Chapter 10 — Light: Reflection and Refraction
Welcome to HSLC Guru! This page contains complete textbook question answers, additional practice questions, glossary, and formula table for Class 10 Science Chapter 10 — Light: Reflection and Refraction (ASSEB Board, English Medium). Light is one of the most fascinating topics in physics. In this chapter, we study how light behaves when it hits a polished surface (reflection) and when it travels from one transparent medium to another (refraction). You will also learn about spherical mirrors, lenses, the mirror formula, the lens formula, magnification, and the power of a lens. Practice the ray diagrams and numerical problems carefully — they appear regularly in HSLC examinations.
Chapter Summary
Light and Reflection: Light is a form of energy that produces the sensation of vision. It travels in straight lines and has a very high speed (3 × 10⁸ m/s in vacuum). When light falls on a polished surface and bounces back into the same medium, the phenomenon is called reflection. The two laws of reflection state that (i) the angle of incidence is equal to the angle of reflection, and (ii) the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane. These laws apply to every type of reflecting surface, whether plane or spherical.
Spherical Mirrors: A spherical mirror is a part of a hollow sphere whose one side is silvered. There are two types — concave mirror (reflecting surface curved inward) and convex mirror (reflecting surface curved outward). Important terms include pole (P), centre of curvature (C), radius of curvature (R), principal axis, principal focus (F), and focal length (f). The relation R = 2f connects the radius of curvature with the focal length. Ray diagrams are drawn using two principal rays to find the position, size, and nature of the image. The mirror formula is 1/v + 1/u = 1/f and magnification is m = −v/u = h’/h. The New Cartesian Sign Convention is used: distances measured from the pole, with the principal axis as the x-axis.
Refraction of Light: When light passes obliquely from one transparent medium to another, it changes its direction at the interface. This bending of light is called refraction. It occurs because the speed of light is different in different media. The two laws of refraction are: (i) the incident ray, the refracted ray, and the normal at the point of incidence lie in the same plane; and (ii) Snell’s law — the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media (sin i / sin r = n₂₁). The refractive index of a medium with respect to vacuum is called the absolute refractive index and is given by n = c/v, where c is the speed of light in vacuum and v is its speed in the medium.
Lenses and Power: A lens is a transparent material bounded by two surfaces, of which at least one is curved. Convex (converging) lenses are thicker in the middle, while concave (diverging) lenses are thicker at the edges. The lens formula is 1/v − 1/u = 1/f and magnification is m = v/u = h’/h. The power of a lens is the reciprocal of its focal length in metres, P = 1/f, and is measured in dioptre (D). Convex lenses have positive power; concave lenses have negative power. Mirrors and lenses have many practical uses — concave mirrors are used in shaving mirrors, headlights, and solar furnaces; convex mirrors as rear-view mirrors; convex lenses in cameras, microscopes, and the human eye; and concave lenses in spectacles to correct myopia.
Textbook Question and Answers
A. Very Short Answer Type Questions (1 mark)
Q1. What is the SI unit of power of a lens?
Answer: The SI unit of power of a lens is dioptre, denoted by D. 1 D = 1 m⁻¹.
Q2. Define the principal focus of a concave mirror.
Answer: The principal focus of a concave mirror is the point on the principal axis where rays of light parallel to the principal axis converge after reflection from the mirror.
Q3. What is the relation between focal length and radius of curvature of a spherical mirror?
Answer: The focal length is half of the radius of curvature, i.e. f = R/2 or R = 2f.
Q4. Name the type of mirror used as a rear-view mirror in vehicles.
Answer: A convex mirror is used as a rear-view mirror in vehicles because it gives an erect, diminished image and provides a wider field of view.
Q5. What is the refractive index of a medium?
Answer: The refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium, n = c/v.
Q6. State Snell’s law of refraction.
Answer: Snell’s law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media: sin i / sin r = n₂₁.
Q7. Why do we use a convex lens in a magnifying glass?
