 2: Colours on Your Television

A television screen can create a wide range of colours by changing the brightness of red, green and blue light.

Fact 3: The Aurora

Auroras are colourful light displays in the sky caused by interactions between energetic particles and gases in Earth's atmosphere. Oxygen commonly produces green and red emissions, while nitrogen contributes blue and purple colours.

Fact 4: The Colour of an Object Can Change

A red flower may look bright red in sunlight but appear dark under blue light. Its appearance depends on the light falling on it.

11. Become a Colour Scientist!

Activity A: The Colour Detective

Materials: A red object, a green object, a white sheet of paper and a torch.

Procedure:

1. Observe the objects under normal white light.
2. Note their colours.
3. Discuss what might happen if only blue light falls on the red object.
4. Explain why the colour of an object depends on the light falling on it.

Expected learning: An object's appearance depends on the colours of light it reflects and the illumination available.

Activity B: The RGB Challenge

Imagine you have three torches fitted with red, green and blue filters. Shine the lights onto a white screen, allowing the beams to overlap.

• Overlap red and green light.
• Overlap green and blue light.
• Overlap blue and red light.
• Finally, overlap all three lights.

Predict the colours you expect to see before checking the answer table in Section 6.

Safety: Never shine a torch or any other light directly into anyone's eyes. Do not look directly at the Sun.

12. Think Beyond the Textbook!

Question 1: The Red Rose

A red rose is placed in a room illuminated only by green light. Will it appear bright red?

Think: Is there any red light available for the rose to reflect?

Question 2: The White Wall

A white wall is illuminated by blue light. What colour is it likely to appear?

Think: Which colour of light is reaching the wall and being reflected?

Question 3: The Mobile Screen

Why can a mobile screen show yellow even though its light-producing system uses red, green and blue as its primary colours?

Think: What happens when red and green light combine?

Question 4: The Black Shirt

Why can a black shirt become warmer in sunlight than a similar white shirt?

Think: Which shirt absorbs more of the incident visible light?

Reveal the Answers

1. No. A red rose reflects red light. Under green illumination, it has no red light to reflect and may appear dark.

2. It is likely to appear blue because it reflects the blue light falling on it.

3. Red and green light combine to produce yellow in additive colour mixing.

4. A black shirt generally absorbs more incident light energy, while a white shirt reflects more light. Other factors, such as the fabric and surroundings, also affect temperature.

13. Values We Learn Through Science

Observe Before You Conclude

Aarav did not simply accept the tomato's changing appearance. He observed, questioned and used scientific reasoning.

Curiosity Leads to Discovery

Everyday objects can teach us valuable scientific lessons when we ask thoughtful questions.

Respect Different Perspectives

Just as different colours combine to create something beautiful, people with different ideas and abilities can work together to achieve meaningful goals.

Use Knowledge Responsibly

Science helps us understand the world. We should use scientific knowledge carefully, honestly and safely.

14. Interactive Quiz: Test Your Understanding!

Choose the correct answer for each question. There are 15 questions, including application-based and thinking-based MCQs.

Scoring: Each correct answer carries 1 mark. Maximum marks: 15.

1. White light is made up of:

Only red and blue light Different colours of visible light Only green light Only violet light

2. Which colour comes immediately after green in VIBGYOR?

Orange Red Yellow Blue

3. Which is a primary colour of light?

Red Orange Yellow Magenta

4. Red light mixed with green light produces:

Blue Yellow Violet Black

5. Green light mixed with blue light produces:

Red Orange Yellow Cyan

6. Blue light mixed with red light produces:

Green Yellow Magenta Orange

7. The primary colours used in television screens are:

Red, green and blue Red, yellow and blue Green, orange and violet Yellow, cyan and red

8. A green leaf appears green mainly because it:

Produces green light by itself Reflects green light Absorbs all green light Reflects only red light

9. A black object generally appears black because it:

Reflects all colours equally Produces black light Reflects only white light Absorbs most visible light

10. The band of colours formed by separating white light is called:

A shadow An image A spectrum A reflection

11. A red tomato is illuminated only by blue light. It will most likely appear:

Bright red Very dark or nearly black Bright yellow Pure white

12. Which combination can produce white light in additive colour mixing?

Red, green and blue light Red and black light Green and orange light only Violet and black light

13. Which statement correctly describes colour subtraction?

It occurs only when three torches overlap It always produces white light It means adding more light to a screen Some colours are absorbed or removed from the light

14. A white sheet of paper is illuminated only by blue light. What colour will it generally appear?

Red Black Blue Orange

15. Which process helps form a natural rainbow?

Refraction, dispersion and internal reflection in raindrops Only absorption of sunlight Only the reflection of light from a mirror Mixing paint colours in the sky

Your Quiz Report Card

15. Quick Revision: Remember These Points

• White light is a mixture of different visible colours.
• The visible spectrum is commonly represented by VIBGYOR.
• The primary colours of light are red, green and blue.
• Red + Green = Yellow.
• Green + Blue = Cyan.
• Blue + Red = Magenta.
• Red + Green + Blue, in suitable proportions, can produce white light.
• Objects appear coloured because they reflect some colours and absorb others.
• Additive colour mixing combines coloured lights.
• Colour subtraction involves the absorption or removal of selected colours.

Your Mission as a Young Scientist!

The next time you see a rainbow, a colourful flower or a mobile screen, remember that you are witnessing the fascinating science of light.

Keep asking questions. Keep experimenting. Keep discovering!

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