Navigating through the CBSE Class 10 Science curriculum requires an in-depth understanding of rectilinear propagation of light, spherical mirror reflections, Snell’s law of refraction, lens image formations, Cartesian sign conventions, magnification, and lens power calculations. Chapter 9 of Class 10 Physics, “Light – Reflection and Refraction”, forms the mathematical and conceptual core of secondary optics. It investigates how concave and convex mirrors form real and virtual images; explains the bending of light through optically denser and rarer media; establishes the mirror and lens formulas; and details the quantitative determination of image position, size, nature, and optical power. To help students master every aspect of this high-weightage chapter, this comprehensive solutions guide offers textbook-accurate, highly structured, and pedagogically sound responses strictly aligned with the latest CBSE evaluation standards.
Every question presented in the official NCERT textbook—ranging from all four in-text question sets (Pages 168, 171, 176, and 184) to the complete chapter-end exercises (Questions 1 to 17 on Pages 185–186)—has been solved with exhaustive detail. Numerical problems follow a step-by-step box format with explicit sign conventions, standard formulas, intermediate algebraic substitutions, and final answers with units. Key scoring terms, official CBSE board year tags, and optical summary tables have been highlighted to ensure students secure maximum marks in their CBSE Board Examinations.
Master Formula & Concept Summary Tables
1. Master Formula Sheet for Reflection & Refraction
| Optical Device / Concept | Standard Formula | Variable Legend | Cartesian Sign Convention Rule |
|---|---|---|---|
| Focal Length & Radius | R = 2f (or f = R / 2) | R = Radius of curvature, f = Focal length | f is Negative (-) for Concave; Positive (+) for Convex. |
| Mirror Formula | 1/f = 1/v + 1/u | u = Object distance, v = Image distance, f = Focal length | u is always Negative (-); v is Negative (-) for Real, Positive (+) for Virtual. |
| Mirror Magnification | m = h'/h = -v/u | h' = Image height, h = Object height | m is Negative (-) for Real/Inverted; Positive (+) for Virtual/Erect. |
| Snell’s Law | n₂₁ = sin i / sin r | i = Angle of incidence, r = Angle of refraction | n₂₁ = v₁ / v₂ = n₂ / n₁ |
| Absolute Refractive Index | n = c / v | c = 3 × 10⁸ m/s, v = Speed of light in medium | n ≥ 1 always (Dimensionless quantity). |
| Lens Formula | 1/f = 1/v - 1/u | u = Object distance, v = Image distance, f = Focal length | f is Positive (+) for Convex (converging); Negative (-) for Concave (diverging). |
| Lens Magnification | m = h'/h = +v/u | h' = Image height, h = Object height | m is Negative (-) for Real/Inverted; Positive (+) for Virtual/Erect. |
| Power of a Lens | P = 1 / f (in m) or 100 / f (in cm) | P = Power in Dioptres (D), f = Focal length | P is Positive (+) for Convex lens; Negative (-) for Concave lens. |
2. Image Formation by Concave and Convex Mirrors
| Mirror Type | Position of Object | Position of Image | Relative Size of Image | Nature of Image |
|---|---|---|---|---|
| Concave Mirror | At Infinity | At Focus F | Highly diminished (Point-sized) | Real and Inverted |
| Concave Mirror | Beyond C | Between F and C | Diminished | Real and Inverted |
| Concave Mirror | At C | At C | Same size as object | Real and Inverted |
| Concave Mirror | Between C and F | Beyond C | Magnified (Enlarged) | Real and Inverted |
| Concave Mirror | At Focus F | At Infinity | Highly magnified | Real and Inverted |
| Concave Mirror | Between Pole P and F | Behind the mirror | Magnified (Enlarged) | Virtual and Erect |
| Convex Mirror | At Infinity | Behind mirror at F | Highly diminished (Point-sized) | Virtual and Erect |
| Convex Mirror | Between Infinity and P | Behind mirror between P and F | Diminished | Virtual and Erect |
NCERT In-Text Questions: Set 1 (Page No. 168)
Question 1 Define the principal focus of a concave mirror. [CBSE 2024, 2022]
Answer: The principal focus of a concave mirror is a specific point on its principal axis where all incident light rays traveling parallel and close to the principal axis converge and intersect in reality after undergoing reflection from the mirror surface. It is denoted by the capital letter F.
