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Spherical Mirror

A spherical mirror is a mirror whose reflecting surface forms part of a sphere.

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Student-friendly explanation

If the inner surface reflects, it is a concave mirror; if the outer surface reflects, it is a convex mirror. These mirrors are used because they can converge or diverge light and form different images depending on the object position.

How to write this in exams

  1. 1

    Start with the exact idea

    A spherical mirror is a mirror whose reflecting surface forms part of a sphere.

  2. 2

    Then show how to use it

    1. Check which surface reflects. 2. Decide concave or convex. 3. Match the mirror to the required image or field of view.

  3. 3

    Add one concrete example

    A dentist mirror is usually concave, while a vehicle rear-view mirror is usually convex.

  4. 4

    Avoid this incomplete answer

    Saying a convex mirror gives a magnified image in rear-view use. In fact it gives a diminished image but shows a wider area.

Definition

A spherical mirror is a mirror whose reflecting surface forms part of a sphere.

Example

A dentist mirror is usually concave, while a vehicle rear-view mirror is usually convex.

Rule to remember

For a spherical mirror, the principal focus lies on the principal axis; for a small aperture mirror, focal length is half the radius of curvature, f = R/2.

Memory hook

Inner surface curves in and can magnify; outer surface curves out and widens the view.

Examples and method

Worked example

A dentist wants a magnified upright image of a tooth. A concave mirror is chosen, and the tooth is placed close to the mirror so the image appears enlarged and erect.

Method to apply

1. Check which surface reflects. 2. Decide concave or convex. 3. Match the mirror to the required image or field of view.

Diagram support

Draw the pole, principal axis, centre of curvature, focus, and show whether the mirror is concave or convex by marking the reflecting surface.

How CBSE asks it

Questions ask you to identify mirror type from its use, label parts of the mirror, or predict image nature.

Avoid common mistakes

Common confusion

Students often mix up concave and convex by looking only at the shape, not the reflecting surface.

Common wrong answer

Saying a convex mirror gives a magnified image in rear-view use. In fact it gives a diminished image but shows a wider area.

Exam tip

Always identify the reflecting surface first; inner surface means concave, outer surface means convex.

Quick check

Why is a convex mirror used as a rear-view mirror in vehicles?

A convex mirror is used because it gives a wide field of view and forms an erect diminished image, so the driver can see more traffic behind.

Answer writing and exam use

1-mark answer

A spherical mirror is a mirror whose reflecting surface forms part of a sphere.

2-mark answer

A spherical mirror is a mirror whose reflecting surface forms part of a sphere. For a spherical mirror, the principal focus lies on the principal axis; for a small aperture mirror, focal length is half the radius of curvature, f = R/2. A dentist mirror is usually concave, while a vehicle rear-view mirror is usually convex.

3-mark answer

If the inner surface reflects, it is a concave mirror; if the outer surface reflects, it is a convex mirror. These mirrors are used because they can converge or diverge light and form different images depending on the object position. For a spherical mirror, the principal focus lies on the principal axis; for a small aperture mirror, focal length is half the radius of curvature, f = R/2. A dentist wants a magnified upright image of a tooth. A concave mirror is chosen, and the tooth is placed close to the mirror so the image appears enlarged and erect. Questions ask you to identify mirror type from its use, label parts of the mirror, or predict image nature. Saying a convex mirror gives a magnified image in rear-view use. In fact it gives a diminished image but shows a wider area.
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