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In which of the following scenarios does the focal length of a concave mirror vary?
When placed in a vacuum
When placed in air
Focal length does not vary in any media
When placed in water
Focal length does not vary in any media
The focal length of a spherical mirror (concave or convex) is determined solely by its radius of curvature (R). As the medium does not affect the reflection process, the focal length remains constant regardless of whether the mirror is in air, water, or a vacuum.
The focal length of a spherical mirror (concave or convex) is determined solely by its radius of curvature (R). As the medium does not affect the reflection process, the focal length remains constant regardless of whether the mirror is in air, water, or a vacuum.
Think of a shiny metal spoon; the way it curves light to form an image doesn't change just because you dunk it in a glass of waterтАФit's the physical shape of the metal that does the work, not the air around it.
Mirror = Shape, Lens = Refraction. Mirrors don't care about the medium!
f=2RтАЛ тАФ The fundamental relation where focal length (f) is half the radius of curvature (R).
The focal length of a concave mirror is given by the relation f=2RтАЛ. Since the reflection of light occurs at the polished surface of the mirror, it is independent of the refractive index of the surrounding medium, unlike lenses which undergo refraction and thus depend on the medium.
Focal length depends only on the curvature of the mirror surface.
Refraction (which depends on the medium) is not involved in mirror reflection.
Lenses, unlike mirrors, change focal length if the surrounding medium changes.
Allows for consistent optical performance in various environments (e.g., underwater cameras, telescopes).
Headlights of vehicles
Solar furnaces
Dentist's mirrors
| Feature | Constant | Variable |
|---|---|---|
Dependency | Mirror (focal length) | Lens (focal length) |
The refractive index of the medium affects light speed but not the angle of reflection (i=r).
Option D is a common distractor because students confuse mirrors with lenses (lens maker's formula involves the medium's refractive index).
C is correct тАФ The focal length of a concave mirror depends solely on its radius of curvature and remains unchanged in any medium.
Always distinguish between reflection (mirrors - medium independent) and refraction (lenses - medium dependent) for exam questions.
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