Answer: A convex lens forms a virtual, erect, and magnified image of an object placed between the lens and its principal focus, so it is used in a magnifying glass.
Q8. What is the sign of the focal length of a concave lens?
Answer: The focal length of a concave lens is taken as negative according to the New Cartesian Sign Convention.
Q9. What is meant by 1 dioptre?
Answer: 1 dioptre is the power of a lens whose focal length is 1 metre. So 1 D = 1 m⁻¹.
Q10. Why does light bend on entering water from air?
Answer: Light bends because its speed decreases when it travels from a rarer medium (air) to a denser medium (water). This change in speed at the boundary causes refraction.
B. Short Answer Type Questions (2-3 marks)
Q1. State the two laws of reflection of light.
Answer: The two laws of reflection are:
- The angle of incidence is equal to the angle of reflection (∠i = ∠r).
- The incident ray, the reflected ray, and the normal to the reflecting surface at the point of incidence all lie in the same plane.
These laws are valid for all reflecting surfaces — plane as well as spherical.
Q2. Distinguish between a concave mirror and a convex mirror.
Answer:
| Concave Mirror | Convex Mirror |
|---|---|
| Reflecting surface curves inward. | Reflecting surface curves outward. |
| It is a converging mirror. | It is a diverging mirror. |
| Focal length is taken as negative. | Focal length is taken as positive. |
| Forms both real and virtual images. | Always forms a virtual, erect, diminished image. |
| Used in headlights, shaving mirrors, solar furnaces. | Used as rear-view mirror, in street lamps. |
Q3. Why is a convex mirror preferred as a rear-view mirror in vehicles?
Answer: A convex mirror is preferred as a rear-view mirror because:
- It always produces an erect (upright) image of the objects behind.
- It forms a diminished image, so a wider area can be seen in a small mirror.
- Its field of view is much larger than that of a plane mirror.
Q4. Define refractive index. Why has it no unit?
Answer: The refractive index of a medium is the ratio of the speed of light in vacuum (c) to the speed of light in that medium (v). It is given by n = c/v. Since both c and v are speeds and have the same unit (m/s), their ratio is dimensionless. Hence, refractive index has no unit.
Q5. The refractive index of glass is 1.5 and that of water is 1.33. In which medium does light travel faster, and by how much?
Answer: Speed in glass: v₁ = c/n₁ = (3 × 10⁸)/1.5 = 2 × 10⁸ m/s. Speed in water: v₂ = c/n₂ = (3 × 10⁸)/1.33 ≈ 2.26 × 10⁸ m/s. Light travels faster in water than in glass. Difference ≈ 0.26 × 10⁸ m/s = 2.6 × 10⁷ m/s.
Q6. Define power of a lens. State its SI unit. What is the power of a convex lens of focal length 50 cm?
Answer: The power of a lens is the reciprocal of its focal length expressed in metres: P = 1/f(m). Its SI unit is the dioptre (D). For f = 50 cm = 0.5 m, P = 1/0.5 = +2 D.
C. Long Answer Type Questions (5-6 marks)
Q1. With the help of a ray diagram, explain the formation of an image by a concave mirror when the object is placed between the focus (F) and the centre of curvature (C). State the nature, size, and position of the image.
Answer: When an object AB is placed between F and C of a concave mirror, two principal rays are drawn:
- A ray parallel to the principal axis from B passes through F after reflection.
- A ray passing through C from B is reflected back along the same path.
The two reflected rays meet at a point B’ beyond C. From B’ a perpendicular A’B’ is drawn on the principal axis, which is the image of AB.
Position of the image: Beyond C. Nature: Real and inverted. Size: Magnified (larger than the object). This property is used in projectors and headlights of vehicles.
Q2. Draw ray diagrams to show the image formation by a convex lens when the object is placed (a) at 2F₁, (b) between F₁ and the optical centre. State the nature of the image in each case.
