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What are the laws of reflection of light?

Class 7 ScienceNCERT chapter: Light: Shadows and Reflectionsइसे हिंदी में पढ़ें

Exam-ready answer

Reflection follows two laws. First, the angle of incidence is equal to the angle of reflection. Second, the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane. These laws hold for every reflecting surface, such as a plane mirror, still water or polished steel.

Explanation

When light falls on a shiny surface like a plane mirror, it bounces back. This is called reflection of light. The ray that falls on the mirror is the incident ray, and the ray that bounces back is the reflected ray. At the point where the incident ray touches the mirror, we draw a line at 90° to the mirror. This line is called the normal.

The angle between the incident ray and the normal is the angle of incidence (∠i). The angle between the reflected ray and the normal is the angle of reflection (∠r). In a simple activity, a torch beam passed through a comb slit is made to fall on a mirror kept on a sheet of paper. Measuring with a protractor shows that ∠i and ∠r are always equal.

If the paper sheet is bent so that part of it tilts away, the reflected ray can no longer be seen on the tilted part. This shows the second law: the incident ray, the normal and the reflected ray lie in one flat plane. These two laws explain why we see a clear image in a mirror and how a periscope uses mirrors to show objects behind a wall.

Points to remember

  1. Reflection: the bouncing back of light when it falls on a shiny, smooth surface such as a mirror.
  2. Normal: the line drawn at 90° to the mirror at the point where the incident ray strikes it.
  3. First law: angle of incidence (∠i) = angle of reflection (∠r).
  4. Second law: the incident ray, the reflected ray and the normal all lie in the same plane.
  5. Both angles are always measured from the normal, not from the mirror surface.

Exam tip

Draw a neat ray diagram with a mirror, a dotted normal, arrows on both rays, and mark ∠i and ∠r from the normal. Then write both laws as two separate numbered points.

Questions students also ask

What happens when light falls along the normal on a mirror?
If light falls along the normal, the angle of incidence is 0°. So the angle of reflection is also 0°, and the light bounces straight back along the same path it came from.
Do the laws of reflection work on rough surfaces?
Yes. Each tiny part of a rough surface still obeys the laws, but the normals point in different directions. So the reflected rays scatter in many directions and no clear image is formed. This is called irregular or diffused reflection.
What is the angle of reflection if the incident ray makes 30° with the mirror?
The angle with the mirror is not the angle of incidence. The normal is at 90° to the mirror, so the angle of incidence is 90° − 30° = 60°. By the first law, the angle of reflection is also 60°.

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