Light: Mirrors and Lenses – Important Questions and Answers
Premium CBSE Class 8 Science revision post based on the chapter Light: Mirrors and Lenses. Includes spherical mirrors, laws of reflection, concave and convex mirrors, lenses, converging and diverging light beams, daily-life uses, case studies, HOTS, diagram-based questions, FAQs, and an interactive quiz.
Short Introduction
Light helps us see objects around us. In earlier classes, students studied plane mirrors, shadows, and reflections. This chapter extends the idea to curved mirrors and lenses. Concave and convex mirrors form different kinds of images. Lenses also change the way objects appear because they allow light to pass through curved surfaces. The chapter also explains the laws of reflection and important uses of mirrors and lenses in vehicles, torches, dentists’ mirrors, spectacles, cameras, microscopes, telescopes, and solar devices.
Chapter Overview
Spherical Mirrors
Spherical mirrors have reflecting surfaces shaped like a part of a sphere. They may be concave or convex.
Concave Mirror
A concave mirror has a reflecting surface curved inwards. It can form enlarged, diminished, erect, or inverted images depending on object distance.
Convex Mirror
A convex mirror has a reflecting surface curved outwards. It always forms an erect and diminished image.
Laws of Reflection
The angle of incidence equals the angle of reflection, and the incident ray, normal, and reflected ray lie in the same plane.
Convex and Concave Lenses
A convex lens is thicker in the middle and converges light. A concave lens is thicker at the edges and diverges light.
Uses of Mirrors and Lenses
Mirrors and lenses are used in vehicles, torches, dental instruments, eyeglasses, cameras, telescopes, microscopes, and solar cookers.
Important Laws and Rules
This is the first law of reflection.
This is the second law of reflection.
They can concentrate light into a small bright spot.
They spread out parallel beams of light.
Important Keywords with Meanings
| Keyword | Meaning |
|---|---|
| Light ray | A straight line with an arrow used to show the path of light. |
| Reflection | The bouncing back of light from a reflecting surface. |
| Plane mirror | A flat mirror that forms an erect image of the same size as the object. |
| Spherical mirror | A mirror whose reflecting surface is spherical. |
| Concave mirror | A spherical mirror whose reflecting surface curves inwards. |
| Convex mirror | A spherical mirror whose reflecting surface curves outwards. |
| Reflecting surface | The polished surface of a mirror from which light reflects. |
| Non-reflecting surface | The shaded or coated side of a mirror that does not reflect light. |
| Image | The optical appearance of an object formed by a mirror or lens. |
| Erect image | An image that appears upright. |
| Inverted image | An image that appears upside down. |
| Enlarged image | An image bigger than the object. |
| Diminished image | An image smaller than the object. |
| Lateral inversion | Sideways reversal of an image, as seen in mirrors. |
| Incident ray | The ray of light that falls on a mirror. |
| Reflected ray | The ray of light that comes back from a mirror after reflection. |
| Normal | A line drawn perpendicular to the mirror 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. |
| Converge | To come closer or meet at a small region. |
| Diverge | To spread out. |
| Lens | A transparent material, usually glass or plastic, with curved surfaces. |
| Convex lens | A lens thicker at the middle than at the edges. |
| Concave lens | A lens thicker at the edges than at the middle. |
| Solar concentrator | A device that concentrates sunlight into a small area using mirrors or lenses. |
Very Short Answer Questions
1. What is a spherical mirror?
Answer: A spherical mirror is a mirror whose reflecting surface is spherical.
2. What is a concave mirror?
Answer: A concave mirror is a spherical mirror whose reflecting surface curves inwards.
3. What is a convex mirror?
Answer: A convex mirror is a spherical mirror whose reflecting surface curves outwards.
4. Which side of a spoon behaves like a concave mirror?
Answer: The inner curved side of a shiny spoon behaves like a concave mirror.
5. Which side of a spoon behaves like a convex mirror?
Answer: The outer bulging side of a shiny spoon behaves like a convex mirror.
6. What type of image is always formed by a convex mirror?
Answer: A convex mirror always forms an erect and diminished image.
7. Which mirror is used in vehicle side-view mirrors?
Answer: A convex mirror is used in vehicle side-view mirrors.
8. Which mirror is used by dentists to see enlarged teeth?
Answer: A concave mirror is used by dentists.
9. What is an incident ray?
Answer: The ray of light that falls on a mirror is called the incident ray.
