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Light: Mirrors and Lenses

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Chapter Notes

Overview
This chapter explores the fascinating world of spherical mirrors (concave and convex) and lenses (convex and concave). It explains how these curved surfaces form images that can be enlarged, diminished, erect, or inverted, depending on the distance of the object. The chapter also revisits and extends the laws of reflection, showing that they apply to all mirrors. The one big idea is that the shape of a reflecting or refracting surface determines how light behaves and what kind of image we see, which is why curved mirrors and lenses are used in everyday objects like torches, car headlights, side-view mirrors, magnifying glasses, and eyeglasses.
Key Concepts (14)
Spherical mirror
A mirror whose reflecting surface is spherical, shaped like a part of a hollow glass sphere.
e.g. The shiny curved surface of a metallic spoon acts like a spherical mirror.
Concave mirror
A spherical mirror whose reflecting surface curves inwards.
e.g. The inner curved surface of a spoon is a concave mirror.
Convex mirror
A spherical mirror whose reflecting surface curves outwards.
e.g. The outer curved surface of a spoon is a convex mirror.
Incident ray
The ray of light that falls on the mirror.
e.g. The beam of light from a torch that strikes a plane mirror is an incident ray.
Reflected ray
The ray of light that comes back from the mirror after reflection.
e.g. The beam of light that bounces off the mirror and hits the wall is a reflected ray.
Normal
A line drawn perpendicular to the reflecting surface at the point of incidence.
e.g. If you draw a line at 90° to the mirror at the point where light hits it, that line is the normal.
Angle of incidence (i)
The angle between the incident ray and the normal.
e.g. If the incident ray makes a 40° angle with the normal, the angle of incidence is 40°.
Angle of reflection (r)
The angle between the reflected ray and the normal.
e.g. If the reflected ray makes a 40° angle with the normal, the angle of reflection is 40°.
Lens
A piece of transparent material, usually made of glass or plastic, which has curved surfaces.
e.g. A magnifying glass is a lens that makes small print appear bigger.
Convex lens
A lens which is thicker at the middle as compared to the edges.
e.g. The lens in a magnifying glass is a convex lens.
Concave lens
A lens which is thicker at the edges as compared to the middle.
e.g. The lens used in some eyeglasses for nearsightedness is a concave lens.
Converging lens
Another name for a convex lens because it converges (brings together) light beams.
e.g. A convex lens focuses sunlight to a point and can burn paper.
Diverging lens
Another name for a concave lens because it diverges (spreads out) light beams.
e.g. A concave lens spreads out parallel light rays.
Lateral inversion
The phenomenon where the left and right sides of an object appear swapped in the image formed by a mirror.
e.g. When you raise your right hand in front of a mirror, the image appears to raise its left hand.
How It Works
Image formation by a concave mirror
  1. Place the object close to the concave mirror (3-4 cm away).
  2. Observe the image: it is erect and larger than the object (enlarged).
  3. Move the object farther away from the mirror.
  4. Observe the image: it becomes inverted, initially enlarged, then keeps getting smaller.
💡 Students often forget that a concave mirror can form both erect and inverted images depending on the distance of the object. When the object is close, the image is erect and enlarged; when far, it is inverted.
Image formation by a convex mirror
  1. Place the object in front of the convex mirror at any distance.
  2. Observe the image: it is always erect and smaller than the object (diminished).
  3. Move the object farther away.
  4. Observe that the image size decreases slightly but remains erect and diminished.
💡 The image in a convex mirror is ALWAYS erect and diminished, regardless of the object's distance. This is a key exam point.
Laws of reflection (Activity 10.4)
  1. Place a plane mirror upright on a white sheet of paper.
  2. Using a comb and torch, obtain a thin beam of light and let it fall on the mirror.
  3. Draw the incident ray and the reflected ray on the paper.
  4. Remove the mirror and draw the normal at the point of incidence (a line at 90° to the mirror).
  5. Measure the angle of incidence (i) between the incident ray and the normal.
  6. Measure the angle of reflection (r) between the reflected ray and the normal.
  7. Repeat with different angles and note that i = r in each case.
