CBSE Class 10 Physics Mixed Topics Test Paper MCQs Set 12

Answer: (b) electromagnetic waves

Explanation:
1. Light is an electromagnetic wave. It needs no medium and travels fastest in vacuum, with different speeds in different media.

Teacher's Note:
1. Mechanical waves such as sound need a medium.
2. Light can travel through empty space from the sun.

Answer: (c) 36°

Explanation:
1. Number of images = 360°/θ − 1 when 360°/θ is an even whole number. So 9 = 360°/θ − 1, giving 360°/θ = 10 and θ = 36°.

Teacher's Note:
1. Check: 90° gives 3 images, 60° gives 5 and 72° gives 4.
2. Use the formula n = 360°/θ − 1.

Answer: (a) at 2F

Explanation:
1. For an object at 2F the lens equation gives v = 2F: the image is real, inverted and the same size as the object.

Teacher's Note:
1. An object at F gives an image at infinity.
2. Remember the symmetrical case: object at 2F, image at 2F.

Answer: (d) both (a) and (b)

Explanation:
1. In a longitudinal wave, the wavelength is the distance between two successive compressions or two successive rarefactions. The compression-to-rarefaction distance is only half a wavelength.

Teacher's Note:
1. Half-wavelength links compression to rarefaction.
2. Both (a) and (b) give the same length.

Answer: (d) is both proportional to the area and inversely proportional to the distance

Explanation:
1. C = ε₀A/d, so capacitance grows with plate area A and falls with separation d.

Teacher's Note:
1. A medium between the plates raises C further.
2. Statement (c) is the reverse of the truth.

Answer: (a) A plane mirror forms only virtual images of real objects

Explanation:
1. The image in a plane mirror is virtual, erect, laterally inverted, of the same size and as far behind the mirror as the object is in front.

Teacher's Note:
1. A virtual image cannot be caught on a screen.
2. Plane mirrors cannot form real images of real objects.

Answer: (b) Light travels faster in air than in glass

Explanation:
1. Glass is optically denser than air, and the speed of light in glass (about 2 × 10⁸ m/s) is less than in air (about 3 × 10⁸ m/s).

Teacher's Note:
1. Refractive index = c/v, so larger refractive index means slower light.
2. Light bends towards the normal when it slows down.

Answer: (c) depends on the focal lengths of both the objective and the eyepiece

Explanation:
1. The total magnification of a compound microscope is the product of the magnification of the objective and that of the eyepiece, so it depends on both.

Teacher's Note:
1. Short focal lengths of both lenses give high magnification.
2. A telescope has a similar two-lens structure.

Answer: (b) one-fourth

Explanation:
1. Force between poles follows an inverse-square law, F ∝ 1/d². If d is doubled, F becomes 1/4.

Teacher's Note:
1. The same law holds for electric charges.
2. Inverse square means halving distance gives four times force.

Answer: (a) potential difference and current

Explanation:
1. Ohm's law states that at constant temperature the current through a conductor is proportional to the potential difference across it: V = IR.

Teacher's Note:
1. The ratio V/I is the resistance.
2. It holds for metallic conductors at constant temperature.

Answer: (b) principal focus and centre of curvature

Explanation:
1. A concave mirror forms a real, inverted and magnified image when the object lies between F and C. Between P and F the image is virtual and erect.

Teacher's Note:
1. Beyond C the image is real but diminished.
2. At C the image is real and the same size.

Answer: (a) converging, diverging

Explanation:
1. A convex lens brings parallel rays to a focus (converging), while a concave lens spreads them out (diverging).

Teacher's Note:
1. A convex lens is thicker at the middle.
2. Use convex lens for a magnifying glass.

Answer: (a) myopia, hypermetropia

Explanation:
1. In short sight (myopia) distant objects are blurred and a concave lens corrects it. In long sight (hypermetropia) near objects are blurred and a convex lens corrects it.

Teacher's Note:
1. Myopia means the image forms in front of the retina.
2. Match each defect with the correct lens.

Answer: (a) faster

Explanation:
1. Sound travels at about 1500 m/s in water and about 340 m/s in air, because the particles of a liquid are closer together and transmit the disturbance faster.

Teacher's Note:
1. Sound travels fastest in solids.
2. It cannot travel in vacuum.

Answer: (a) positive

Explanation:
1. Glass loses electrons to silk, so the glass becomes positively charged and the silk negatively charged.

Teacher's Note:
1. Ebonite rubbed with fur becomes negative.
2. Electrons are the charges that move.