REFLECTION OF LIGHT
Why can you see your face in a mirror but not clearly on a wall? Why does a spoon sometimes show a distorted image? Discover the fascinating science behind reflection!
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1. Learning Objectives
By the end of this lesson, you will be able to:
- Define reflection of light.
- Identify the incident ray, reflected ray and normal.
- State and apply the two laws of reflection.
- Differentiate between regular and irregular reflection.
- Describe the characteristics of an image formed by a plane mirror.
- Explain lateral inversion using everyday examples.
- Apply the concepts of reflection to real-life situations.
2. An Interesting Story: The Mirror Mystery!
One morning, Aarav stood in front of his mirror and waved his right hand. To his surprise, the person in the mirror seemed to wave the left hand!
"Is my mirror playing tricks on me?" he wondered.
His sister Meera laughed and said, "No! Your mirror is showing an image. The apparent left-right reversal is called lateral inversion."
Later, Aarav went to the playground. He noticed his reflection in a smooth window but could not see a clear image of himself on the rough wall.
His science teacher explained that a smooth surface produces regular reflection, while a rough surface scatters reflected light in different directions.
Aarav realised that reflection was not magic. It was science happening right before his eyes!
3. What Is Reflection of Light?
When light falls on a surface and returns to the same medium, the phenomenon is called reflection of light.
A mirror reflects light that falls on it. When the reflected light enters our eyes, we can see the image of an object.
Important Terms
Incident ray: The ray of light that falls on a surface.
Reflected ray: The ray of light that returns from the surface after reflection.
Point of incidence: The point where the incident ray strikes the reflecting surface.
Normal: An imaginary line drawn perpendicular to the reflecting surface 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.
4. The Two Laws of Reflection
First Law
The angle of incidence is equal to the angle of reflection.
Angle of incidence = Angle of reflection
Second Law
The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
Let's Solve a Quick Numerical!
A ray of light strikes a plane mirror at an angle of incidence of 40°. What is the angle of reflection?
Click to reveal the answer
According to the first law of reflection:
Angle of reflection = Angle of incidence
Therefore, the angle of reflection = 40°.
Remember: Both angles are measured from the normal, not from the mirror surface.
5. Regular and Irregular Reflection
A. Regular Reflection
When parallel rays of light fall on a smooth surface and are reflected in an orderly manner, the reflection is called regular reflection.
Examples include:
- A plane mirror.
- A polished metal surface.
- A calm, smooth water surface.
A smooth mirror can form a clear image because of regular reflection.
B. Irregular Reflection
When parallel rays of light fall on a rough surface and are reflected in different directions, the reflection is called irregular reflection or diffuse reflection.
Examples include:
- A rough wall.
- A sheet of paper.
- An uneven road surface.
We can see a book because light falling on it is reflected in different directions, allowing some of the reflected light to reach our eyes.
6. Image Formed by a Plane Mirror
A plane mirror is a mirror with a flat reflecting surface. It forms an image with the following characteristics:
- The image is virtual.
- The image is erect.
- The image is the same size as the object.
- The image appears as far behind the mirror as the object is in front of it.
- The image is laterally inverted.
What Is Lateral Inversion?
Lateral inversion is the apparent reversal of the left and right sides of an object in its image formed by a plane mirror.
For example, when you raise your right hand in front of a mirror, the image appears to raise its left hand.
This is why the word AMBULANCE is often written in a laterally reversed form on the front of emergency vehicles so that drivers can read it correctly in their rear-view mirrors.
7. Case Study: The Mystery of the Two Reflections
Ananya was getting ready for school. She stood in front of a plane mirror to comb her hair. She noticed that her image appeared to be the same size as herself and seemed to be standing behind the mirror.
Later, in her science laboratory, her teacher placed a torch in front of a plane mirror. Light from the torch fell on the mirror and was reflected in a different direction.
The teacher then demonstrated reflection using a smooth plane mirror and a rough sheet of paper. The mirror produced a clear image, whereas the paper scattered reflected light in different directions.
Ananya was fascinated to learn that reflection helps us see objects and is useful in many everyday situations.
Think and Answer
(i) What is meant by reflection of light?
(ii) Why does Ananya see her image in a plane mirror?
(iii) State any two characteristics of the image formed by a plane mirror.