10. What is a reflected ray?
Answer: The ray of light that comes back from a mirror after reflection is called the reflected ray.
11. What is normal?
Answer: Normal is a line drawn at 90° to the reflecting surface at the point of incidence.
12. What is angle of incidence?
Answer: The angle between the incident ray and the normal is called the angle of incidence.
13. What is angle of reflection?
Answer: The angle between the reflected ray and the normal is called the angle of reflection.
14. State the first law of reflection.
Answer: The angle of incidence is equal to the angle of reflection.
15. State the second law of reflection.
Answer: The incident ray, the normal, and the reflected ray all lie in the same plane.
16. Which mirror converges parallel beams of light?
Answer: A concave mirror converges parallel beams of light.
17. Which mirror diverges parallel beams of light?
Answer: A convex mirror diverges parallel beams of light.
18. What is a lens?
Answer: A lens is a transparent material, usually glass or plastic, with curved surfaces.
19. What is a convex lens?
Answer: A convex lens is thicker at the middle than at the edges.
20. What is a concave lens?
Answer: A concave lens is thicker at the edges than at the middle.
21. Which lens is called a converging lens?
Answer: A convex lens is called a converging lens.
22. Which lens is called a diverging lens?
Answer: A concave lens is called a diverging lens.
23. Which lens is used in a magnifying glass?
Answer: A convex lens is used in a magnifying glass.
24. Name two devices that use lenses.
Answer: Eyeglasses and cameras use lenses.
Short Answer Questions
1. How is a spherical mirror different from a plane mirror?
Answer: A plane mirror has a flat reflecting surface and forms an erect image of the same size as the object. A spherical mirror has a curved reflecting surface and can form images of different sizes depending on the type of mirror and distance of the object.
2. How can you identify a concave and convex mirror by side view?
Answer: A concave mirror has its reflecting surface curved inwards, while a convex mirror has its reflecting surface curved outwards. Viewing the mirror from the side helps identify whether the reflecting surface bends inward or outward.
3. What image is formed by a concave mirror when the object is close to it?
Answer: When an object is placed close to a concave mirror, the image is erect and enlarged.
4. What happens when an object is moved farther away from a concave mirror?
Answer: When the object is moved farther away from a concave mirror, the image becomes inverted. It may first be enlarged and then becomes smaller as the distance increases.
5. Why are convex mirrors used as side-view mirrors in vehicles?
Answer: Convex mirrors form erect and diminished images and provide a wider area of view. This helps drivers see more traffic behind them.
6. Why is the warning “Objects in mirror are closer than they appear” written on vehicle mirrors?
Answer: Vehicle side-view mirrors are convex mirrors. They form diminished images, so vehicles behind appear smaller and farther away than they actually are.
7. Why are concave mirrors used in torches and vehicle headlights?
Answer: Concave mirrors can converge and direct light into a strong beam. Therefore, they are used as reflectors in torches and headlights.
8. What are the two laws of reflection?
Answer: The two laws of reflection are: (1) The angle of incidence is equal to the angle of reflection. (2) The incident ray, normal, and reflected ray all lie in the same plane.
9. What happens when light falls along the normal on a mirror?
Answer: When light falls along the normal, the angle of incidence is 0° and the angle of reflection is also 0°. The light retraces its path after reflection.
10. How does a plane mirror reflect multiple parallel beams of light?
Answer: When multiple parallel beams of light fall on a plane mirror, the reflected beams also remain parallel.
11. How does a concave mirror reflect multiple parallel beams of light?
Answer: A concave mirror makes multiple parallel reflected beams come closer. Thus, it converges light.
12. How does a convex mirror reflect multiple parallel beams of light?
Answer: A convex mirror makes multiple parallel reflected beams spread out. Thus, it diverges light.
13. How is a lens different from a mirror?
Answer: A mirror reflects light and we see images in it. A lens allows light to pass through it and we see objects through it.
14. What image is formed by a concave lens?
Answer: A concave lens always forms an erect and diminished image of the object.
15. Why can a convex lens burn paper in sunlight?
Answer: A convex lens converges sunlight into a small bright spot. The concentrated sunlight produces enough heat to burn paper.
Long Answer Questions
1. Explain spherical mirrors and their two types.
Answer: Spherical mirrors are mirrors whose reflecting surfaces are spherical, like a part of a hollow sphere. They are of two main types:
- Concave mirror: Its reflecting surface curves inwards. It can form enlarged, diminished, erect, or inverted images depending on object distance.
