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Laws Of Reflection

The laws of reflection state that the incident ray, reflected ray, and normal lie in one plane, and angle of incidence equals angle of reflection.

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Student-friendly explanation

The first law tells us that the incident ray, reflected ray, and the normal must remain in the same plane. The second law says the angle made by the incident ray with the normal is equal to the angle made by the reflected ray with the normal. This is always measured from the normal, not from the mirror surface.

How to write this in exams

  1. 1

    Start with the exact idea

    The laws of reflection state that the incident ray, reflected ray, and normal lie in one plane, and angle of incidence equals angle of reflection.

  2. 2

    Then show how to use it

    1. Identify the normal. 2. Measure both angles from the normal. 3. Keep i = r. 4. Put the rays in the same plane.

  3. 3

    Add one concrete example

    If a ray strikes a mirror at 40° to the normal, the reflected ray also makes 40° to the normal.

  4. 4

    Avoid this incomplete answer

    Many students write the angle from the mirror surface instead of the normal, which gives the wrong value.

Definition

The laws of reflection state that the incident ray, reflected ray, and normal lie in one plane, and angle of incidence equals angle of reflection.

Example

If a ray strikes a mirror at 40° to the normal, the reflected ray also makes 40° to the normal.

Rule to remember

Law 1: incident ray, reflected ray, and normal lie in the same plane. Law 2: i = r.

Memory hook

Normal decides the angle; mirror surface does not.

Examples and method

Worked example

If a ray makes 40° with the normal, the reflected ray also makes 40° with the normal. The angle between the incident and reflected rays becomes 80°.

Method to apply

1. Identify the normal. 2. Measure both angles from the normal. 3. Keep i = r. 4. Put the rays in the same plane.

Diagram support

Show a mirror line, normal at point of incidence, equal angle marks i and r, and label the same plane clearly.

How CBSE asks it

You may be asked to state the laws, complete a ray diagram, or find the angle of reflection from a given angle of incidence.

Avoid common mistakes

Common confusion

Students often measure the angle with the mirror surface instead of the normal.

Common wrong answer

Many students write the angle from the mirror surface instead of the normal, which gives the wrong value.

Exam tip

In ray diagrams, always draw the normal first; most marks are lost because of wrong angle marking.

Quick check

If a ray strikes a mirror at 25° to the normal, what is the reflected angle?

The reflected angle is 25° to the normal because the law of reflection says the two angles are equal, both measured from the normal.

Answer writing and exam use

1-mark answer

The laws of reflection state that the incident ray, reflected ray, and normal lie in one plane, and angle of incidence equals angle of reflection.

2-mark answer

The laws of reflection state that the incident ray, reflected ray, and normal lie in one plane, and angle of incidence equals angle of reflection. Law 1: incident ray, reflected ray, and normal lie in the same plane. Law 2: i = r. If a ray strikes a mirror at 40° to the normal, the reflected ray also makes 40° to the normal.

3-mark answer

The first law tells us that the incident ray, reflected ray, and the normal must remain in the same plane. The second law says the angle made by the incident ray with the normal is equal to the angle made by the reflected ray with the normal. This is always measured from the normal, not from the mirror surface. Law 1: incident ray, reflected ray, and normal lie in the same plane. Law 2: i = r. If a ray makes 40° with the normal, the reflected ray also makes 40° with the normal. The angle between the incident and reflected rays becomes 80°. You may be asked to state the laws, complete a ray diagram, or find the angle of reflection from a given angle of incidence. Many students write the angle from the mirror surface instead of the normal, which gives the wrong value.
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