Light
1. Nature of Light
Light is a form of ENERGY that enables us to SEE objects.
Key Properties
- Light travels in STRAIGHT LINES (rectilinear propagation).
- Speed of light in vacuum = 3 × 10⁸ m/s.
- Light travels MUCH faster than sound.
Luminous and Non-Luminous Objects
| Type | Definition | Examples |
|---|---|---|
| Luminous | Produce their own light | Sun, stars, bulb, candle |
| Non-luminous | Reflect light from other sources | Moon, table, book, person |
2. Reflection of Light
REFLECTION is the bouncing back of light when it strikes a smooth surface.
Laws of Reflection
- The INCIDENT ray, the REFLECTED ray, and the NORMAL all lie in the SAME plane.
- The ANGLE OF INCIDENCE (∠i) is ALWAYS EQUAL to the ANGLE OF REFLECTION (∠r).
Terms
| Term | Meaning |
|---|---|
| Incident ray | Light ray that strikes the surface |
| Reflected ray | Light ray that bounces off |
| Normal | Imaginary line perpendicular to the surface at the point of incidence |
| Angle of incidence | Angle between incident ray and normal |
| Angle of reflection | Angle between reflected ray and normal |
Types of Reflection
| Type | Surface | Example |
|---|---|---|
| Regular/Specular | Smooth surface | Mirror, still water |
| Diffuse/Irregular | Rough surface | Wall, paper, cloth |
3. Plane Mirror
A PLANE MIRROR is a flat, smooth surface that reflects light regularly.
Image Formation by a Plane Mirror
- The image is VIRTUAL (cannot be obtained on a screen).
- The image is ERECT (same orientation as object).
- The image is the SAME SIZE as the object.
- The image is LATERALLY INVERTED (left and right swapped).
- The DISTANCE of the image BEHIND the mirror equals the DISTANCE of the object IN FRONT of the mirror.
Worked Example (ICSE 2024, 2 marks)
'A person stands 2 m in front of a plane mirror. Where is the image located?'
Solution: The image is 2 m BEHIND the mirror. So the person is 4 m away from their own image.
4. Spherical Mirrors
A SPHERICAL MIRROR is a mirror whose reflecting surface is part of a SPHERE.
Types
| Type | Shape | Reflecting Surface |
|---|---|---|
| Concave mirror | Curved INWARD | Inside of the sphere |
| Convex mirror | Curved OUTWARD | Outside of the sphere |
Key Terms for Spherical Mirrors
| Term | Meaning |
|---|---|
| Centre of curvature (C) | Centre of the sphere of which the mirror is a part |
| Pole (P) | Centre point of the mirror |
| Principal axis | Line joining P and C |
| Focus (F) | Point where parallel rays converge (or appear to diverge from) |
| Focal length (f) | Distance from P to F. f = R/2 |
5. Concave Mirror
Image Formation
| Position of Object | Position of Image | Nature | Size | Use |
|---|---|---|---|---|
| At infinity | At F | Real, inverted | Very small | — |
| Beyond C | Between C and F | Real, inverted | Diminished | — |
| At C | At C | Real, inverted | Same size | — |
| Between C and F | Beyond C | Real, inverted | Enlarged | — |
| At F | At infinity | Real, inverted | Highly enlarged | — |
| Between F and P | Behind mirror | VIRTUAL, ERECT | Enlarged | Shaving mirror, dentist |
Uses of Concave Mirrors
- Shaving mirrors (enlarged virtual image).
- Dentist's mirror (enlarged view of teeth).
- Solar concentrators (focus sunlight to a point).
- Headlights and torches (diverging beam from focus).
6. Convex Mirror
Image Formation (ALWAYS virtual, erect, and diminished)
| Position of Object | Position of Image | Nature | Use |
|---|---|---|---|
| Anywhere | Between P and F, behind mirror | Virtual, erect, diminished | Rear-view mirror |
Uses of Convex Mirrors
- Rear-view mirrors in vehicles (gives a wider field of view).
- Security mirrors in shops.
- At road intersections to see around corners.
Comparison: Concave vs Convex
| Feature | Concave | Convex |
|---|---|---|
| Shape | Curved inward | Curved outward |
| Image type | Real or virtual | Always virtual |
| Image size | Diminished / Same / Enlarged | Always diminished |
| Field of view | Narrower | Wider |
7. Real vs Virtual Images
| Feature | Real Image | Virtual Image |
|---|---|---|
| Can be obtained on screen | YES | NO |
| Formed by | Actual intersection of rays | Apparent intersection |
| Nature | Usually inverted | Usually erect |
| Mirrors that form | Concave | Plane, Convex (and Concave in some cases) |
8. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Laws of reflection | 2 marks | Very High |
| Plane mirror properties | 2-3 marks | Very High |
| Concave mirror uses and ray diagrams | 3 marks | High |
| Convex mirror uses | 2 marks | High |
| Real vs virtual images | 2 marks | Medium |
Common Mistakes
- Saying plane mirror image is REAL — it is VIRTUAL (cannot be obtained on screen).
- Confusing concave and convex — concave is 'cave' (curving inward).
- Forgetting that convex mirror always gives diminished images.
- Drawing ray diagrams incorrectly (wrong angle or wrong path).
Self-Test (5 Questions)
Q1. Which mirror is used as a rear-view mirror in cars? (1 mark)
- A) Plane mirror
- B) Concave mirror
- C) Convex mirror
- D) All of these
Q2. State the second law of reflection. (2 marks)
Q3. 'What type of image does a plane mirror form?' (1 mark)
Q4. 'A concave mirror forms a real, inverted, enlarged image. Where is the object placed?' (2 marks)
Q5. 'An object is placed 10 cm in front of a plane mirror. How far is the image from the object?' (2 marks)
Answers
A1. C) Convex mirror (gives wider field of view). A2. The angle of incidence is EQUAL to the angle of reflection (∠i = ∠r). A3. Virtual, erect, same size, laterally inverted. A4. Between C and F (centre of curvature and focus). A5. 20 cm. (Image is 10 cm behind the mirror. Distance from object to image = 10 + 10 = 20 cm.)