- Convex mirror: Its reflecting surface curves outwards. It always forms an erect and diminished image.
- The non-reflecting surface of a spherical mirror is shown as shaded in diagrams.
2. Describe the spoon activity used to understand spherical mirrors.
Answer: A shiny metallic spoon acts like a curved mirror. When we look at the inner curved side, it behaves like a concave mirror. When we look at the outer bulging side, it behaves like a convex mirror. The image changes when the spoon is moved closer or farther. This shows that curved reflecting surfaces form images different from plane mirrors.
3. Compare images formed by plane, concave and convex mirrors.
| Mirror | Image Characteristics |
|---|---|
| Plane mirror | Always erect and same size as object; shows lateral inversion. |
| Concave mirror | Can be enlarged, diminished, same size, erect or inverted depending on object distance. |
| Convex mirror | Always erect and diminished; provides a wider field of view. |
4. Explain the uses of concave mirrors.
Answer: Concave mirrors are used in many places because they can enlarge nearby objects and converge light.
- Dentists use concave mirrors to get an enlarged view of teeth.
- Torches and headlights use concave reflectors to direct light into a strong beam.
- Reflecting telescopes use large concave mirrors to collect light.
- Solar concentrators use concave mirrors to focus sunlight and produce heat.
5. Explain the uses of convex mirrors.
Answer: Convex mirrors always form erect and diminished images and provide a wider area of view. Therefore, they are used:
- As side-view mirrors in vehicles.
- At road intersections and sharp bends to prevent collisions.
- In big stores as surveillance mirrors to monitor a large area.
6. Explain the laws of reflection with ray terms.
Answer: The ray of light that falls on a mirror is called the incident ray. The ray that comes back from the mirror is called the reflected ray. A normal is drawn perpendicular to the mirror at the point of incidence. The angle between the incident ray and normal is angle of incidence, and the angle between the reflected ray and normal is angle of reflection.
- First law: Angle of incidence = Angle of reflection.
- Second law: Incident ray, normal, and reflected ray all lie in the same plane.
7. How do plane, concave and convex mirrors reflect parallel beams of light?
Answer: When multiple parallel beams fall on a plane mirror, the reflected beams remain parallel. When they fall on a concave mirror, the reflected beams come closer and converge. When they fall on a convex mirror, the reflected beams spread out and diverge. Each ray still follows the laws of reflection, but the curved surface changes the direction of reflected rays.
8. What is a lens? Explain convex and concave lenses.
Answer: A lens is a piece of transparent material, usually glass or plastic, with curved surfaces. Unlike mirrors, lenses allow light to pass through them.
- Convex lens: Thicker at the middle and thinner at the edges. It converges light.
- Concave lens: Thicker at the edges and thinner at the middle. It diverges light.
9. Compare convex lens and concave lens.
| Convex Lens | Concave Lens |
|---|---|
| Thicker at the middle. | Thicker at the edges. |
| Converges light beams. | Diverges light beams. |
| Also called converging lens. | Also called diverging lens. |
| Can form enlarged, diminished, erect or inverted images depending on object distance. | Always forms erect and diminished image. |
| Used in magnifying glass, cameras, eye, microscopes and telescopes. | Used in some eyeglasses and optical devices. |
10. Why should we follow safety precautions while focusing sunlight with mirrors or lenses?
Answer: Concave mirrors and convex lenses can concentrate sunlight into a small bright spot. This can produce enough heat to burn paper. Looking directly at the Sun or reflected sunlight can seriously damage the eyes. Therefore, such activities should be done only under adult or teacher supervision, and reflected sunlight must never be directed toward anyone’s face or eyes.
Case-Study Based Questions
Case Study 1: Science Centre Mirrors
Meena visited a science centre and saw unusual curved mirrors. In one mirror her face appeared very large, while in another she saw a tiny version of herself.
Q1. What kind of mirrors did Meena see?
Answer: She saw spherical mirrors.
Q2. Which mirror can make a nearby face appear enlarged?
Answer: A concave mirror can make a nearby face appear enlarged.
Q3. Which mirror always gives a diminished image?
Answer: A convex mirror always gives a diminished image.
Case Study 2: Vehicle Side-View Mirror
A driver notices that the side-view mirror shows a wide area of the road behind, but vehicles appear smaller.
