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Which mirror has negative focal length?
Plane mirror
Both convex and concave
Concave
Convex
Concave
A concave mirror has a negative focal length because its focus lies in front of the mirror, consistent with the Cartesian sign convention. In this convention, distances measured against the direction of incident light are considered negative.
A concave mirror has a negative focal length because its focus lies in front of the mirror, consistent with the Cartesian sign convention. In this convention, distances measured against the direction of incident light are considered negative.
Think of a concave mirror as a 'cave' where light rays are gathered inside; since you have to go 'into' the cave (against the incoming light) to reach the focus, the measurement is negative.
Concave = Cave = Negative (caves are dark/negative spaces).
f=2RтАЛ тАФ Relationship where f is focal length and R is radius of curvature, obeying signs.
f1тАЛ=v1тАЛ+u1тАЛ тАФ Mirror formula for relating object/image distance.
According to the sign convention for spherical mirrors, the pole is treated as the origin (0,0). Since light travels from left to right, distances measured to the left of the pole (in front of the mirror) are negative. Because the focal point of a concave mirror is located in front of the reflecting surface, its focal length f is always negative.
Concave mirror: Real focus, f<0.
Convex mirror: Virtual focus, f>0.
Plane mirror: Focal length is infinite (f=тИЮ).
Concave mirrors provide magnified images used in makeup mirrors and telescopes.
Concave mirrors can cause spherical aberration if the aperture is too large.
Dentist head mirrors
Car headlights (reflector bowls)
Sign convention follows the Cartesian coordinate system: Light travels from left to right, so the left side is negative.
Option D (Convex) has a positive focal length because the focus is behind the mirror, appearing on the right side.
C is correct тАФ A concave mirror has a negative focal length because its principal focus lies in front of its reflecting surface.
Remember: For mirrors, 'Concave' (having a cave) results in a negative f, while 'Convex' (bulging out) results in a positive f.
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