Question 2 The radius of curvature of a spherical mirror is 20 cm. What is its focal length? [CBSE 2023]
Answer:
================================================================================ CALCULATION: -------------------------------------------------------------------------------- Given: Radius of curvature (R) = 20 cm Formula: Focal length (f) = R / 2 Calculation: f = 20 / 2 = 10 cm Final Answer: The focal length of the spherical mirror is 10 cm. ================================================================================
Question 3 Name a mirror that can give an erect and enlarged image of an object. [CBSE 2024, 2020]
Answer: A Concave Mirror gives an erect, virtual, and magnified (enlarged) image of an object when the object is placed between the Pole (P) and the Principal Focus (F) of the mirror.
Question 4 Why do we prefer a convex mirror as a rear-view mirror in vehicles? [CBSE 2024, 2023, 2020]
Answer: Convex mirrors are universally preferred as rear-view (wing) mirrors in automobiles for two critical optical reasons:
- Always Produces Erect Images: A convex mirror always forms an erect and virtual image of vehicles approaching from behind, regardless of their distance.
- Wider Field of View: Because a convex mirror is curved outwards, it provides a much wider field of view than a plane mirror, enabling the driver to view a large traffic area behind the vehicle.
NCERT In-Text Questions: Set 2 (Page No. 171)
Question 1 Find the focal length of a convex mirror whose radius of curvature is 32 cm. [CBSE 2023]
Answer:
================================================================================ CALCULATION: -------------------------------------------------------------------------------- Given: Radius of curvature (R) = +32 cm (Positive for convex mirror) Formula: f = R / 2 Calculation: f = +32 / 2 = +16 cm Final Answer: The focal length of the convex mirror is +16 cm. ================================================================================
Question 2 A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located? [CBSE 2024, 2020]
Answer:
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NUMERICAL SOLUTION (MIRROR MAGNIFICATION):
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GIVEN DATA:
• Object distance (u) = -10 cm (Always negative)
• Magnification (m) = -3 (Negative because the image is real and inverted)
CALCULATION:
Formula: m = -v / u
-3 = -v / (-10)
-3 = v / 10
v = -3 × 10
v = -30 cm
FINAL ANSWER:
The image is located at a distance of 30 cm in front of the concave mirror
(on the same side as the object). The negative sign confirms the image is real.
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NCERT In-Text Questions: Set 3 (Page No. 176)
Question 1 A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why? [CBSE 2024, 2022]
Answer: The light ray will bend towards the normal.
- Scientific Reason: Water is optically denser than air (which is an optically rarer medium). When a light ray travels from an optically rarer medium into an optically denser medium, its speed decreases, causing the refracted ray to bend toward the normal at the interface.
Question 2 Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 × 10⁸ m/s. [CBSE 2023, 2020]
Answer:
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CALCULATION (REFRACTIVE INDEX & SPEED OF LIGHT):
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GIVEN DATA:
• Speed of light in vacuum (c) = 3 × 10⁸ m/s
• Refractive index of glass (n) = 1.50
CALCULATION:
Formula: n = c / v
v = c / n
v = (3 × 10⁸) / 1.50
v = 2 × 10⁸ m/s
FINAL ANSWER:
The speed of light in glass is 2 × 10⁸ m/s.
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Question 3 Find out, from the NCERT table, the medium having highest optical density. Also find the medium with lowest optical density. [CBSE 2023]
Answer:
- Medium with Highest Optical Density: Diamond (Refractive Index,
n = 2.42). - Medium with Lowest Optical Density: Air (Refractive Index,
n = 1.0003).
Question 4 You are given kerosene, turpentine and water. In which of these does the light travel fastest? (Refractive index of water = 1.33, kerosene = 1.44, turpentine = 1.47). [CBSE 2024, 2020]
Answer: Light travels fastest in Water:
- Scientific Principle: Speed of light in a medium is inversely proportional to its optical refractive index (
v = c / n). - Since water has the lowest refractive index (
n = 1.33) compared to kerosene (n = 1.44) and turpentine oil (n = 1.47), light travels with the maximum speed in water.
Question 5 The refractive index of diamond is 2.42. What is the meaning of this statement? [CBSE 2024, 2022]
Answer: This statement means that the ratio of the speed of light in vacuum (or air) to the speed of light in diamond is 2.42.
