Explain why a ray of light passing through the centre of curvature of a concave mirror gets reflected along the same path.
Explain why a ray of light passing through the centre of curvature of a concave mirror gets reflected along the same path.
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 , and according to the laws of reflection, the angle of reflection must also be .
Marking scheme
- 1.Mentioning that the ray falls along the normal to the reflecting surface. (0.5 marks)
- 2.Stating 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 to the tangent), meaning the angle of incidence is zero. Therefore, the angle of reflection is also zero.
Common mistakes
- 1.Confusing the centre of curvature with the principal focus.
- 2.Failing to mention that the ray acts as a normal to the spherical surface.
Analysis and 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 . By the first law of reflection, the angle of reflection must also be , causing the ray to retrace its path.
Interesting facts
The principal axis is also a normal to the mirror at its pole (P).
Question details
Q3 of 27 questions
- Board
- CBSE
- Class
- Class 10
- Subject
- Science
- Year
- 2010
- Section
- Section A
- Marks
- 1
- Type
- Very Short Answer
- Chapter
- Light — Reflection and Refraction
- Topic
- Representation of Images Formed by Spherical Mirrors Using Ray Diagrams
- Bloom's level
- Understanding
- Syllabus topic
- Reflection of light by curved surfaces; Images formed by spherical mirrors; centre of curvature, principal axis, principal focus, focal length
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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.