Answer:
(a) Object at 2F₁: One ray parallel to principal axis passes through F₂ after refraction. Another ray through the optical centre passes undeviated. The two rays meet at 2F₂. Nature of image: Real, inverted, same size as object, located at 2F₂ on the other side of the lens.
(b) Object between F₁ and optical centre: A ray parallel to the axis passes through F₂ after refraction. A ray through the optical centre passes undeviated. The two refracted rays appear to diverge from a point on the same side of the lens as the object. Nature of image: Virtual, erect, magnified, on the same side as the object. This is the principle of the magnifying glass.
Q3. An object 4 cm in size is placed at a distance of 25 cm from a concave mirror of focal length 15 cm. Find the position, nature, and size of the image.
Answer: Given: h = 4 cm, u = −25 cm, f = −15 cm.
Mirror formula: 1/v + 1/u = 1/f → 1/v = 1/f − 1/u = 1/(−15) − 1/(−25) = −1/15 + 1/25.
1/v = (−5 + 3)/75 = −2/75 → v = −37.5 cm. The image is formed 37.5 cm in front of the mirror (real and inverted).
Magnification: m = −v/u = −(−37.5)/(−25) = −1.5. Image height: h’ = m × h = −1.5 × 4 = −6 cm. So the image is real, inverted, magnified, 6 cm in size, formed at 37.5 cm in front of the mirror.
Q4. A convex lens forms a real image of an object placed at a distance of 30 cm in front of it. The image is formed at 60 cm on the other side of the lens. Find the focal length and the power of the lens.
Answer: Given: u = −30 cm, v = +60 cm.
Lens formula: 1/v − 1/u = 1/f → 1/f = 1/60 − 1/(−30) = 1/60 + 1/30 = (1 + 2)/60 = 3/60 = 1/20.
Therefore f = +20 cm = 0.20 m. Power P = 1/f(m) = 1/0.20 = +5 D. Magnification m = v/u = 60/(−30) = −2 (image is real, inverted, magnified twice).
Q5. Derive the relation between the focal length and the radius of curvature of a spherical mirror.
Answer: Consider a concave mirror of small aperture. Let a ray AB parallel to the principal axis strike the mirror at B and be reflected through the focus F. The line CB is the normal at B (since C is the centre of curvature). By the law of reflection, ∠ABC = ∠CBF. Also, ∠ABC = ∠BCP (alternate angles), so ∠BCP = ∠CBF. Therefore the triangle BCF is isosceles, giving BF = CF.
For small aperture, B lies very close to P, so BF ≈ PF = f and CF = CP − PF = R − f. From BF = CF: f = R − f → 2f = R → f = R/2. Hence the focal length of a spherical mirror is half its radius of curvature.
Additional Practice Questions
Multiple Choice Questions (MCQs)
Q1. The image formed by a plane mirror is:
(a) real and inverted (b) virtual and erect (c) real and magnified (d) virtual and diminished
Answer: (b) virtual and erect.
Q2. A concave mirror has focal length 20 cm. Its radius of curvature is:
(a) 10 cm (b) 20 cm (c) 40 cm (d) 80 cm
Answer: (c) 40 cm.
Q3. The mirror used in solar concentrators and headlights is:
(a) plane (b) convex (c) concave (d) cylindrical
Answer: (c) concave.
Q4. The refractive index of a medium of speed of light 2 × 10⁸ m/s is:
(a) 0.67 (b) 1.0 (c) 1.5 (d) 2.0
Answer: (c) 1.5.
Q5. Power of a lens is +2 D. The lens is:
(a) concave with f = 50 cm (b) convex with f = 50 cm (c) convex with f = 2 m (d) concave with f = 2 m
Answer: (b) convex with f = 50 cm.
Q6. A ray of light strikes a plane mirror at an angle of 30° with the mirror surface. The angle of reflection is:
(a) 30° (b) 60° (c) 90° (d) 120°
Answer: (b) 60° (angle of incidence is measured from the normal, so i = 90° − 30° = 60°, and r = 60°).
