Q4
2 marksShort AnswerSection A

“The magnification produced by a spherical mirror is –3”. List four informations you obtain from this statement about the mirror/image.

Light — Reflection and Refraction
Magnification by Spherical Mirrors
Official Answer

From the statement "The magnification produced by a spherical mirror is −3-3", we obtain the following four pieces of information:


  1. Nature of the Image: The negative sign of the magnification (m=−3m = -3) indicates that the image formed is real and inverted.
  2. Size of the Image: Since the magnitude of magnification is greater than 11 (∣m∣=3>1|m| = 3 > 1), the image is magnified (enlarged). Specifically, the size of the image is three times the size of the object.
  3. Type of Mirror: The mirror is a concave mirror. A convex mirror always produces virtual, erect, and diminished images (where magnification is positive and less than 11), and a plane mirror always produces a magnification of +1+1. Only a concave mirror can produce a real, inverted, and magnified image.
  4. Position of the Object: The object is placed between the principal focus (FF) and the centre of curvature (CC) of the concave mirror, as this is the only position that yields a real, inverted, and magnified image.
real and invertedmagnifiedconcave mirrorbetween F and C

Marking Scheme

  • 1State that the image is real and inverted (due to the negative sign). (0.5 marks)
  • 2State that the image is magnified/enlarged (three times the size of the object). (0.5 marks)
  • 3Identify the mirror as a concave mirror. (0.5 marks)
  • 4Identify the position of the object (between FF and CC). (0.5 marks)

Hint

Recall the sign convention for magnification: a negative sign indicates a real and inverted image, while a magnitude greater than 1 indicates an enlarged image. Think about which mirror can form such an image.

Quick Oral Answer

The negative sign indicates that the image is real and inverted, and the value 3 indicates that the image is three times larger than the object. This can only happen with a concave mirror.

Analysis & Explanation

The magnification (mm) produced by a spherical mirror is defined as the ratio of the height of the image (h′h') to the height of the object (hh):

m=h′h=−vum = \frac{h'}{h} = -\frac{v}{u}


  • Sign Analysis: A negative sign in magnification means h′h' (image height) is negative (measured downwards, below the principal axis) while hh (object height) is positive (measured upwards). This corresponds to a real and inverted image.
  • Magnitude Analysis: ∣m∣=3|m| = 3 means the image height is three times the object height (h′=−3hh' = -3h).
  • Mirror Identification: Since convex mirrors only form virtual, erect, and diminished images (0<m<10 < m < 1), and plane mirrors form virtual, erect, and same-sized images (m=+1m = +1), a magnification of −3-3 can only be achieved using a concave mirror.
  • Object Position: According to the ray diagrams of a concave mirror, when the object is placed between CC and FF, the image is formed beyond CC, and it is real, inverted, and magnified.

Common Mistakes

  1. 1Confusing the negative sign of magnification with a diminished image.
  2. 2Identifying the mirror as convex because of the negative sign.

Interesting Facts

Dentists use concave mirrors in this exact configuration (though with the object placed closer than the focus to get a virtual, erect, and magnified view) to inspect teeth clearly.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2016?

This question carries 2 marks in the CBSE Class 10 Science 2016 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Light — Reflection and Refraction" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Magnification by Spherical Mirrors" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2016 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2016 paper.