In other words, light travels 2.42 times slower in diamond than in vacuum (v_diamond = c / 2.42 = 1.24 × 10⁸ m/s), making diamond an exceptionally optically dense medium.
NCERT In-Text Questions: Set 4 (Page No. 184)
Question 1 Define 1 dioptre of power of a lens. [CBSE 2024, 2020]
Answer: One Dioptre (1 D) is defined as the optical power of a lens whose focal length is exactly 1 metre (1 D = 1 m⁻¹).
Question 2 A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens. [CBSE 2024, 2023]
Answer:
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NUMERICAL SOLUTION (LENS & POWER):
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GIVEN DATA:
• Image distance (v) = +50 cm (Real image formed behind the lens)
• Magnification (m) = -1 (Real image having equal size to object)
STEP 1: Finding Object Distance (u)
Formula: m = v / u
-1 = 50 / u
u = -50 cm
(The needle is placed 50 cm in front of the lens at 2F₁).
STEP 2: Finding Focal Length (f)
Since object is at 2F, 2f = 50 cm ===> f = +25 cm = +0.25 m
STEP 3: Calculating Power (P)
Formula: P = 1 / f (in m)
P = 1 / (+0.25)
P = +4 D
FINAL ANSWER:
• Position of Needle (u) : 50 cm in front of the lens (at 2F₁).
• Power of Lens (P) : +4 Dioptres (+4 D).
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Question 3 Find the power of a concave lens of focal length 2 m. [CBSE 2023]
Answer:
================================================================================ CALCULATION: -------------------------------------------------------------------------------- Given: Focal length of concave lens (f) = -2 m (Negative for concave/diverging lens) Formula: P = 1 / f (in m) Calculation: P = 1 / (-2) = -0.5 D Final Answer: The power of the concave lens is -0.5 Dioptre (-0.5 D). ================================================================================
NCERT Chapter-End Exercises (Page No. 185-186)
Question 1 Which one of the following materials cannot be used to make a lens? (a) Water (b) Glass (c) Plastic (d) Clay
Answer: (d) Clay Explanation: A lens must be made of an optically transparent material that allows light to pass through and undergo refraction. Clay is an opaque substance that blocks light completely.
Question 2 The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object? (a) Between the principal focus and the centre of curvature (b) At the centre of curvature (c) Beyond the centre of curvature (d) Between the pole of the mirror and its principal focus
Answer: (d) Between the pole of the mirror and its principal focus Explanation: When an object is placed between the pole (P) and principal focus (F) of a concave mirror, the reflected rays diverge and appear to meet behind the mirror, forming a virtual, erect, and magnified image.
Question 3 Where should an object be placed in front of a convex lens to get a real image of the size of the object? (a) At the principal focus of the lens (b) At twice the focal length (c) At infinity (d) Between the optical centre of the lens and its principal focus
Answer: (b) At twice the focal length (at 2F₁) Explanation: Placing an object at 2F₁ of a convex lens produces a real, inverted image of the same size at 2F₂ on the opposite side of the lens (m = -1).
Question 4 A spherical mirror and a thin spherical lens have each a focal length of -15 cm. The mirror and the lens are likely to be: (a) both concave (b) both convex (c) the mirror is concave and the lens is convex (d) the mirror is convex, but the lens is concave
Answer: (a) both concave Explanation: According to the Cartesian sign convention, focal length is always negative (-) for both a concave mirror and a concave lens.
Question 5 No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be: (a) only plane (b) only concave (c) only convex (d) either plane or convex
Answer: (d) either plane or convex Explanation: A plane mirror always forms a virtual, erect image of the same size, and a convex mirror always forms a virtual, erect, and diminished image at all object distances.
Question 6 Which of the following lenses would you prefer to use while reading small letters found in a dictionary? (a) A convex lens of focal length 50 cm (b) A concave lens of focal length 50 cm (c) A convex lens of focal length 5 cm (d) A concave lens of focal length 5 cm
Answer: (c) A convex lens of focal length 5 cm Explanation: A convex lens functions as a magnifying glass when the object is within its focal length. Shorter focal length (f = 5 cm) produces higher optical power (P = 100/5 = +20 D) and greater magnification than a 50 cm lens.