Q7. A convex lens forms an image of the same size as the object when the object is placed at:
(a) F₁ (b) 2F₁ (c) infinity (d) optical centre
Answer: (b) 2F₁.
Q8. Magnification produced by a plane mirror is:
(a) less than 1 (b) more than 1 (c) equal to 1 (d) zero
Answer: (c) equal to 1.
Q9. A concave lens always forms an image which is:
(a) real, inverted (b) virtual, erect, diminished (c) real, magnified (d) virtual, magnified
Answer: (b) virtual, erect, diminished.
Q10. Speed of light is maximum in:
(a) water (b) glass (c) diamond (d) vacuum
Answer: (d) vacuum.
Fill in the Blanks
Q1. The angle of incidence is always ______ to the angle of reflection.
Answer: equal.
Q2. The focal length of a spherical mirror is ______ of its radius of curvature.
Answer: half.
Q3. The SI unit of power of a lens is ______.
Answer: dioptre.
Q4. A ______ lens always produces a virtual, erect, and diminished image.
Answer: concave.
Q5. The bending of light when it passes from one medium to another is called ______.
Answer: refraction.
True or False
Q1. A convex mirror always forms a real image.
Answer: False. A convex mirror always forms a virtual, erect, and diminished image.
Q2. Light travels faster in a denser medium than in a rarer medium.
Answer: False. Light travels slower in a denser medium.
Q3. The power of a concave lens is negative.
Answer: True. Because its focal length is negative.
Q4. The mirror formula is 1/v − 1/u = 1/f.
Answer: False. The mirror formula is 1/v + 1/u = 1/f.
Q5. Refractive index has no unit because it is a ratio of two similar quantities.
Answer: True.
Glossary
| Term | Meaning |
|---|---|
| Reflection | Bouncing back of light from a polished surface into the same medium. |
| Refraction | Bending of light when it passes from one transparent medium to another. |
| Pole (P) | The geometrical centre of the reflecting surface of a spherical mirror. |
| Centre of Curvature (C) | The centre of the sphere of which the mirror is a part. |
| Radius of Curvature (R) | The radius of the sphere of which the mirror is a part. |
| Principal Axis | The straight line passing through the pole and centre of curvature. |
| Principal Focus (F) | The point where parallel rays converge (concave) or appear to diverge (convex) after reflection. |
| Focal Length (f) | The distance between the pole and the principal focus. |
| Magnification (m) | The ratio of the height of the image to the height of the object. |
| Refractive Index | Ratio of speed of light in vacuum to speed of light in the medium. |
| Optical Centre | The geometrical centre of a lens through which a ray passes undeviated. |
| Power of a Lens | Reciprocal of the focal length of the lens in metres; measured in dioptre. |
| Dioptre (D) | SI unit of power of a lens; 1 D = 1 m⁻¹. |
Important Formulae
| Quantity | Formula | Remark |
|---|---|---|
| Focal length and radius | f = R/2 | For spherical mirrors and lenses (small aperture). |
| Mirror formula | 1/v + 1/u = 1/f | Cartesian sign convention applied. |
| Magnification (mirror) | m = −v/u = h’/h | Negative m means real, inverted image. |
| Snell’s law | sin i / sin r = n₂₁ | Constant for a given pair of media. |
| Refractive index | n = c/v | Higher n means denser optical medium. |
| Lens formula | 1/v − 1/u = 1/f | Sign of f decides type of lens. |
| Magnification (lens) | m = v/u = h’/h | Negative m → real, inverted image. |
| Power of a lens | P = 1/f (m) | Unit: dioptre (D); +ve for convex, −ve for concave. |
This completes the Class 10 Science Chapter 10 — Light: Reflection and Refraction notes for ASSEB Board. For more chapter-wise notes, MCQs, and previous year questions, keep visiting HSLC Guru. Practice the ray diagrams on a separate sheet and solve at least five numericals daily for full mastery.