Reflection — Set 2

Correct Answer: A. 60 cm

• **60 cm** = The image in a plane mirror is 30 cm behind the mirror; the object is 30 cm in front; so the total object-to-image distance is 30 + 30 = 60 cm. • **Object-to-image = 2 × object distance** — for a plane mirror, the separation between object and image is always twice the object distance from the mirror. • 💡 Wrong-option analysis: 50 cm: this has no geometric basis for a plane mirror; 45 cm: this is 1.5 times the object distance, which has no standard derivation here; 30 cm: this is only the image distance from the mirror (equal to object distance), not the object-to-image distance.

Correct Answer: B. 70°

• **70°** = The angle of reflection equals the angle of incidence (35°); the total angle between the two rays is 35° + 35° = 70°. • **Angle between rays = 2i** — the angle between incident and reflected rays is always twice the angle of incidence since both are measured from the same normal. • 💡 Wrong-option analysis: 90°: 90° between rays would require an incidence angle of 45°, not 35°; 35°: this is the angle of incidence alone, not the total angle between the two rays; 55°: this is 90° − 35°, which is the complement of the incidence angle, not the angle between the two rays.

Correct Answer: B. Inverted

• **Inverted** = A real image is formed where reflected rays actually converge; convergence by a concave mirror always produces an image that is inverted (upside-down) relative to the object. • **Obtainable on a screen** — because rays physically meet at the real image position, it can be projected onto a screen, unlike a virtual image. • 💡 Wrong-option analysis: Erect: erect images formed by mirrors are always virtual; real images from concave mirrors are always inverted; Laterally inverted: lateral inversion is a feature of plane mirror images, not of real images from curved mirrors; Always diminished: real images from a concave mirror can be magnified (object between f and 2f), same-size (object at 2f), or diminished (object beyond 2f).

Correct Answer: C. Principal axis

• **Principal axis** = The principal focus is defined as the point on the principal axis where rays parallel to it converge (concave) or appear to diverge from (convex) after reflection. • **F = R/2 along the axis** — the focus is located halfway between the pole and the centre of curvature along the principal axis. • 💡 Wrong-option analysis: Mirror surface only: the focus is a point in front of (or behind for convex) the mirror, not on the mirror surface itself; Pole: the pole is where the principal axis meets the mirror surface, which is a different reference point; Normal: the normal at any point is perpendicular to the surface at that point; only the normal at the pole coincides with the principal axis.

Correct Answer: C. Regular (specular) reflection

• **Regular (specular) reflection** = On a smooth surface, nearby normals are nearly parallel so all reflected rays leave at the same angle, producing a sharp image. • **Mirror-like** — specular reflection preserves the wavefront structure of the incident light, which is why polished metal and glass act as mirrors. • 💡 Wrong-option analysis: Scattering without any rule: scattering violates the law of reflection; smooth surfaces obey it precisely; Refraction: refraction occurs when light passes through a medium boundary, not when it reflects off a surface; Diffuse reflection: diffuse reflection occurs from rough surfaces where normals vary randomly, scattering reflected rays in many directions.

Correct Answer: C. Decibel (dB)

• **Decibel (dB)** = Sound intensity level is a logarithmic measure defined as L = 10 log₁₀(I/I₀), where I₀ = 10⁻¹² W/m²; the unit of this scale is the decibel. • **Logarithmic scale** — a 10 dB increase corresponds to a tenfold increase in intensity; normal conversation is about 60 dB, a jet engine about 140 dB. • 💡 Wrong-option analysis: Joule (J): joules measure energy, not intensity level; intensity level is a dimensionless logarithmic ratio expressed in dB; Newton (N): newtons are the SI unit of force and have no role in measuring sound intensity level; Hertz (Hz): hertz measure frequency (cycles per second), not intensity.

Correct Answer: A. 68 m

• **68 m** = For an echo the sound travels to the wall and back, so d = v × t / 2 = 340 × 0.4 / 2 = 68 m. • **Divide by 2** — the echo time is the round-trip time; dividing by 2 gives the one-way distance to the wall. • 💡 Wrong-option analysis: 34 m: this is d = v × t / 4, which incorrectly halves the distance again; 170 m: this is d = v × t / 2 × 2.5, an arithmetic error; 136 m: this is d = v × t = 340 × 0.4 without dividing by 2, forgetting that sound makes a return trip.

Correct Answer: C. Frequency of light

• **Frequency of light** = Reflection changes the direction of the light ray but does not alter its frequency; the reflected photon has the same energy and colour as the incident photon. • **Colour preserved** — since frequency is unchanged, wavelength in the same medium is also unchanged, which is why objects keep their colour when seen in a mirror. • 💡 Wrong-option analysis: Speed of light in vacuum: the speed in vacuum is always c regardless, but this is a property of vacuum, not something that changes or stays due to reflection; Direction of propagation: direction definitely changes in reflection — that is the whole purpose of a mirror; Path length in space: path length changes because the reflected ray travels in a new direction after the point of incidence.

Correct Answer: D. Pole

• **Pole** = The pole (P) is the geometric centre of the mirror surface, where the principal axis intersects the mirror; all mirror distances are measured from this point. • **Reference point** — the pole is chosen as the origin in Cartesian sign convention; all distances u, v, f, and R are measured from P along the principal axis. • 💡 Wrong-option analysis: Center of curvature: the centre of curvature (C) is a separate point located at distance R from the pole, not on the mirror surface itself; Focus: the focus (F) is at distance f = R/2 from the pole, not where the axis meets the surface; Aperture: aperture refers to the diameter of the mirror, not a specific point on it.

Correct Answer: D. Angle of incidence

• **Angle of incidence** = The angle of incidence (i) is measured between the incoming ray and the normal drawn at the point where the ray strikes the surface. • **Normal reference** — using the normal rather than the surface itself as reference is standard; the surface angle is the complement (90° − i) of the incidence angle. • 💡 Wrong-option analysis: Angle of reflection: the angle of reflection is the angle between the reflected ray and the normal, not the incident ray; Angle of deviation: the deviation angle is the angle between the original direction of incidence and the final direction of the reflected ray; Angle of polarization: the polarization angle (Brewster angle) is a special incidence angle related to polarisation of reflected light, not the general incidence angle.