Reflection of Light
How do mirrors show our faces? Why do vehicle headlights use special mirrors? Discover the science behind reflected light!
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Observe • Explore • Think • Discover
Welcome to SpeedUpScience!
1. Welcome to the World of Reflection!
Stand in front of a mirror. You can see your face, your clothes and even the objects behind you when they appear in the mirror.
But does the mirror produce your image by itself? No! The image is formed because light from you reflects from the mirror and reaches your eyes.
Reflection of light helps us understand mirrors, shiny surfaces, vehicle headlights and many everyday optical devices.
2. 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 the two laws of reflection.
- Understand the relationship between the angles of incidence and reflection.
- Distinguish between real and virtual images.
- Describe the image formed by a plane mirror.
- Identify the parts of a spherical mirror.
- Differentiate between concave and convex mirrors.
- Explain the basic ray rules for spherical mirrors.
- Describe practical applications of concave and convex mirrors.
3. What Is Reflection of Light?
Reflection of light is the bouncing back of light into the same medium after it strikes a surface.
For example, when light falls on a plane mirror, it is reflected from the mirror's surface.
Important Terms
Incident ray: The ray of light that falls on a reflecting surface.
Reflected ray: The ray of light that bounces back from the surface.
Normal: An imaginary line drawn perpendicular to the reflecting surface at the point of incidence.
Angle of incidence (i): The angle between the incident ray and the normal.
Angle of reflection (r): The angle between the reflected ray and the normal.
4. The Laws of Reflection
First Law
The angle of incidence is equal to the 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.
Worked Example
A ray of light strikes a plane mirror at an angle of incidence of 35°. Find the angle of reflection.
According to the law of reflection:
Angle of reflection = Angle of incidence
Answer: 35°
Think and Answer
If the angle of incidence is 50°, what is the angle between the incident ray and the reflected ray?
Reveal the answer
Angle between the rays = 50° + 50° = 100°.
5. Regular and Diffuse Reflection
Regular Reflection
When parallel rays of light fall on a smooth surface, they are reflected in an orderly manner and remain parallel.
Example: Reflection from a plane mirror or a smooth polished surface.
Diffuse Reflection
When parallel rays fall on a rough surface, they are reflected in different directions because the surface has irregularities.
Example: Reflection from a wall, paper or an uneven road.
A Surprising Fact!
Diffuse reflection does not mean that the laws of reflection stop working. Each ray still obeys the laws of reflection at the point where it strikes the surface.
6. Image Formed by a Plane Mirror
A plane mirror is a flat reflecting surface.
The image formed by a plane mirror has the following characteristics:
- It is virtual.
- It is 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 of it, when the object and image positions are measured perpendicular to the mirror.
What Is Lateral Inversion?
Lateral inversion is the apparent interchange of the left and right sides of an object in its mirror image.
For example, when you raise your right hand, the image appears to raise the hand on the opposite side.
Real and Virtual Images
| Real Image | Virtual Image |
|---|---|
| Formed where reflected rays actually meet. | Formed where reflected rays appear to meet when extended backwards. |
| Can be obtained on a screen. | Cannot be obtained on a screen. |
| For a single mirror, a real image is generally inverted. | A plane mirror forms a virtual and erect image. |
7. Introduction to Spherical Mirrors
A spherical mirror is a part of a hollow sphere whose surface reflects light.
There are two types of spherical mirrors:
Concave Mirror
A concave mirror has its reflecting surface on the inner, curved side of the spherical surface.
It can converge parallel rays of light towards a principal focus.
Convex Mirror
A convex mirror has its reflecting surface on the outer, bulging side of the spherical surface.
It diverges parallel rays of light. The reflected rays appear to come from a focus behind the mirror.
Concave mirror: reflecting surface curves inward.
Convex mirror: reflecting surface bulges outward.
8. Important Parts of a Spherical Mirror
Pole (P): The geometric centre of the reflecting surface of the mirror.
Centre of curvature (C): The centre of the sphere of which the mirror is a part.
Radius of curvature (R): The radius of that sphere.
Principal axis: The straight line joining the pole and the centre of curvature.
Principal focus (F): For a concave mirror, the point on the principal axis where rays parallel to the axis converge after reflection. For a convex mirror, it is the point from which such reflected rays appear to diverge.
Focal length (f): The distance between the pole and the principal focus.
For a spherical mirror with a small aperture, the focal length is approximately half its radius of curvature.
9. The Three Important Ray Rules
Ray Rule 1: Parallel Ray
A ray parallel to the principal axis is reflected through the principal focus of a concave mirror.
In a convex mirror, the reflected ray diverges and appears to come from the principal focus behind the mirror.
Ray Rule 2: Ray Through the Focus
A ray passing through the principal focus of a concave mirror is reflected parallel to the principal axis.
For a convex mirror, a ray directed towards its virtual focus behind the mirror is reflected parallel to the principal axis.
Ray Rule 3: Ray Through the Centre of Curvature
A ray directed through the centre of curvature of a spherical mirror strikes the mirror normally and is reflected back along the same path.
10. Image Formation by Concave and Convex Mirrors
Concave Mirror
A concave mirror can form different types of images depending on the position of the object.
| Object Position | Image Characteristics |
|---|---|
| Beyond the centre of curvature | Between C and F; real, inverted and diminished. |
| At the centre of curvature | At C; real, inverted and the same size. |
| Between C and F | Beyond C; real, inverted and enlarged. |
| At F | Image is formed at infinity in the ideal ray model. |
| Between F and P | Behind the mirror; virtual, erect and enlarged. |
Convex Mirror
A convex mirror always forms an image of a real object that is:
- Virtual.
