Which mirror has negative focal length?

Correct option is C

The correct answer is (c) Concave

Explanation:

The focal length (f) of a mirror depends on its type:

• Concave Mirror: The focal length of a concave mirror is negative because the focus lies on the same side as the reflecting surface (left side in the Cartesian coordinate system).
• Convex Mirror: The focal length of a convex mirror is positive because its focus lies behind the mirror (right side in the Cartesian coordinate system).
• Plane Mirror: A plane mirror has no focal point, so its focal length is considered infinite.

In the mirror formula:

                                                              $$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$$​

where f is the focal length, u is the object distance, and v is the image distance. According to the Cartesian sign convention, the focal length of a concave mirror is taken as negative.

Information Booster:

• Sign Convention in Mirrors:
• Concave mirror: f<0f < 0f<0 (negative focal length)
• Convex mirror: f>0f > 0f>0 (positive focal length)
• Plane mirror: No real focal length
• Concave Mirror Uses:
• Used in reflecting telescopes
• Shaving mirrors and makeup mirrors (to produce magnified images)
• Car headlights and torches (to focus light in a parallel beam)

Convex Uses:

• Rearview Mirrors in vehicles (provides a wider field of view).
• Security Mirrors in shops, parking lots, and ATMs for surveillance.
• Street Light Reflectors for better light dispersion.

Nature of Reflection: Light rays diverge after reflection, appearing to come from a virtual focus behind the mirror.