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THE MAGIC OF A PLANE MIRROR

Class 7 ICSE Physics | Light

Discover how mirrors form images, why your left hand appears on the right and how reflection works!

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1. Learning Objectives

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

• Explain how a plane mirror forms an image.
• Understand real and virtual images at a basic level.
• Describe the characteristics of an image formed by a plane mirror.
• Explain lateral inversion with everyday examples.
• Identify the uses of plane mirrors in daily life.
• Apply the laws of reflection to simple situations.
• Develop observation, reasoning and scientific thinking skills.

2. A Small Story: Riya and the Mysterious Mirror

One morning, Riya stood in front of her dressing mirror. She raised her right hand to adjust her hair. To her surprise, the person in the mirror seemed to raise the left hand!

Riya wondered, “Has the mirror changed my right hand into my left hand? Why does my image look exactly as tall as me? And why can I see the image behind the mirror even though there is nothing there?”

Her science teacher smiled and explained that a plane mirror does not perform magic. It reflects light according to the laws of reflection.

The image appears behind the mirror because our eyes trace the reflected rays backwards. The image is virtual, upright and the same size as the object.

Riya then discovered that the mirror was teaching her an important lesson: things that appear mysterious can often be understood through scientific observation.

Can you solve Riya's mirror mystery?

3. What Is a Plane Mirror?

A plane mirror is a mirror with a flat, smooth reflecting surface that forms clear images of objects.

Examples include:

• A dressing mirror.
• A bathroom mirror.
• A mirror used by a barber.
• A mirror used in a periscope.

Remember: A mirror reflects light. When reflected rays enter our eyes, we see the object or its image.

4. How Is an Image Formed by a Plane Mirror?

Imagine placing a small point of light in front of a plane mirror. Light rays travel from the point and strike the mirror. The mirror reflects these rays into different directions.

When the reflected rays reach our eyes, they appear to come from a point behind the mirror. Our brain interprets the rays as travelling in straight lines, so we perceive an image behind the mirror.

Important Terms

• Incident ray: A ray of light that strikes the reflecting surface.
• Reflected ray: A ray of light that bounces back from the reflecting surface.
• Point of incidence: The point where the incident ray strikes the mirror.
• Normal: An imaginary line perpendicular to the mirror at the point of incidence.
• Angle of incidence: The angle between the incident ray and the normal.
• Angle of reflection: The angle between the reflected ray and the normal.

Angle of incidence = Angle of reflection

Why is the image virtual? The reflected rays do not actually meet behind the mirror. Only their backward extensions meet there. Therefore, the image cannot be obtained on a screen and is called a virtual image.

5. Characteristics of an Image Formed by a Plane Mirror

1. Erect: The image appears upright.
2. Same size: The image has the same height as the object.
3. Virtual: The image cannot be formed on a screen.
4. Same distance: The image appears as far behind the mirror as the object is in front.
5. Lateral inversion: The left and right sides appear interchanged.

Object distance = Image distance

For example, if you stand 2 metres in front of a plane mirror, your image appears approximately 2 metres behind it. The distance between you and your image is therefore 4 metres.

6. Lateral Inversion: The Left-Right Mystery

Lateral inversion is the apparent interchange of the left and right sides of an object in the image formed by a plane mirror.

Examples from Daily Life

• Your right hand appears on the left side of your image.
• Your left hand appears on the right side of your image.
• The word AMBULANCE is written in reverse on the front of many ambulances so drivers can read it correctly in their rear-view mirrors.

Think carefully: A plane mirror does not physically swap your hands. The image shows a reversal in the direction perpendicular to the mirror's surface. This is why the effect appears to us as a left-right reversal.

7. Fun Activity: Become a Mirror Scientist!

Aim: To observe lateral inversion.

Materials Required

• A plane mirror.
• A sheet of paper.
• A pencil or marker.

Procedure

1. Stand in front of a plane mirror.
2. Raise your right hand and observe your image.
3. Write the word SCIENCE on a sheet of paper.
4. Hold the paper in front of the mirror.
5. Observe how the word appears in the mirror.

Observation

Your raised right hand appears on the left side of your image. The letters in the word appear laterally reversed.

Conclusion

The image formed by a plane mirror exhibits lateral inversion.

8. Uses of Plane Mirrors

• Looking glasses: Used for grooming and checking our appearance.
• Barbershops: Help customers see different views of their hair.
• Periscopes: Plane mirrors can redirect light so that objects outside the direct line of sight can be seen.
• Kaleidoscopes: Multiple reflections form beautiful patterns.
• Decoration: Mirrors can make rooms appear brighter and more spacious.

9. Thinking-Based MCQs

Try answering these questions before revealing the answers.

Q1. A student stands 3 metres in front of a plane mirror. How far behind the mirror does the image appear?

1. 1.5 metres
2. 3 metres
3. 6 metres
4. 9 metres

Correct answer: (b) 3 metres. The image formed by a plane mirror appears as far behind the mirror as the object is in front of it.

