Q3
1 markVery Short AnswerSection A

Explain why a ray of light passing through the centre of curvature of a concave mirror gets reflected along the same path.

Light — Reflection and Refraction
Representation of Images Formed by Spherical Mirrors Using Ray Diagrams
Official Answer

A ray of light passing through the centre of curvature of a concave mirror is reflected back along the same path because the incident ray falls on the mirror surface along the normal. Since the ray is perpendicular to the reflecting surface at the point of incidence, the angle of incidence is 0∘0^\circ, and according to the laws of reflection, the angle of reflection must also be 0∘0^\circ.

normalsame pathcentre of curvatureangle of incidencelaws of reflection

Marking Scheme

  • 1Mentioning that the ray falls along the normal to the reflecting surface. (0.5 marks)
  • 2Stating that the angle of incidence and reflection are zero/the ray retraces its path. (0.5 marks)

Hint

Recall that the radius of a circle/sphere is always perpendicular to the tangent at the point of contact.

Quick Oral Answer

The ray retraces its path because it hits the mirror surface perpendicularly (at 90∘90^\circ to the tangent), meaning the angle of incidence is zero. Therefore, the angle of reflection is also zero.

Analysis & Explanation

According to the NCERT text in section 9.2.2 (iii), light rays passing through the centre of curvature (C) of a concave mirror are reflected back along the same path. This is because the line joining the centre of curvature to any point on the spherical surface is always a normal to the surface at that point. When a ray travels along the normal, it strikes the surface at an angle of incidence i=0∘i = 0^\circ. By the first law of reflection, the angle of reflection rr must also be 0∘0^\circ, causing the ray to retrace its path.

Common Mistakes

  1. 1Confusing the centre of curvature with the principal focus.
  2. 2Failing to mention that the ray acts as a normal to the spherical surface.

Interesting Facts

The principal axis is also a normal to the mirror at its pole (P).

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2010?

This question carries 1 mark in the CBSE Class 10 Science 2010 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 "Representation of Images Formed by Spherical Mirrors Using Ray Diagrams" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2010 paper?

This is a Very Short Answer question from Section A in the CBSE Class 10 Science 2010 paper.