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How does a concave mirror form images?

Class 10 ScienceNCERT chapter: Light – Reflection and Refractionइसे हिंदी में पढ़ें

Exam-ready answer

A concave mirror forms images by reflecting rays according to the laws of reflection. The image position, size, and nature depend on the object's distance from the pole. Using ray diagrams with principal focus and centre of curvature, we find that images can be real or virtual, inverted or erect, and magnified or diminished.

Explanation

A concave mirror is a spherical mirror with the reflecting surface curved inwards. Its important terms are pole (P), principal focus (F), and centre of curvature (C). The distance PF is the focal length (f), and PC is the radius of curvature (R), with R = 2f. Rays parallel to the principal axis converge at F after reflection, while a ray through F becomes parallel to the axis. A ray through C reflects back along the same path.

To locate the image, we draw any two of these rays from the top of the object. Their intersection gives the image position. For example, when the object is beyond C, the image forms between F and C, is real, inverted, and diminished. When the object is at C, the image is at C, real, inverted, and of the same size. When the object is between F and C, the image is beyond C, real, inverted, and magnified.

When the object is at F, the reflected rays are parallel, so the image is at infinity, highly magnified. When the object is between P and F, the image forms behind the mirror, is virtual, erect, and magnified. This is why a concave mirror is used as a shaving mirror or dentist's mirror. The mirror formula, 1/v + 1/u = 1/f, and magnification m = -v/u, help calculate image distance and size, with sign conventions.

Points to remember

  1. Concave mirror: converging mirror, reflecting surface curved inwards.
  2. Focal length f = R/2, where R is radius of curvature.
  3. Mirror formula: 1/v + 1/u = 1/f, with sign conventions (distances measured from pole).
  4. Magnification m = height of image / height of object = -v/u.
  5. Image nature: real and inverted for object beyond F; virtual and erect for object between P and F.

Exam tip

Always draw a neat ray diagram with labelled pole, focus, and centre of curvature. Write the mirror formula with proper sign convention and mention the nature of the image (real/virtual, inverted/erect, magnified/diminished) to score full marks.

Questions students also ask

What is the difference between a real and a virtual image in a concave mirror?
A real image is formed when reflected rays actually meet, can be taken on a screen, and is inverted. A virtual image is formed when reflected rays appear to meet behind the mirror, cannot be taken on a screen, and is erect. In a concave mirror, virtual image occurs when object is between pole and focus.
When does a concave mirror form a magnified image?
A concave mirror forms a magnified image when the object is placed between the focus and the centre of curvature (real, inverted, magnified) or between the pole and focus (virtual, erect, magnified). The magnification depends on object distance.
What is the sign convention for a concave mirror?
According to the New Cartesian sign convention, the pole is the origin. Distances measured in the direction of incident light are positive, and opposite are negative. For a concave mirror, focal length (f) is negative, object distance (u) is negative, and image distance (v) can be negative (real) or positive (virtual).

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.