Q2. A girl is 150 cm tall. What is the height of her image in a plane mirror?

1. 75 cm
2. 100 cm
3. 150 cm
4. 300 cm

Correct answer: (c) 150 cm. A plane mirror forms an image of the same size as the object.

Q3. Which statement about the image formed by a plane mirror is correct?

1. It is always inverted.
2. It can always be obtained on a screen.
3. It is erect and virtual.
4. It is always smaller than the object.

Correct answer: (c) It is erect and virtual. The image is upright and cannot be obtained on a screen.

Q4. Why is AMBULANCE written in reverse on the front of many ambulances?

1. To make the word look attractive.
2. To allow drivers to read it correctly in their rear-view mirrors.
3. To make the ambulance travel faster.
4. To reduce the brightness of headlights.

Correct answer: (b). The reversed lettering appears in the correct order when seen in a rear-view mirror, helping drivers recognise the ambulance quickly.

Q5. A student moves 1 metre towards a plane mirror. What happens to the distance between the student and the image?

1. It increases by 2 metres.
2. It decreases by 2 metres.
3. It decreases by 1 metre.
4. It remains unchanged.

Correct answer: (b) It decreases by 2 metres. The student moves 1 metre closer to the mirror. The image also moves 1 metre closer to the mirror from behind it. Therefore, the total distance between the student and the image decreases by 2 metres.

Q6. A student says, “The image formed by a plane mirror is real because I can see it clearly.” Is the student correct?

1. Yes, every visible image is real.
2. No, the image is virtual.
3. Yes, because the mirror absorbs light.
4. No, because the image is always inverted.

Correct answer: (b) No, the image is virtual. Being visible does not mean an image is real. The reflected rays only appear to come from behind the mirror. They do not actually meet there.

Q7. If the angle of incidence is 35°, what is the angle of reflection?

1. 17.5°
2. 35°
3. 55°
4. 70°

Correct answer: (b) 35°. According to the law of reflection, the angle of incidence is equal to the angle of reflection.

10. Quick Quiz: Test Your Understanding

Attempt each question, then click its answer button.

1. Name the phenomenon responsible for the formation of an image in a plane mirror.

Answer: Reflection of light.

2. Is the image formed by a plane mirror real or virtual?

Answer: Virtual.

3. What is lateral inversion?

Answer: The apparent interchange of the left and right sides of an object in the image formed by a plane mirror.

4. State one characteristic of the image formed by a plane mirror.

Answer: Any one of the following:

• The image is erect.
• The image is virtual.
• The image is the same size as the object.
• The image appears as far behind the mirror as the object is in front.
• The image shows lateral inversion.

5. Name two devices that use plane mirrors.

Answer: A periscope and a kaleidoscope.

11. Moral Values: What Can a Mirror Teach Us?

Value 1: Self-awareness A mirror helps us observe our appearance. Similarly, self-reflection helps us recognise our strengths and identify areas where we can improve.

Value 2: Honesty Just as a mirror reflects light, we should try to be honest in our words and actions.

Value 3: Observation and patience Scientific discoveries require careful observation. We should investigate a question instead of accepting every assumption without evidence.

Value 4: Social responsibility The reverse lettering on an ambulance helps other drivers recognise an emergency vehicle. Understanding science can help us make practical decisions that benefit society.

Value-Based Question

A student notices that a classmate is confused about lateral inversion. Instead of laughing, the student demonstrates the effect using a mirror and explains it patiently. Which values does the student demonstrate?

Answer: Kindness, patience, cooperation, empathy and a willingness to share knowledge.

12. Did You Know?

The mirror challenge! Stand in front of a mirror and raise your right hand. Your image appears to raise the hand on the opposite side. Now write your name on a piece of paper and hold it in front of the mirror. Observe the reversed lettering. This is an example of lateral inversion. Challenge: Can you write your name so that it appears in the correct order when viewed in the mirror?

13. Homework and Creative Thinking

1. Draw a labelled diagram showing image formation by a plane mirror.
2. Write five characteristics of the image formed by a plane mirror.
3. Explain why the image formed by a plane mirror cannot be obtained on a screen.
4. Describe an activity to demonstrate lateral inversion.
5. Find three examples of plane mirrors used in daily life and explain their purposes.

Creative task: Prepare a small poster titled “The Science Behind My Mirror”. Include a labelled diagram, the characteristics of the image and one example of lateral inversion.

14. Summary: Remember These Five Points

1. A plane mirror has a flat reflecting surface.
2. The image formed is virtual and erect.
3. The image is the same size as the object.
4. The image appears as far behind the mirror as the object is in front of it.
5. The image shows lateral inversion.

Final thought: Science becomes interesting when we ask questions, observe carefully and discover the reasons behind everyday experiences. Keep exploring. Keep questioning. Keep learning!

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Class 7 ICSE Physics | Light | Plane Mirrors