💡 The two laws of reflection are: (1) The angle of incidence is equal to the angle of reflection. (2) The incident ray, the normal, and the reflected ray all lie in the same plane. Students often forget the second law.
Convergence and divergence of light by spherical mirrors
  1. Take a plane mirror, a concave mirror, and a convex mirror.
  2. Using a comb with multiple slits, obtain multiple parallel beams of light.
  3. Let the parallel beams fall on the plane mirror: the reflected beams remain parallel.
  4. Let the parallel beams fall on the concave mirror: the reflected beams converge (come closer together).
  5. Let the parallel beams fall on the convex mirror: the reflected beams diverge (spread out).
💡 Concave mirrors converge light; convex mirrors diverge light. This is why concave mirrors are used in torches and headlights to focus light, and convex mirrors are used as side-view mirrors to give a wider view.
Burning paper using a concave mirror (Activity 10.7)
  1. Hold a concave mirror with its reflecting surface facing the Sun.
  2. Direct the reflected sunlight onto a sheet of paper.
  3. Adjust the distance of the paper until a sharp, bright spot appears on it.
  4. Hold the mirror and paper steady for a few minutes.
  5. Observe that the paper starts to burn and produce smoke.
💡 The bright spot is the point where sunlight is concentrated. This produces enough heat to ignite the paper. Never look directly at the Sun or the reflected beam.
Image formation by a convex lens
  1. Place the object behind the convex lens at a small distance.
  2. Look through the lens from the other side: the object appears erect and enlarged.
  3. Move the object farther from the lens.
  4. Observe that the image becomes inverted, initially enlarged, then diminishes in size.
💡 Like a concave mirror, a convex lens can form both erect and inverted images. When the object is close, the image is erect and enlarged; when far, it is inverted.
Image formation by a concave lens
  1. Place the object behind the concave lens at any distance.
  2. Look through the lens from the other side: the object always appears erect and diminished (smaller).
  3. Move the object farther away.
  4. Observe that the image size changes slightly but remains erect and diminished.
💡 The image in a concave lens is ALWAYS erect and diminished, regardless of the object's distance. This is similar to a convex mirror.
Convergence and divergence of light by lenses
  1. Take a thin glass plate, a convex lens, and a concave lens.
  2. Using a comb with multiple slits, obtain multiple parallel beams of light.
  3. Let the parallel beams fall on the thin glass plate: the beams pass through unchanged.
  4. Let the parallel beams fall on the convex lens: the beams converge (come together).
  5. Let the parallel beams fall on the concave lens: the beams diverge (spread out).
💡 A convex lens is called a converging lens; a concave lens is called a diverging lens. This is a direct parallel to spherical mirrors.
Exam Traps — Don't Make These Mistakes
⚠ Confusing concave and convex mirrors
✗ A concave mirror bulges outwards and a convex mirror curves inwards.
→ This is the exact opposite of the correct definitions. Concave means 'caved in' (curved inwards), and convex means 'bulging out' (curved outwards).
✓ A concave mirror has a reflecting surface that curves inwards (like the inside of a spoon). A convex mirror has a reflecting surface that curves outwards (like the back of a spoon).
⚠ Forgetting that laws of reflection apply to all mirrors
✗ The laws of reflection only apply to plane mirrors, not to spherical mirrors.
→ The chapter explicitly states that the laws of reflection are valid for all kinds of mirrors — plane, concave, and convex. Each ray obeys the laws individually.
✓ The laws of reflection are valid for all mirrors — plane, concave, and convex. The curved surface of spherical mirrors causes parallel rays to converge or diverge, but each ray still obeys the laws.
⚠ Mixing up the image characteristics of concave and convex mirrors
✗ A concave mirror always forms an erect and diminished image, while a convex mirror always forms an inverted and enlarged image.
→ This is completely reversed. A concave mirror can form both erect (when object is close) and inverted (when object is far) images, and the size can be enlarged or diminished. A convex mirror ALWAYS forms an erect and diminished image.
✓ A concave mirror forms an erect and enlarged image when the object is close, and an inverted image (initially enlarged, then smaller) when the object is far. A convex mirror always forms an erect and diminished image regardless of the object's distance.
⚠ Confusing the behaviour of lenses with mirrors
✗ A convex lens diverges light and a concave lens converges light.
