Mirrors and Image Formation
~9 min read
- Mirror types: Plane (flat), concave (converging), convex (diverging).
- Plane mirror: Image is virtual, erect, of same size, laterally inverted, and behind mirror at equal distance.
- Mirror formula: 1/v + 1/u = 1/f. Magnification m = -v/u = h_image/h_object.
Mirrors form images by reflection. CDS/OTA tests image properties for each mirror type, sign conventions, and real-world uses like vehicle side-view mirrors, dental mirrors and reflecting telescopes.
Laws of Reflection
- 1st law: Angle of incidence = angle of reflection (i = r).
- 2nd law: Incident ray, reflected ray and normal lie in the same plane.
- Laws hold for both plane and curved mirrors.
Plane Mirror
- Image is virtual, erect, of same size as object.
- Image distance = object distance (behind mirror).
- Image is laterally inverted (left becomes right).
- To see the full image of yourself, mirror needs to be at least half your height.
Concave and Convex Mirrors
| Feature | Concave (converging) | Convex (diverging) |
|---|---|---|
| Reflecting surface | Inner / hollow side | Outer / bulged side |
| Focal length | Positive (real focus) | Negative (virtual focus) |
| Nature of image | Real or virtual, depends on object distance | Always virtual, erect, diminished |
| Field of view | Narrow | Wide |
| Uses | Shaving mirror, dental mirror, head-light reflector, solar cooker, reflecting telescope | Vehicle rear-view mirror, blind-corner security mirror, street-light reflector |
Concave mirror — image varies with object position (beyond C, at C, between C and F, at F, between F and pole).
Mirror Formula and Sign Convention
- Mirror formula: 1/v + 1/u = 1/f.
- Magnification: m = -v/u = h'/h. Negative m → real, inverted image.
- Cartesian sign convention: distances measured from pole; +ve in direction of incident light (towards object). u is negative; for concave, f is negative; for convex, f is positive (in standard Indian NCERT convention).
- Radius of curvature R = 2f.
CDS/OTA PYQ Examples
Q: The image formed by a plane mirror is:
(a) Real and inverted (b) Virtual and erect (c) Real and erect (d) Virtual and inverted
Answer: (b) Virtual and erect, same size, laterally inverted.
Q: A convex mirror always forms an image that is:
(a) Real and magnified (b) Real and diminished (c) Virtual and diminished (d) Virtual and magnified
Answer: (c) Virtual and diminished — used as vehicle rear-view mirror.
Q: Shaving mirror used to see enlarged image of face is:
(a) Plane (b) Concave (c) Convex (d) Cylindrical
Answer: (b) Concave — when face is between pole and focus.
Q: Rear-view mirror of a vehicle is convex because:
(a) Image is erect (b) Diminished and wider field of view (c) Image is magnified (d) It is cheaper
Answer: (b) Wider field of view and erect image.
Q: For a concave mirror, focal length is:
(a) Equal to radius of curvature (b) Half of radius (c) Twice radius (d) Equal to object distance
Answer: (b) Half of R, i.e. f = R/2.
Drill Mirrors and Image Formation for CDS/OTA
CDS/OTA-pattern items on Mirrors and Image Formation with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is a concave mirror used by dentists?
Held close to the tooth (within focal length), it gives a magnified, erect, virtual image of the tooth surface.
Why are side mirrors of cars convex and not plane?
Convex mirrors give a wider field of view and always form an erect, diminished image — useful for spotting overtaking traffic.
Why is a plane mirror image laterally inverted but not upside down?
Reflection reverses the front-back axis, which when viewed face-on appears as left-right swap. Up-down is unchanged because gravity defines the vertical reference.