(iv) Differentiate between regular and irregular reflection.
(v) Why does a plane mirror form a clear image, whereas a rough sheet of paper does not?
(vi) State the two laws of reflection.
(vii) If the angle of incidence is 35°, calculate the angle of reflection.
(viii) Why can we see a book even though it does not produce its own light?
Click here to check your answers
(i) Reflection is the phenomenon in which light falling on a surface returns to the same medium.
(ii) Light reflected from her face by the mirror enters her eyes, allowing her to see her image.
(iii) Any two: The image is virtual and erect; it is the same size as the object; it is laterally inverted; it appears as far behind the mirror as the object is in front.
(iv) Regular reflection occurs at a smooth surface and produces an orderly reflection of parallel rays. Irregular reflection occurs at a rough surface and scatters reflected rays in different directions.
(v) A smooth mirror produces regular reflection and a clear image. A rough sheet of paper produces diffuse reflection and does not form a clear image.
(vi) First, the angle of incidence equals the angle of reflection. Second, the incident ray, reflected ray and normal at the point of incidence lie in the same plane.
(vii) The angle of reflection is 35°.
(viii) The book reflects light from another source into our eyes.
8. Explore Science: A Simple Mirror Activity
Can You Verify the First Law of Reflection?
Materials required:
- A plane mirror.
- A sheet of white paper.
- A ruler and protractor.
- A pencil.
- A torch, preferably with a narrow beam.
Procedure
- Place the mirror upright on the paper.
- Mark the point where the light ray will strike the mirror.
- Draw a normal perpendicular to the mirror at that point.
- Direct a narrow light beam towards the mirror.
- Observe the direction of the reflected beam.
- Measure the angle of incidence and the angle of reflection from the normal.
Observation: The two angles should be approximately equal.
Conclusion: The experiment supports the first law of reflection.
Safety: Never direct a laser beam into anyone's eyes. Use a torch carefully and follow your teacher's instructions.
9. Where Do We Use Reflection in Everyday Life?
Plane Mirrors
Used in dressing mirrors, periscopes and decorative applications.
Rear-View Mirrors
Mirrors help drivers observe traffic behind their vehicles. Convex mirrors are commonly used because they provide a wider field of view.
Periscopes
Plane mirrors or prisms can redirect light, allowing a person to see over or around an obstacle.
Reflectors
Reflective materials on bicycles, vehicles and road signs help make them more visible when illuminated.
Solar Cookers
Reflectors can direct more sunlight towards a cooking vessel, helping it absorb solar energy.
10. Thinking-Based MCQs
Read each question carefully. Select one option for every question. Click Submit Quiz to check your answers, score and explanations.
11. Your Performance Report Card
- 9–10: Excellent! You are a Reflection Expert.
- 7–8: Very good! Your concepts are strong.
- 5–6: Good effort! Revise the laws of reflection.
- 0–4: Keep practising. Learn from the explanations and try again.
Your score, percentage, grade and feedback will appear after you submit the quiz.
12. Quick Revision
- Reflection is the return of light to the same medium after striking a surface.
- The angle of incidence equals the angle of reflection.
- The incident ray, reflected ray and normal lie in the same plane.
- Regular reflection occurs at smooth surfaces.
- Irregular reflection occurs at rough surfaces.
- A plane mirror forms a virtual, erect image of the same size as the object.
- The image in a plane mirror is laterally inverted.
- We see non-luminous objects because they reflect light into our eyes.
13. Moral Values: Reflect Goodness!
A mirror reflects the light that falls on it. In the same way, our actions influence the people around us.
When we show kindness, honesty and respect, we encourage others to practise these values too.
Just as we need the correct direction of light to understand reflection, we need the right knowledge and values to make good decisions.
Today's message: Reflect kindness, spread knowledge and let your actions inspire others!
14. Challenge of the Day
Become a Science Detective!
Answer these questions in your science notebook:
- Why can you see your face in a mirror but not clearly on a rough wall?
- Why is the word AMBULANCE written in reverse on the front of an emergency vehicle?
- Can irregular reflection obey the laws of reflection? Explain.
- Design a simple periscope using two plane mirrors. Explain how reflection helps you see an object that is not directly in your line of sight.
Discuss your answers with your classmates and share your observations with your teacher.
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