→ This is the opposite of the correct behaviour. A convex lens is thicker in the middle and converges light (converging lens). A concave lens is thinner in the middle and diverges light (diverging lens).
✓ A convex lens converges light (it is a converging lens). A concave lens diverges light (it is a diverging lens).
⚠ Thinking the image in a convex mirror is always the same size
✗ The image in a convex mirror is always the same size regardless of the object's distance.
→ While the image is always diminished, its size does change slightly as the object moves farther away — it becomes even smaller.
✓ The image in a convex mirror is always erect and diminished, but its size decreases slightly as the object is moved farther from the mirror.
⚠ Misidentifying the normal in reflection diagrams
✗ The normal is the line along which the incident ray travels.
→ The normal is a line drawn perpendicular (at 90°) to the mirror at the point of incidence, not the path of the ray.
✓ The normal is a line drawn at 90° to the reflecting surface at the point where the incident ray strikes the mirror.
Compare & Contrast
Shape of reflecting surface
Concave mirror
Convex mirror
A concave mirror curves inwards (like the inside of a bowl), while a convex mirror curves outwards (like the back of a spoon).
Nature of image formed
Concave mirror
Convex mirror
A concave mirror can form both erect (when object is close) and inverted (when object is far) images, and the size can be enlarged or diminished. A convex mirror always forms an erect and diminished image regardless of the object's distance.
Shape of lens
Convex lens
Concave lens
A convex lens is thicker at the middle than at the edges. A concave lens is thicker at the edges than at the middle.
Effect on parallel light beams
Convex lens
Concave lens
A convex lens converges (brings together) parallel light beams, so it is called a converging lens. A concave lens diverges (spreads out) parallel light beams, so it is called a diverging lens.
Image formation when object is close
Concave mirror
Convex lens
Both form an erect and enlarged image when the object is placed close to them. However, a concave mirror reflects light, while a convex lens allows light to pass through it.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (3 marks), 1 Long Answer (5 marks). Total: 11 marks.
Most asked: Laws of reflection (definition and application) · Characteristics of images formed by concave and convex mirrors · Characteristics of images formed by convex and concave lenses · Differences between concave and convex mirrors · Differences between convex and concave lenses · Uses of concave mirrors (torch, headlights, dentist mirror) · Uses of convex mirrors (side-view mirrors, road safety mirrors, surveillance mirrors) · Converging and diverging nature of spherical mirrors and lenses
Don't skip: Long answer question comparing image formation by concave and convex mirrors (5 marks); Short answer question on laws of reflection with diagram (3 marks); MCQ on identifying the type of mirror or lens from its image characteristics (1 mark)
One-Line Revision (read aloud before the exam)
  1. Spherical mirrors have reflecting surfaces that are spherical, shaped like a part of a hollow glass sphere.
  2. A concave mirror curves inwards, and a convex mirror curves outwards.
  3. A concave mirror can form an erect and enlarged image when the object is close, and an inverted image when the object is far.
  4. A convex mirror always forms an erect and diminished image, regardless of the object's distance.
  5. The two laws of reflection are: (i) angle of incidence equals angle of reflection, and (ii) the incident ray, normal, and reflected ray all lie in the same plane.
  6. The laws of reflection are valid for plane, concave, and convex mirrors.
  7. A concave mirror converges parallel light beams, while a convex mirror diverges them.
  8. A lens is a piece of transparent material with curved surfaces, usually made of glass or plastic.
  9. A convex lens is thicker in the middle and converges light; it is called a converging lens.
  10. A concave lens is thinner in the middle and diverges light; it is called a diverging lens.
  11. A convex lens can form an erect and enlarged image when the object is close, and an inverted image when the object is far.
  12. A concave lens always forms an erect and diminished image, regardless of the object's distance.
  13. Concave mirrors are used in torches, car headlights, and dentist mirrors to focus light.
  14. Convex mirrors are used as side-view mirrors on vehicles and at road intersections to provide a wider view.
  15. A magnifying glass is a convex lens used to read small print.
  16. Lateral inversion is seen in plane, concave, and convex mirrors — the left and right sides of the image are swapped.

