Draw ray diagram in each of the following cases to show what happens after reflection to the incident ray when

(a) it is parallel to the principal axis and falling on a convex mirror.

(b) it is falling on a concave mirror while passing through its principal focus.

(c) it is coming oblique to the principal axis and falling on the pole of a convex mirror.

Light — Reflection and Refraction

Representation of Images Formed by Spherical Mirrors Using Ray Diagrams

Official Answer

To represent the behavior of incident rays after reflection, we use the standard rules of reflection for spherical mirrors:

(a) Ray parallel to the principal axis falling on a convex mirror:

According to the rules of reflection, a ray parallel to the principal axis, after reflection from a convex mirror, appears to diverge from its principal focus ( $F$ ) situated behind the mirror.

(b) Ray passing through the principal focus falling on a concave mirror:

A ray passing through the principal focus of a concave mirror, after reflection, will emerge parallel to the principal axis.

(c) Ray incident obliquely to the principal axis falling on the pole of a convex mirror:

A ray incident obliquely to the principal axis towards the pole ( $P$ ) of a convex mirror is reflected obliquely. The incident and reflected rays follow the laws of reflection at the point of incidence (pole $P$ ), making equal angles with the principal axis ( $\angle i = \angle r$ ).

diverge from principal focusparallel to principal axisobliquelylaws of reflectionangle of incidenceangle of reflection

Marking Scheme

11 mark
21 mark
31 mark

Hint

Remember that for a convex mirror, the focus lies behind the mirror, so rays appear to diverge from it. For a concave mirror, the focus is in front. Always draw arrows to show the direction of incident and reflected rays.

Quick Oral Answer

Q: Where does the principal focus of a convex mirror lie? A: The principal focus of a convex mirror lies behind its reflecting surface.

Analysis & Explanation

The behavior of light rays upon striking spherical mirrors is governed by the laws of reflection.

For a convex mirror, the principal focus ( $F$ ) and the centre of curvature ( $C$ ) lie behind the reflecting surface. Thus, rays parallel to the axis cannot physically pass through $F$ ; instead, they diverge outwards along a line that projects back to $F$ .
For a concave mirror, $F$ lies in front of the mirror. A ray passing through $F$ undergoes a symmetric path reversal (principle of reversibility of light) and emerges parallel to the principal axis.
At the pole ( $P$ ), the principal axis acts as the normal to the spherical surface. Therefore, any oblique ray incident at $P$ reflects such that the angle of incidence equals the angle of reflection relative to the principal axis.

Common Mistakes

1Forgetting to draw arrows indicating the direction of light propagation.
2Drawing the focus of a convex mirror in front of the mirror instead of behind it.
3Not extending the diverged ray backwards with dotted lines to the focus in the convex mirror diagram.

Interesting Facts

The principle of reversibility of light ensures that if the direction of a reflected ray is reversed, it will retrace its path as the incident ray. This is why a ray passing through the focus becomes parallel, which is the exact reverse of a parallel ray passing through the focus.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 3 marks in the CBSE Class 10 Science 2020 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 2020 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.