Class 10 Science Chapter 9 Light – Reflection and Refraction NCERT Solutions 2026 PDF Download

Ans: The principal focus of a concave mirror is a point on its principal axis where light rays coming parallel to the principal axis actually meet (converge) after reflection.
[Image of Concave Mirror Focus]

Ans: 1. It always forms an erect (upright) and diminished image.
2. It has a wider field of view (it is curved outwards), allowing the driver to see a larger area of the traffic behind.

Ans: Given: Magnification ($m$) = -3 (Real image is inverted), Object distance ($u$) = -10 cm.
Formula: $m = -\frac{v}{u}$
Calculation: $-3 = -\frac{v}{-10} \Rightarrow -3 = \frac{v}{10} \Rightarrow v = -30 \text{ cm}$.
Result: The image is located at 30 cm in front of the mirror.

Ans: Refractive Index ($n_g$) = $\frac{\text{Speed in Vacuum (c)}}{\text{Speed in Glass (v)}}$
$1.50 = \frac{3 \times 10^8}{v}$
$v = \frac{3 \times 10^8}{1.5} = 2 \times 10^8 \text{ m/s}$.

Ans: Focal length of concave lens is negative. So, $f = -2 \text{ m}$.
Power ($P$) = $\frac{1}{f} = \frac{1}{-2} = -0.5 \text{ Dioptre (D)}$.

Ans: $u = -10 \text{ cm}, f = +15 \text{ cm}$ (Convex mirror).
$\frac{1}{v} + \frac{1}{u} = \frac{1}{f} \Rightarrow \frac{1}{v} - \frac{1}{10} = \frac{1}{15}$
$\frac{1}{v} = \frac{1}{15} + \frac{1}{10} = \frac{2+3}{30} = \frac{5}{30} = \frac{1}{6}$
$v = +6 \text{ cm}$.
Since $v$ is positive, the image is formed behind the mirror. It is virtual and erect.

Ans: A Convex Lens of focal length 1 meter has a power of +1 Dioptre.
Definition: 1 Dioptre is the power of a lens whose focal length is 1 meter.

Ans: $P = +1.5 \text{ D}$.
$f = \frac{1}{P} = \frac{1}{1.5} = \frac{10}{15} = 0.67 \text{ meters} = +67 \text{ cm}$.
Since power is positive, it is a Convex (Converging) lens.

Ans:

• Real Image: Formed when light rays actually meet. Can be obtained on a screen. Always inverted. (e.g., Cinema screen).
• Virtual Image: Formed when light rays appear to meet. Cannot be obtained on a screen. Always erect. (e.g., Plane mirror).

Ans: This is due to Refraction. Light rays coming from the part of the pencil inside water bend away from the normal as they move from water (denser) to air (rarer). Our eyes project these rays backwards, making the pencil appear raised/bent at the interface.

Ans: 1. The incident ray, the refracted ray, and the normal to the interface at the point of incidence all lie in the same plane.
2. 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 (Snell's Law): $\frac{\sin i}{\sin r} = \text{constant}$.

Ans: It means that the speed of light in diamond is $\frac{1}{2.42}$ times the speed of light in a vacuum. It indicates that diamond is optically very dense and slows down light significantly.

Ans:
Image Nature: Real, Inverted, and Magnified.
Position: Beyond 2F.

Ans: $h = +5, u = -25, f = +10$ (Convex).
Formula: $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
$\frac{1}{v} = \frac{1}{10} + \frac{1}{-25} = \frac{5-2}{50} = \frac{3}{50}$.
$v = \frac{50}{3} = +16.67 \text{ cm}$. (Real image).
Magnification: $m = \frac{v}{u} = \frac{16.67}{-25} = -0.66$.
Size ($h'$) = $m \times h = -0.66 \times 5 = -3.33 \text{ cm}$.
Result: Image is Real, Inverted, diminished (3.33 cm high), formed 16.67 cm behind the lens.

Ans: [Image of Refraction through Glass Slab]
Light refracts twice: once entering the glass (Air to Glass, bends towards normal) and once leaving it (Glass to Air, bends away from normal).
Since the two refracting surfaces are parallel, the bending effect at the first surface is exactly reversed at the second. Hence, the emergent ray is parallel to the incident ray but laterally displaced.

Ans:

• Concave Mirror: Can form both Real (inverted) and Virtual (erect) images depending on object position. Can magnify or diminish. Used in torches, shaving mirrors.
• Convex Mirror: Always forms Virtual, Erect, and Diminished images regardless of object position. Used as rear-view mirrors.

Ans: A concave lens diverges light rays. Rays coming from an object never actually meet on the other side. They always appear to diverge from a point on the same side as the object (Focus). Therefore, the image formed is always Virtual, Erect, and Diminished.
[Image of Concave Lens Ray Diagram]

Ans: Speed of light is inversely proportional to refractive index ($v \propto \frac{1}{n}$).
Water has the lowest refractive index (1.33). Therefore, light travels fastest in Water.

Ans: It is the perpendicular distance between the incident ray and the emergent ray in a glass slab.
Factors: (i) Thickness of slab, (ii) Angle of incidence, (iii) Refractive index of material.

Ans: Magnification +1 means the image is Virtual, Erect (+), and the same size as the object (1). This is characteristic of a Plane Mirror.

Ans: Refractive Index $n = \frac{c}{v}$. Since the speed of light in vacuum ($c$) is the maximum possible speed in the universe, the speed in any medium ($v$) will always be less than $c$. Hence, the ratio is always $>1$.

Ans: The refractive index of a medium with respect to vacuum (or air) is called its absolute refractive index.

Ans: $f = \frac{1}{P} = \frac{1}{-2} = -0.5 \text{ m} = -50 \text{ cm}$.
Negative sign indicates it is a Concave Lens.