Q1. Which mirror is used as a side-view mirror?
Answer: A convex mirror is used.
Q2. Why does it show a wide area?
Answer: A convex mirror has a curved outward surface and provides a wider field of view.
Q3. Why do vehicles appear farther than they are?
Answer: Convex mirrors form diminished images, so objects appear smaller and farther away.
Case Study 3: Reflection Experiment
A student shines a narrow beam of light on a plane mirror. The angle between the incident ray and normal is 35°.
Q1. What is the angle of incidence?
Answer: The angle of incidence is 35°.
Q2. What is the angle of reflection?
Answer: The angle of reflection is also 35°.
Q3. Which law explains this?
Answer: The first law of reflection explains this.
Case Study 4: Burning Paper with a Convex Lens
A student focuses sunlight on a paper using a convex lens. A sharp bright spot appears and the paper starts producing smoke.
Q1. Why does a bright spot form?
Answer: The convex lens converges sunlight into a small area.
Q2. Why does the paper start burning?
Answer: Concentrated sunlight produces enough heat to ignite the paper.
Q3. What safety rule must be followed?
Answer: Do not look at the Sun directly or through the lens.
Case Study 5: Water Drop Lens
A student places a small water drop on an oiled transparent plastic strip and looks at printed letters through it. The letters appear larger.
Q1. Why do the letters look larger?
Answer: The curved water drop behaves like a simple convex lens and magnifies the letters.
Q2. What is a lens?
Answer: A lens is a transparent material with curved surfaces.
Q3. Name one device that uses a convex lens.
Answer: A magnifying glass uses a convex lens.
Assertion-Reason Questions
Choose the correct option:
A. Both Assertion and Reason are true and Reason is the correct explanation.
B. Both Assertion and Reason are true but Reason is not the correct explanation.
C. Assertion is true but Reason is false.
D. Assertion is false but Reason is true.
1. Assertion: A concave mirror has a reflecting surface curved inwards. Reason: The inner side of a shiny spoon behaves like a concave mirror.
Answer: A
2. Assertion: A convex mirror always forms an erect and diminished image. Reason: Convex mirrors are used as vehicle side-view mirrors.
Answer: B
3. Assertion: Convex mirrors are preferred for observing traffic behind vehicles. Reason: Convex mirrors provide a significantly larger view area than plane mirrors.
Answer: A
4. Assertion: A concave mirror is used by dentists. Reason: A concave mirror can provide an enlarged image when held close to teeth.
Answer: A
5. Assertion: Angle of incidence is equal to angle of reflection. Reason: This is one of the laws of reflection.
Answer: A
6. Assertion: The incident ray, normal and reflected ray lie in different planes. Reason: Bending the paper makes the reflected beam disappear from the new plane.
Answer: D
7. Assertion: A concave mirror converges parallel beams of light. Reason: A convex mirror diverges parallel beams of light.
Answer: B
8. Assertion: A convex lens is called a converging lens. Reason: It converges parallel beams of light.
Answer: A
9. Assertion: A concave lens always forms an erect and diminished image. Reason: A concave lens diverges light beams.
Answer: A
10. Assertion: It is safe to look directly at the Sun through a convex lens. Reason: A convex lens converges sunlight.
Answer: D
Competency-Based and Critical Thinking / HOTS Questions
1. A ray makes an angle of incidence of 40° with the normal. What is the angle of reflection?
Answer: The angle of reflection is 40°, because angle of incidence is equal to angle of reflection.
2. A ray makes an angle of incidence of 40° with the normal. What angle does the reflected ray make with the mirror?
Answer: The reflected ray makes 50° with the mirror because it makes 40° with the normal, and the normal is perpendicular to the mirror. So, 90° − 40° = 50°.
3. If light falls along the normal on a plane mirror, what are the angle of incidence and reflection?
Answer: Both angles are 0°. The light ray reflects back along the same path.
4. Why can a convex mirror show more area than a plane mirror?
Answer: A convex mirror has an outward curved reflecting surface. It diverges reflected rays and forms smaller images, allowing a wider field of view.
5. Why is a concave mirror not used as a general vehicle side-view mirror?
Answer: A concave mirror can form inverted or enlarged images depending on object distance. This can confuse drivers. A convex mirror gives a consistent erect and wider view.