- Erect.
- Diminished.
- Located behind the mirror, between the pole and the principal focus.
11. Mirrors in Everyday Life
1. Rear-View Mirrors
Convex mirrors are used in vehicles because they provide a wider field of view and form erect, diminished images of objects behind the vehicle.
2. Vehicle Headlights and Torches
Concave mirrors can be used as reflectors. When a light source is placed near the focus, the reflected light can form a strong, nearly parallel beam.
3. Shaving and Makeup Mirrors
A concave mirror can produce an enlarged, erect, virtual image when the face is placed between the mirror's pole and principal focus.
4. Streetlights
Convex reflectors can spread light over a wider area in some lighting arrangements.
5. Security Mirrors
Convex mirrors are used in shops and corridors to help people observe a wider area.
12. Story Time: The Mystery of the Two Mirrors
Meet Rohan, the Young Investigator!
One Sunday, Rohan visited a science exhibition with his sister, Meera.
At the first stall, they looked into a plane mirror. They saw erect images that were the same size as themselves.
At the next stall, Rohan looked into a concave mirror. When he moved closer to it, his face appeared enlarged and erect. When he moved farther away, the image became inverted.
Meera then asked him to look into a convex mirror. No matter where he stood in front of it, his image remained erect and smaller.
Rohan realised that the shape of a mirror changes the way reflected rays meet or appear to meet.
He wrote in his notebook:
"A mirror is not just a shiny surface. Its shape determines the kind of image it forms."
Story-Based Questions
- Which mirror forms an image of the same size as the object?
- Why can a concave mirror produce an enlarged, erect image when the object is close to it?
- Why is a convex mirror useful as a rear-view mirror?
Reveal the answers
1. A plane mirror.
2. When the object is between the pole and focus, the reflected rays diverge and appear to meet behind the mirror, forming a virtual, erect and enlarged image.
3. It provides a wider field of view and forms erect, diminished images.
13. Become a Reflection Scientist!
Activity A: Verify the Law of Reflection
Materials: A plane mirror, paper, ruler, pencil and protractor.
- Place the mirror upright on a sheet of paper.
- Draw a normal at a point on the mirror.
- Draw an incident ray towards the point.
- Draw the reflected ray.
- Measure the angle of incidence and angle of reflection using a protractor.
- Compare the two angles.
Expected observation: The angle of incidence is equal to the angle of reflection.
Activity B: Explore Different Mirrors
Observe your image in a plane mirror, a concave mirror and a convex mirror, where available.
Record whether the image is erect or inverted, enlarged or diminished.
For a concave mirror, move closer to and farther from the mirror to observe how the image changes.
Use mirrors safely. Never direct reflected sunlight into anyone's eyes.
14. Thinking Beyond the Textbook
Question 1: The Changing Angle
A ray of light strikes a plane mirror at an angle of incidence of 30°. If the angle of incidence increases to 45°, what happens to the angle of reflection?
Question 2: The Wider View
Why is a convex mirror preferred over a plane mirror for vehicle rear-view mirrors?
Question 3: The Enlarged Face
Which mirror can be used to see an enlarged, erect image of your face?
Question 4: The Hidden Image
Can you obtain the virtual image formed by a plane mirror on a screen? Explain your answer.
Question 5: The Reflector
Why is a concave mirror useful as a reflector in a torch?
Reveal the answers
1. The angle of reflection becomes 45°.
2. A convex mirror provides a wider field of view and forms erect, diminished images.
3. A concave mirror, when the face is placed between its pole and principal focus.
4. No. A virtual image is formed where reflected rays appear to meet, not where they actually meet.
5. When the light source is placed near the principal focus, the reflected rays can form a strong, nearly parallel beam.
15. Values We Learn Through Science
Curiosity
Asking why different mirrors form different images encourages us to investigate the world.
Observation
Careful observation helps us identify patterns and understand scientific laws.
Accuracy
Measuring angles carefully teaches us the importance of precision.
Responsibility
Mirrors help us see our surroundings, but we must use them responsibly and remain alert to areas they cannot show.
Safety
Always handle glass mirrors carefully and never use a mirror to direct sunlight into someone's eyes.
16. Interactive MCQ Challenge
Test your understanding with 20 questions on the laws of reflection, plane mirrors, spherical mirrors, image formation and applications.
Each correct answer carries 1 mark. Maximum score: 20.
Your Physics Report Card
17. Quick Revision
- Reflection is the bouncing back of light from a surface.
- The angle of incidence equals the angle of reflection.
- The incident ray, reflected ray and normal lie in the same plane.
- A plane mirror forms a virtual, erect image of the same size.
- A concave mirror can form real or virtual images depending on object position.
- A convex mirror always forms a virtual, erect and diminished image of a real object.
- Focal length is approximately half the radius of curvature for a spherical mirror with a small aperture.
- Convex mirrors provide a wide field of view.
- Concave mirrors are used as reflectors in torches and headlights.
- Regular reflection occurs from smooth surfaces, while diffuse reflection occurs from rough surfaces.
Your Mission as a Young Scientist!
The next time you look into a mirror, remember that a simple reflection reveals important scientific laws.
Observe carefully. Think clearly. Learn continuously.
SpeedUpScience.com — Making Physics simple, interesting and meaningful.
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