Question 7 We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case. [CBSE 2024, 2020]
Answer:
- Range of Distance: The object must be placed between the Pole (P) and Principal Focus (F), which means the distance must be between 0 cm and 15 cm in front of the mirror (
0 < u < 15 cm). - Nature of Image: Virtual and Erect.
- Size of Image: Magnified (larger than the object), formed behind the mirror.
================================================================================ RAY DIAGRAM DESCRIPTION: -------------------------------------------------------------------------------- 1. Object AB is placed between Pole (P) and Focus (F) (e.g., at 10 cm). 2. Ray 1: From point A traveling parallel to principal axis reflects through Focus (F). 3. Ray 2: From point A passing through Centre of Curvature (C) reflects back along the same path. 4. Both reflected rays diverge in front; producing them backward behind the mirror causes them to intersect at point A', forming a magnified, erect virtual image A'B'. ================================================================================
Question 8 Name the type of mirror used in the following situations: [CBSE 2024, 2022] (a) Headlights of a car (b) Side/rear-view mirror of a vehicle (c) Solar furnace Support your answer with reason.
Answer:
- (a) Headlights of a Car: Concave Mirror. Reason: When a high-intensity light bulb is placed at the principal focus of a concave reflector, the reflected light emerges as a powerful, parallel beam that illuminates the road ahead over long distances.
- (b) Side/Rear-View Mirror: Convex Mirror. Reason: It always forms an erect, diminished image and provides a very wide field of view, enabling the driver to monitor traffic behind.
- (c) Solar Furnace: Large Concave Mirror. Reason: Concave mirrors are converging mirrors that concentrate massive amounts of parallel incident sunlight onto a single focal point, producing high temperatures (over 2000°C) for industrial heating.
Question 9 One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations. [CBSE 2024, 2020]
Answer: Yes, the convex lens will still produce a complete, full image of the object.
- Experimental Verification: Place a lighted candle in front of a convex lens on an optical bench and project its image onto a screen. Cover the lower half of the lens with opaque black paper. A complete, full image of the candle flame is still seen on the screen.
- Scientific Explanation: Every portion of a convex lens refracts light rays coming from all points of an object to form the image. Covering half the lens reduces the total number of light rays passing through the lens by 50%, which reduces the brightness (intensity) of the image, but the complete image is formed without any missing parts.
Question 10 An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Find the position, size and the nature of the image formed. [CBSE 2024, 2023]
Answer:
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NUMERICAL SOLUTION (CONVEX LENS FORMULA):
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GIVEN DATA:
• Height of object (h) = +5 cm
• Object distance (u) = -25 cm
• Focal length of lens (f) = +10 cm (Converging/Convex lens)
STEP 1: Finding Image Distance (v)
Formula: 1/f = 1/v - 1/u
1/10 = 1/v - 1/(-25)
1/10 = 1/v + 1/25
1/v = 1/10 - 1/25
1/v = (5 - 2) / 50 = 3 / 50
v = +50 / 3 = +16.67 cm
STEP 2: Finding Height of Image (h')
Formula: m = h'/h = v/u
h'/5 = (50/3) / (-25)
h'/5 = -2/3
h' = -10/3 = -3.33 cm
FINAL RESULT SUMMARY:
• Position of Image (v) : +16.67 cm on the other side (behind the lens).
• Nature of Image : Real and Inverted (indicated by negative h' and positive v).
• Size of Image (h') : Diminished (3.33 cm in height).
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Question 11 A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? [CBSE 2024, 2022]
Answer:
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NUMERICAL SOLUTION (CONCAVE LENS):
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GIVEN DATA:
• Focal length (f) = -15 cm (Negative for concave lens)
• Image distance (v) = -10 cm (Concave lens always forms virtual image in front)
CALCULATION (LENS FORMULA):
Formula: 1/f = 1/v - 1/u
1/u = 1/v - 1/f
1/u = 1/(-10) - 1/(-15)
1/u = -1/10 + 1/15
1/u = (-3 + 2) / 30
1/u = -1 / 30
u = -30 cm
FINAL ANSWER:
The object is placed at a distance of 30 cm in front of the concave lens.