6. Why can the same concave mirror form both erect and inverted images?
Answer: The image formed by a concave mirror depends on the distance of the object from the mirror. When the object is close, the image is erect and enlarged. When it is farther away, the image becomes inverted.
7. Why does a flat transparent glass plate not behave like a lens?
Answer: A flat glass plate has plane surfaces and does not significantly converge or diverge light beams. A lens has curved surfaces that change the direction of light.
8. Why does a water drop act like a simple lens?
Answer: A water drop has a curved surface and is transparent. It bends light passing through it and can make letters below it appear enlarged.
9. Why do solar cookers use mirrors?
Answer: Mirrors in solar cookers converge sunlight into a smaller area, producing heat that can be used for cooking and reducing dependence on electricity or fuels.
10. A pencil appears bent when seen through a glass tumbler filled with water. Why?
Answer: The curved water-filled glass acts like a lens and bends light passing through it. Due to this change in light path, the pencil appears changed or bent.
Diagram-Based Questions
Diagram 1: Concave and Convex Mirror Shape
Concave Mirror: Reflecting surface curves inwards
Convex Mirror: Reflecting surface curves outwards
Question: Which mirror is used by dentists?
Answer: A concave mirror is used by dentists.
Diagram 2: Laws of Reflection
Question: If angle of incidence is 30°, what is angle of reflection?
Answer: Angle of reflection is 30°.
Diagram 3: Reflection of Parallel Rays
Question: Which mirror converges parallel rays?
Answer: A concave mirror converges parallel rays.
Diagram 4: Convex and Concave Lens
Convex Lens: Thicker in middle
Concave Lens: Thicker at edges
Question: Which lens is used in a magnifying glass?
Answer: A convex lens is used in a magnifying glass.
Diagram 5: Light Beam Through Lenses
Question: Which lens is called a diverging lens?
Answer: A concave lens is called a diverging lens.
Quick Revision Box
- Spherical mirrors have curved reflecting surfaces.
- Concave mirror curves inwards.
- Convex mirror curves outwards.
- Concave mirror can form enlarged, diminished, erect or inverted images.
- Convex mirror always forms erect and diminished image.
- Convex mirrors are used as vehicle side-view mirrors.
- Concave mirrors are used in torches, headlights and dental mirrors.
- Incident ray falls on the mirror.
- Reflected ray comes back from the mirror.
- Normal is perpendicular to the mirror at the point of incidence.
- Angle of incidence equals angle of reflection.
- Incident ray, normal and reflected ray lie in the same plane.
- Concave mirror converges light.
- Convex mirror diverges light.
- Convex lens is thicker in the middle and converges light.
- Concave lens is thicker at the edges and diverges light.
- A convex lens is used in magnifying glass.
- Our eye also has a convex lens.
Important Exam Tips
Interactive Quiz
Choose the correct answer and click submit.
Frequently Asked Questions
1. What is a spherical mirror?
A spherical mirror is a mirror whose reflecting surface is spherical.
2. What is the difference between concave and convex mirrors?
A concave mirror curves inwards, while a convex mirror curves outwards.
3. Which mirror is used in vehicle side-view mirrors?
A convex mirror is used because it gives an erect, diminished image and a wider field of view.
4. Which mirror is used by dentists?
A concave mirror is used by dentists because it gives an enlarged image when held close to teeth.
5. What are the laws of reflection?
The angle of incidence equals the angle of reflection, and the incident ray, normal and reflected ray lie in the same plane.
6. What is the difference between incident ray and reflected ray?
The incident ray falls on the mirror, while the reflected ray comes back from the mirror.
7. What is a convex lens?
A convex lens is thicker at the middle and thinner at the edges. It converges light.
8. What is a concave lens?
A concave lens is thicker at the edges and thinner at the middle. It diverges light.
9. Why can a convex lens burn paper in sunlight?
A convex lens focuses sunlight into a small region, producing enough heat to burn paper.
10. Where are lenses used?
Lenses are used in eyeglasses, cameras, telescopes, microscopes, magnifying glasses and the human eye.
Final Conclusion
The chapter Light: Mirrors and Lenses explains how curved mirrors and lenses form different types of images. Concave mirrors and convex lenses converge light, while convex mirrors and concave lenses diverge light. The laws of reflection apply to all mirrors. These concepts are useful in understanding vehicle mirrors, dental mirrors, torches, headlights, spectacles, cameras, telescopes, microscopes, solar cookers and even the human eye.
Leave a Reply