Light - Reflection and Mirrors PYQs – Physics, SSC & Railway

Answer: Option 3 — In front of the mirror
A concave mirror's reflecting surface curves inward, so the centre of the sphere it is cut from lies on the reflecting side — in front of the mirror.
C lies at a distance R = 2f from the pole, beyond the focus.
Revision note — Spherical mirror terms
• Pole (P): centre of the mirror's surface; principal axis: line through P and C.
• Centre of curvature (C): centre of the sphere of which the mirror is a part — in front for a concave mirror, behind for a convex mirror.
• Radius of curvature (R): distance PC; focal length f = R/2.
• Principal focus (F): lies midway between P and C; real for concave, virtual for convex.
• A ray through C strikes the mirror along the normal and retraces its path.
• Sign convention: for a concave mirror f and R are negative (measured against the incident light).
Do not mix these up
• Concave vs convex — C and F are in front of a concave mirror, behind a convex mirror.
Also asked from this topic: the path of a ray passing through the centre of curvature (it retraces its path); where the focus of a convex mirror of R = 50 cm lies (25 cm behind the mirror).

Answer: Option 3 — real, inverted and diminished
With the object beyond C, a concave mirror forms the image between F and C — real, inverted and diminished.
Revision note — Image formation by a concave mirror
• Object at infinity → image at F; real, inverted, highly diminished (point-sized).
• Object beyond C → image between F and C; real, inverted, diminished.
• Object at C → image at C; real, inverted, same size.
• Object between C and F → image beyond C; real, inverted, enlarged.
• Object at F → image at infinity; real, inverted, highly enlarged.
• Object between P and F → image behind the mirror; virtual, erect, enlarged (shaving / make-up mirror).
• R = 2f; a concave mirror is converging — used in torches, headlights, solar furnaces, dentists' mirrors.
Do not mix these up
• Beyond C vs between C and F — diminished vs enlarged; the image and object "swap" regions.
Also asked from this topic: the image when the object is between C and F (real, inverted and enlarged, beyond C); the change as the object moves from beyond C towards F (image grows and moves away).

Answer: Option 1 — behind the mirror
A convex mirror makes parallel rays diverge; the reflected rays only appear to come from a point behind the mirror.
So its principal focus is virtual and lies behind the mirror, midway between the pole and the centre of curvature.
Revision note — Spherical mirror terms (Class 10)
• Pole (P) — centre of the reflecting surface; principal axis — line through P and C.
• Centre of curvature (C) — centre of the sphere of which the mirror is a part; radius of curvature R = PC.
• Focal length f = R/2.
• Concave mirror: focus is real, in front of the mirror (rays actually meet there).
• Convex mirror: focus and C are behind the mirror; image always virtual, erect, diminished.
• Sign convention: f is negative for concave and positive for convex mirrors.
Also asked from this topic: where a parallel beam appears to diverge from for a convex mirror of R = 50 cm (25 cm behind the mirror); the nature of image in a convex mirror (virtual and diminished).

Answer: Option 1 — At the centre of curvature
With the object at C, the image forms at C itself — real, inverted and the same size as the object.
By the mirror formula, u = −2f gives v = −2f.
Revision note — Image formation by a concave mirror (Class 10)
• Object at infinity → image at F, highly diminished, real, inverted.
• Beyond C → between F and C, diminished, real, inverted.
• At C → at C, same size, real, inverted.
• Between C and F → beyond C, enlarged, real, inverted.
• At F → at infinity, highly enlarged.
• Between P and F → behind the mirror, enlarged, virtual and erect (shaving / make-up mirror).
Also asked from this topic: the image when the object is between C and F (real, inverted, enlarged beyond C); where to keep the face in a shaving mirror (between P and F).

Answer: Option 2 — Measure the distance between the mirror and the image of a distant object formed by it.
Rays from a distant object (a far tree, the Sun) arrive almost parallel to the principal axis.
A concave mirror brings parallel rays to a point at its principal focus, so the sharp image on a screen lies at F.
The mirror-to-screen distance is therefore the focal length.
Revision note — Concave mirror and focal length
• Object at infinity ⇒ image at F, real, inverted and highly diminished (point-sized).
• Focal length f = R/2, where R is the radius of curvature; F lies midway between P and C.
• Object at C ⇒ image at C, real, inverted, same size.
• Object between P and F ⇒ image virtual, erect, magnified, behind the mirror (shaving / make-up mirror).
• Concave mirrors converge light, so they are used in solar furnaces, torches / headlights (bulb at F) and by dentists.
• Sign convention: f of a concave mirror is negative.
Do not mix these up
• Focal length vs radius of curvature — the pole-to-C distance is R = 2f, not f.
Also asked from this topic: why concave mirrors are used in solar furnaces (they converge sunlight to the focus); object position for a virtual, erect, magnified image (between P and F).

Answer: Option 1 — Focal length does not vary in any media
A mirror works by reflection, and the angle of reflection equals the angle of incidence in every medium.
Its focal length depends only on its shape: f = R/2 — so it stays the same in air, water or vacuum.
Revision note — Spherical mirrors
• Pole (P) — centre of the mirror surface; centre of curvature (C) — centre of the sphere it is part of.
• Radius of curvature (R) = PC; focal length (f) = PF; R = 2f for mirrors of small aperture.
• Concave mirror — converging; rays parallel to the axis meet at a real focus in front of the mirror.
• Convex mirror — diverging; the focus is virtual, behind the mirror.
• Mirror formula 1/v + 1/u = 1/f; magnification m = −v/u.
• Uses of concave mirrors: torches and headlights, shaving mirrors, dentist's mirror, solar furnaces; convex mirrors: rear-view mirrors (wider field of view).
Do not mix these up
• Mirror vs lens — a lens works by refraction, so its focal length changes with the surrounding medium (a glass lens in water has a longer focal length); a mirror's focal length does not.
Also asked from this topic: where a parallel beam focuses for a convex mirror of radius of curvature 50 cm (25 cm behind the mirror).