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Question 12 An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image. [CBSE 2023, 2020]
Answer:
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NUMERICAL SOLUTION (CONVEX MIRROR):
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GIVEN DATA:
• Object distance (u) = -10 cm
• Focal length (f) = +15 cm (Positive for convex mirror)
CALCULATION (MIRROR FORMULA):
Formula: 1/f = 1/v + 1/u
1/v = 1/f - 1/u
1/v = 1/15 - 1/(-10)
1/v = 1/15 + 1/10
1/v = (2 + 3) / 30 = 5 / 30 = 1 / 6
v = +6 cm
MAGNIFICATION (m):
Formula: m = -v / u = -(+6) / (-10) = +0.6
FINAL RESULT SUMMARY:
• Image Position (v) : +6 cm behind the mirror.
• Nature of Image : Virtual and Erect (positive v and positive m).
• Size of Image : Diminished (0.6 times the size of the object).
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Question 13 The magnification produced by a plane mirror is +1. What does this mean? [CBSE 2024, 2020]
Answer: A magnification of m = +1 conveys two distinct physical meanings:
- The Positive Sign (
+): Indicates that the image formed is Virtual and Erect. - The Numerical Value (
1): Indicates that the size of the image is exactly equal to the size of the object (h' = h), formed at an equal distance behind the mirror.
Question 14 An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size. [CBSE 2024, 2022]
Answer:
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NUMERICAL SOLUTION (CONVEX MIRROR):
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GIVEN DATA:
• Object height (h) = +5.0 cm
• Object distance (u) = -20 cm
• Radius of Curvature (R) = +30 cm ===> Focal length (f) = R / 2 = +15 cm
STEP 1: Finding Image Distance (v)
Formula: 1/f = 1/v + 1/u
1/15 = 1/v + 1/(-20)
1/v = 1/15 + 1/20
1/v = (4 + 3) / 60 = 7 / 60
v = +60 / 7 = +8.57 cm
STEP 2: Finding Height of Image (h')
Formula: m = h'/h = -v/u
h'/5 = -(60/7) / (-20)
h'/5 = 3/7
h' = 15 / 7 = +2.14 cm
FINAL RESULT SUMMARY:
• Position of Image (v) : +8.57 cm behind the convex mirror.
• Nature of Image : Virtual and Erect (positive sign of h' and v).
• Size of Image (h') : Diminished (2.14 cm tall).
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Question 15 An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focused image can be obtained? Find the size and the nature of the image. [CBSE 2024, 2020]
Answer:
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NUMERICAL SOLUTION (CONCAVE MIRROR):
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GIVEN DATA:
• Object height (h) = +7.0 cm
• Object distance (u) = -27 cm
• Focal length (f) = -18 cm (Concave mirror)
STEP 1: Finding Screen Distance (Image Distance, v)
Formula: 1/f = 1/v + 1/u
1/v = 1/f - 1/u
1/v = 1/(-18) - 1/(-27)
1/v = -1/18 + 1/27
1/v = (-3 + 2) / 54 = -1 / 54
v = -54 cm
STEP 2: Finding Image Size (h')
Formula: m = h'/h = -v/u
h'/7 = -(-54) / (-27)
h'/7 = -2
h' = -14.0 cm
FINAL RESULT SUMMARY:
• Screen Distance : The screen must be placed 54 cm in front of the concave mirror.
• Nature of Image : Real and Inverted (negative h').
• Size of Image (h') : Enlarged / Magnified (14.0 cm tall, twice the object size).
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Question 16 Find the focal length of a lens of power -2.0 D. What type of lens is this? [CBSE 2023]
Answer:
================================================================================ CALCULATION: -------------------------------------------------------------------------------- Given: Power of lens (P) = -2.0 D Formula: f = 1 / P (in metres) Calculation: f = 1 / (-2.0) = -0.5 m = -50 cm Final Answer: • The focal length is -0.5 m (or -50 cm). • Since the power and focal length are negative, it is a Concave Lens (Diverging Lens). ================================================================================
Question 17 A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging? [CBSE 2024, 2022]
Answer:
================================================================================ CALCULATION: -------------------------------------------------------------------------------- Given: Power of corrective lens (P) = +1.5 D Formula: f = 1 / P (in metres) Calculation: f = 1 / (+1.5) = +10 / 15 = +2 / 3 m = +0.67 m (or +66.7 cm) Final Answer: • The focal length of the lens is +0.67 m (or +66.7 cm). • Since the power and focal length are positive, it is a Converging Lens (Convex Lens), prescribed for correcting Hypermetropia (farsightedness). ================================================================================
Frequently Asked Questions (FAQs) – Class 10 Physics Chapter 9
Question 1: State the laws of reflection of light. [CBSE 2024] Answer:
- The incident ray, the reflected ray, and the normal to the reflecting surface at the point of incidence all lie in the same geometric plane.
