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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.

In one line: Normal decides the angle; mirror surface does not.

Practice This Concept

Understand it

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.

Example

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

Formula and rule

Law 1: incident ray, reflected ray, and normal lie in the same plane.

Law 2: i = r.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1If a ray makes 40° with the normal, the reflected ray also makes 40° with the normal.
2The angle between the incident and reflected rays becomes 80°.

Method you can reuse

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

Sketch it in your answer

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer

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

Common wrong answer

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

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

1
Enough for 1 mark
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
Add for 2 marks
Example: If a ray strikes a mirror at 40° to the normal, the reflected ray also makes 40° to the normal.
3
Add for 3 marks
The first law tells us that the incident ray, reflected ray, and the normal must remain in the same plane.

MCQ Quiz

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