CBSE Class 10 Science
Board Paper & Answer Key 2025

(a) Speed: Speed of light in medium X is greater than in medium Y. When light travels from an optically denser to an optically rarer medium, it speeds up and bends away from the normal.

(b) Optical Density: Medium X is optically rarer compared to medium Y. A ray bends away from normal only when it enters a medium with a lower refractive index (lower optical density) from a medium with a higher refractive index.

(i) To see nearby objects clearly — process called Accommodation:

• Ciliary muscles contract
• Eye lens becomes thicker and more convex (curvature increases)
• Focal length decreases
• Increased converging power focuses diverging rays from nearby object onto the retina

(ii) Range of Accommodation: Far point = Infinity (relaxed eye). Near point = 25 cm. The eye can increase its optical power by 4 Dioptres (P = 1/0.25 = +4D).

The old man suffers from Presbyopia (near point shifted to 1 m). A convex (converging) lens is used to correct this. The ray diagram shows: rays from an object at 25 cm appear to diverge, the convex lens converges them so they appear to come from 1 m (the person's near point), making the image fall on the retina.

(a) Ray diagram labels: PE = Incident ray, EF = Refracted ray (inside prism), FS = Emergent ray. ∠i = angle of incidence, ∠r = angle of refraction, ∠e = angle of emergence, ∠A = angle of prism, ∠d = angle of deviation.

(b) The angle of deviation remains unchanged when the path is reversed.

According to the Principle of Reversibility of Light, the path of a light ray is reversible. If a ray travels from A to B through a prism, it will follow the same path when reversed. Since angles of incidence and refraction remain the same, the angle of deviation does not change.

Given: h = 6 cm, u = –30 cm, f = –15 cm (concave lens)

(a) Using lens formula: 1/f = 1/v – 1/u

1/(–15) = 1/v – 1/(–30)  ⇒  1/v = 1/(–15) – 1/30 = –2/30 – 1/30 = –3/30

v = –10 cm (image is on the same side as the object)

(b) m = v/u = (–10)/(–30) = 1/3

h' = m × h = 1/3 × 6 = 2 cm

The image is virtual, erect and diminished (formed at 10 cm from optical centre).

(a) The heater draws a large current when switched on. This causes a significant voltage drop across the internal resistance of the supply line. Since the bulb is in parallel, the available voltage across the bulb decreases. By P = V²/R, reduced voltage → reduced power → bulb glows dim.

(b) Connect two 8 Ω resistors in parallel, then connect the third 8 Ω resistor in series with this combination.

Parallel part: 1/Rₚ = 1/8 + 1/8 = 2/8 → Rₚ = 4 Ω

Series with third: Rs = 4 + 8 = 12 Ω ✓

⊢─[8Ω]─⊣
⊢─────⊢─[8Ω]─⊣ = 4Ω + 8Ω = 12Ω
⊢─[8Ω]─⊣

(a) R ∝ l → doubling length doubles resistance. I = V/R → current becomes half. Ammeter reading = X/2 A

(b) R ∝ 1/A → doubling area halves resistance. I = V/R → current becomes double. Ammeter reading = 2X A

(c) Resistivity (ρ): The resistance offered by a conductor of unit length and unit cross-sectional area. Formula: ρ = RA/l. SI unit: Ω·m (ohm-metre).

Comparison: An alloy has higher resistivity than its constituent metals. (Example: Nichrome has higher resistivity than nickel and chromium.)

(i) Tungsten for lamp filaments: Very high melting point (≈3400°C) — can be heated to white-hot without melting. High resistivity and low evaporation rate at high temperature — glows brightly for a long time.

(ii) Alloy for bread-toasters: Alloys have much higher resistivity than pure metals → produce more heat for same current. High melting point and do not oxidise easily at high temperature → can be heated repeatedly without damage.

(i) Fleming's Left-Hand Rule: Hold the thumb, forefinger and middle finger of the left hand mutually perpendicular. Align forefinger → direction of magnetic field; middle finger → direction of current. Thumb gives the direction of force on the conductor.

(ii) Two devices: Electric Motor, Loudspeaker.

(iii) Case I (Perpendicular): Electron moves perpendicular to magnetic field → current is also perpendicular → Force is maximum.

Case II (Parallel): Electron moves parallel to magnetic field → Force is zero. (Force = Bqv sinθ; θ=0° → sin0°=0 → F=0)

(i) Three reasons for short-circuiting:
1. Damage to insulation of wires — live and neutral come in contact.
2. Loose or faulty wiring connections.
3. Faulty or damaged electrical appliances — internal defects create direct connection.

(ii) Fuse in overload: Excessive current generates heat that melts the fuse wire, breaking the circuit and stopping current before it damages wiring or appliances.

(iii) Fuse rating for 220 V, 2200 W heater:
I = P/V = 2200/220 = 10 A. Minimum fuse rating = 10 A.