- The angle of incidence is always equal to the angle of reflection (
∠i = ∠r).
Question 2: State the laws of refraction of light (including Snell’s Law). [CBSE 2024, 2022] Answer:
- The incident ray, the refracted ray, and the normal to the interface of two transparent media at the point of incidence all lie in the same plane.
- The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for light of a given color and for a given pair of media (
sin i / sin r = constant = n₂₁).
Question 3: What is the optical center of a lens? [CBSE 2023] Answer: The optical center (denoted by O) is the central geometric point of a lens through which a ray of light passes straight without suffering any angular deviation.
Question 4: What is the difference between real and virtual images? [CBSE 2024, 2020] Answer:
- Real Image: Formed when light rays actually intersect after reflection/refraction; it is always inverted and can be obtained on a physical screen.
- Virtual Image: Formed when light rays appear to diverge from a point; it is always erect and cannot be obtained on a screen.
Question 5: Why is the absolute refractive index of a medium always greater than 1? [CBSE 2023] Answer: Absolute refractive index is given by n = c / v. Since the speed of light in vacuum (c = 3 × 10⁸ m/s) is the ultimate cosmic speed limit and is always greater than the speed of light in any material medium (v), the ratio c / v is always strictly greater than 1.
Question 6: What happens to a light ray passing through the focus of a concave mirror? [CBSE 2022] Answer: An incident light ray passing through the principal focus of a concave mirror emerges parallel to the principal axis after reflection.
Question 7: What happens to a light ray entering perpendicular (normal) to a glass slab? [CBSE 2024] Answer: When angle of incidence is zero (∠i = 0°), the angle of refraction is also zero (∠r = 0°). The light ray passes straight through the glass slab without any bending or refraction.
Question 8: What is lateral displacement in a glass slab? [CBSE 2023] Answer: Lateral displacement is the perpendicular shift between the original path of the incident light ray and the emergent light ray after undergoing refraction through a rectangular glass slab with parallel faces.
Question 9: What is total internal reflection? [BOARD EXAM FAVORITE] Answer: When light traveling from an optically denser medium to a rarer medium strikes the interface at an angle of incidence greater than the critical angle, the entire light reflects back into the denser medium.
Question 10: Which lens is used as a magnifying glass and why? [CBSE 2024] Answer: A Convex lens of short focal length is used as a magnifying glass because when an object is held within its focal length (u < f), it forms an erect, virtual, and highly magnified image on the same side.
Question 11: Why does a pencil immersed partially in water appear bent at the water surface? [CBSE 2024, 2020] Answer: Light rays coming from the submerged portion of the pencil travel from optically denser water to rarer air, bending away from the normal. The eye traces them straight back, creating a virtual image higher than the actual position, making the pencil appear bent.
Question 12: What is the relation between the power and focal length of a combination of lenses? [CBSE 2023] Answer: The total optical power of thin lenses placed in contact is the algebraic sum of their individual powers: P = P₁ + P₂ + P₃ + ...
Question 13: Why is a concave lens called a diverging lens? [CBSE 2022] Answer: A concave lens is thinner at the center and thicker at the edges; it refracts parallel incident rays of light outwards, causing them to diverge away from the principal axis.
Question 14: How does the focal length of a lens change when placed in water? [BOARD EXAM FAVORITE] Answer: The refractive index difference between glass and water is smaller than that between glass and air, which reduces the light-bending power of the lens and causes its focal length to increase.
Question 15: What is the focal length of a plane mirror? [CBSE 2024] Answer: The focal length of a plane mirror is Infinity (∞), and its optical power is Zero (0 D).
Mastering the NCERT Solutions for Class 10 Science Chapter 9, “Light – Reflection and Refraction”, equips students with the ray diagram constructions, optical formula substitutions, and sign conventions required for the CBSE Board Examination. Review the mirror and lens formulas, the solved numerical box layouts, and the 15 board-level FAQs above to secure full marks in your physics evaluations.