Light — Reflection at Plane Surfaces
Introduction
Reflection is the phenomenon by which light rays bounce off a surface. In ICSE Class 10 Physics, you study the laws of reflection, image formation by plane mirrors, multiple reflections, and practical devices like the periscope.
Laws of Reflection
- The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
- The angle of incidence (i) is equal to the angle of reflection (r): i = r.
Plane Mirror — Image Characteristics
| Property | Description |
|---|---|
| Nature | Virtual (cannot be obtained on a screen) |
| Size | Same size as the object |
| Orientation | Laterally inverted (left-right reversed) |
| Distance | Image distance = Object distance behind the mirror |
Lateral Inversion
The reversal of left and right when an object is reflected in a plane mirror. For example, when you raise your right hand, the image appears to raise its left hand.
Multiple Reflections
When two plane mirrors are placed at an angle θ to each other, the number of images formed is:
n = (360° / θ) − 1
| Angle between mirrors (θ) | Number of images (n) |
|---|---|
| 90° | 3 |
| 60° | 5 |
| 45° | 7 |
| 0° (parallel) | Infinite |
Parallel mirrors: When two mirrors are placed parallel and facing each other, an infinite number of images are formed (e.g., barber shop mirrors).
Periscope
A periscope uses two plane mirrors (or prisms) placed at 45° to the line of sight.
Working: Light from the object enters the upper mirror, reflects downward to the lower mirror, and then reflects horizontally into the observer's eye.
Uses: Submarines (to see above water), in trenches during warfare, in crowd observation.
Worked Numericals
Example 1: Image Distance
A person stands 2 m in front of a plane mirror. Where is the image formed?
Solution: Image distance = object distance = 2 m behind the mirror. The image is 2 m behind the mirror.
Example 2: Number of Images
Two mirrors are placed at 60°. How many images are formed?
Solution: n = (360°/60°) − 1 = 6 − 1 = 5 images
Example 3: Field of View
A plane mirror has a field of view determined by the area visible in the mirror. If a person 1.6 m tall stands 1 m from a 0.8 m tall mirror, the top half of the mirror can view the full height of the person because the mirror needs only be half the object's height to view the full object.
Comparison: Real vs Virtual Images
| Feature | Real Image | Virtual Image |
|---|---|---|
| Can be obtained on a screen | Yes | No |
| Formed by | Converging rays | Diverging rays |
| Orientation | Inverted | Erect |
| Example | Image on a cinema screen | Image in a plane mirror |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Drawing the reflected ray on the wrong side of the normal | Incident and reflected rays are on opposite sides of the normal |
| Confusing lateral inversion with upside-down inversion | Lateral inversion is left-right reversal, not vertical |
| Thinking image size changes with distance | Image size in a plane mirror is always equal to object size |
ICSE Exam Focus
This chapter carries 4–6 marks. Key topics: laws of reflection, image characteristics, multiple reflections formula, ray diagrams.
Marks Blueprint: Laws of reflection — 2 marks, Multiple reflections — 2 marks, Periscope/ray diagrams — 2 marks.
Self-Test Questions
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State the two laws of reflection of light.
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Explain lateral inversion with an example.
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How many images are formed when two mirrors are placed at 45°?
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Draw a ray diagram showing the image formation in a plane mirror.
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Describe the construction and working of a periscope.
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A man stands 1.5 m in front of a plane mirror. How far behind the mirror is his image? If he moves 0.5 m towards the mirror, what is the new distance between the man and his image?