MCQs — 10 Questions with Answers

Q1. Which of the following best describes a concave mirror?

  • A. A mirror with a reflecting surface that curves inward like a bowl ✓
  • B. A mirror with a reflecting surface that bulges outward like a dome
  • C. A flat mirror with a normal reflecting surface
  • D. A mirror that only reflects light at right angles

Answer: A — A concave mirror has a reflecting surface that curves inward, like the inner surface of a hollow sphere.

Q2. When you look at the outer curved surface of a shiny metallic spoon, what kind of image do you see?

  • A. Inverted and enlarged
  • B. Erect and enlarged
  • C. Erect and diminished ✓
  • D. Inverted and diminished

Answer: C — The outer curved surface of a spoon acts as a convex mirror, which always produces erect and smaller (diminished) images.

Q3. What is the angle of incidence if a light ray falls along the normal to a mirror surface?

  • A. 90 degrees
  • B. 45 degrees
  • C. 0 degrees ✓
  • D. 180 degrees

Answer: C — The angle of incidence is measured between the incident ray and the normal; if the ray travels along the normal itself, the angle is 0 degrees.

Q4. In Activity 10.3, when an object is placed very close to a concave mirror, the image appears:

  • A. Inverted and enlarged
  • B. Erect and enlarged ✓
  • C. Erect and diminished
  • D. Inverted and diminished

Answer: B — When an object is very close to a concave mirror, the image is erect and magnified (enlarged).

Q5. Which type of mirror is used as a reflector in a torch to focus light?

  • A. Plane mirror
  • B. Convex mirror
  • C. Concave mirror ✓
  • D. Cylindrical mirror

Answer: C — Concave mirrors are used in torch and car headlight reflectors because they can focus and concentrate light rays into a bright beam.

Q6. A dentist needs to examine a patient's teeth and wants an enlarged, clear view. Which mirror should the dentist use and how should it be positioned?

  • A. A convex mirror held close to the teeth
  • B. A concave mirror held close to the teeth ✓
  • C. A plane mirror held close to the teeth
  • D. A concave mirror held far from the teeth

Answer: B — A concave mirror held close produces an erect, enlarged image, which is ideal for the dentist to inspect teeth in detail.

Q7. Why are convex mirrors preferred for vehicle side-view mirrors instead of plane mirrors?

  • A. They produce inverted images which are safer
  • B. They provide a wider field of view and allow the driver to see more of the road behind ✓
  • C. They produce enlarged images of nearby vehicles
  • D. They do not show lateral inversion

Answer: B — Convex mirrors curve outward, providing a much wider field of view and helping drivers see a larger area of the road behind them safely.

Q8. At a sharp road bend, a convex mirror is installed at the intersection. Compared to the actual traffic behind, the images in this mirror appear:

  • A. Larger and inverted
  • B. Smaller and inverted
  • C. Larger and erect
  • D. Smaller and erect ✓

Answer: D — Convex mirrors always form erect, diminished (smaller) images, helping drivers see around the bend while making vehicles appear farther away than they actually are.

Q9. In the law of reflection experiment, if the angle of incidence is 30 degrees, what should the angle of reflection be?

  • A. 15 degrees
  • B. 30 degrees ✓
  • C. 45 degrees
  • D. 60 degrees

Answer: B — According to the law of reflection, the angle of incidence always equals the angle of reflection; therefore, if i = 30°, then r = 30°.

Q10. A student observes that when she moves a small toy farther and farther from a mirror, the image in the mirror changes from erect to inverted. Which type of mirror is this student looking at?

  • A. Plane mirror
  • B. Convex mirror
  • C. Concave mirror ✓
  • D. Spherical mirror with unknown curvature

Answer: C — Only a concave mirror produces an image that changes from erect (when object is close) to inverted (when object is far away) as the object distance increases.

Flashcards

What is a spherical mirror?

A mirror whose reflecting surface is shaped like a part of a hollow glass sphere, either curving inward (concave) or outward (convex).

How is a concave mirror different from a convex mirror in terms of reflecting surface?

A concave mirror has a reflecting surface that curves inward, while a convex mirror has a reflecting surface that curves outward.

What image does a concave mirror form when an object is placed very close to it?

An erect, enlarged (magnified) image of the object.

What image does a convex mirror always form?

An erect, diminished (smaller) image of the object, regardless of the object's distance from the mirror.

Define the angle of incidence.

The angle between the incident ray and the normal to the reflecting surface at the point where the ray strikes the mirror.

What is the law of reflection?

The angle of incidence is equal to the angle of reflection, measured from the normal to the reflecting surface.

Why is a warning written on vehicle side-view mirrors that 'Objects in mirror are closer than they appear'?

Because convex mirrors form diminished (smaller) images, making objects appear farther away than they actually are.

Name one use of a concave mirror in daily life.

Concave mirrors are used as reflectors in torches and car headlights to focus and direct light beams.

Why are convex mirrors used at road intersections?

Convex mirrors provide a wide field of view and form erect, diminished images, helping drivers from both sides see around sharp bends and prevent collisions.

What happens to the image in a concave mirror as the object moves away from the mirror?

The image changes from erect and enlarged to inverted, and its size gradually decreases as the object moves farther.

Important Board Questions

What is a concave mirror? [1 mark]

Define based on the shape of the reflecting surface—curved inward like a part of a hollow sphere.

How does the image formed by a concave mirror change as the object moves away from the mirror? Explain with at least two observations. [2 marks]

Describe how image changes from erect-enlarged (close) to inverted-diminished (far); size reduces gradually with distance.

State the law of reflection and explain how angle of incidence and angle of reflection are measured. Draw a simple diagram with labels showing incident ray, reflected ray, normal, and both angles. [3 marks]

Law: angle of incidence = angle of reflection. Angles measured from the normal (perpendicular to mirror surface at point of incidence). Include labeled diagram with incident ray, reflected ray, normal line, point O, and angle i and r clearly marked.

Explain why convex mirrors are installed at sharp road bends and in shops for surveillance, giving specific advantages they provide in each location. Also explain why the warning 'Objects in mirror are closer than they appear' is written on vehicle side-view mirrors. Discuss how the properties of convex mirrors make them suitable for these applications. [5 marks]

Convex mirrors give wider field of view, always erect-diminished images, help see around bends, prevent collisions; shop surveillance monitors large area; warning exists because diminished images make objects appear farther; discuss how these properties serve safety and surveillance purposes.

Assertion-Reason Questions for Light: Mirrors and Lenses

Q1. Assertion (A):

A concave mirror can form an inverted and enlarged image of an object placed far from the mirror.

Reason (R):

When an object is placed at a distance greater than the focal length of a concave mirror, the mirror forms a real, inverted, and diminished image.

Answer: D — A is false but R is true — The assertion is false because a concave mirror forms an inverted and enlarged image only when the object is very close (between the mirror and its focal point), not when placed far away. When far away, the image is inverted but diminished. The reason is also false in stating enlarged images form at distance, so answer is D.

Q2. Assertion (A):

Convex mirrors are used in vehicle side-view mirrors because they provide a wide field of view and always form erect, diminished images.

Reason (R):

The outward curvature of a convex mirror causes light rays to diverge, which reduces the size of the image but increases the visible area, allowing drivers to see more of the traffic behind the vehicle.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why convex mirrors are preferred for side-view mirrors. The diverging property of convex mirrors creates the wide field of view while reducing image size, which is why the warning label is necessary.

Q3. Assertion (A):

A plane mirror and a convex mirror always form erect images of objects placed in front of them.

Reason (R):

Plane mirrors have flat reflecting surfaces that produce lateral inversion, while convex mirrors have outward-curving surfaces that diverge light rays.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both the assertion and reason are true. Plane mirrors do form erect images, and convex mirrors also form erect images. However, the reason does not fully explain why both form erect images. The reason describes properties but is not the correct explanation of the assertion itself.

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Case-Based Questions (CBQ) for Light: Mirrors and Lenses

Case Study:

Meena conducted an experiment in science class where she studied the behavior of a concave mirror. She placed a small toy at distances of 5 cm, 10 cm, 15 cm, 20 cm, and 30 cm from the mirror and observed the images formed. At 5 cm distance, the image was erect and enlarged. At 10 cm and 15 cm, the image became inverted but still quite large. At 20 cm, the image was inverted but noticeably smaller. At 30 cm, the image was barely visible and very small. Based on these observations, Meena concluded that the focal length of the mirror was somewhere between 5 cm and 10 cm.

Q1. At what distance from the concave mirror was the image formation consistent with Meena's observation of an inverted and enlarged image?

  • A. 5 cm
  • B. 10 cm and 15 cm ✓
  • C. 20 cm
  • D. 30 cm

Answer: B

Q2. What type of image is formed when the object is placed very close to the concave mirror at 5 cm distance?

  • A. Erect and diminished
  • B. Erect and enlarged ✓
  • C. Inverted and enlarged
  • D. Inverted and diminished

Answer: B

Q3. Based on Meena's observation that the image becomes smaller as distance increases from 10 cm onwards, which statement is correct about the behavior of a concave mirror?

  • A. The image size remains constant regardless of object distance
  • B. The image size increases beyond 20 cm distance
  • C. The image size decreases as the object moves farther from the mirror ✓
  • D. The image becomes erect at larger distances

Answer: C

Q4. Meena's conclusion about the focal length being between 5 cm and 10 cm is based on which observation?

  • A. The image changes from erect (at 5 cm) to inverted (at 10 cm and beyond) ✓
  • B. The image becomes very small at 30 cm
  • C. The image is enlarged at 10 cm and 15 cm distances
  • D. The image disappears at distances greater than 20 cm

Answer: A

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