Light Energy
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 FASTER than sound
'Luminous objects PRODUCE their own light (Sun, bulb, candle). Non-luminous objects REFLECT light (Moon, book, table).'
2. Reflection of Light
The bouncing back of light when it strikes a surface.
Laws of Reflection
- The INCIDENT ray, the REFLECTED ray, and the NORMAL all lie in the SAME plane.
- The ANGLE of incidence (i) = ANGLE of reflection (r).
∠i = ∠r
Types of Reflection
| Type | Surface | Example |
|---|---|---|
| REGULAR reflection | Smooth, polished | Mirror, still water |
| DIFFUSE reflection | Rough, uneven | Wall, paper, cloth |
3. Plane Mirror
| Aspect | Description |
|---|---|
| Image type | VIRTUAL (cannot be projected on a screen) |
| Orientation | LATERALLY INVERTED (left-right reversed) |
| Size | SAME size as object |
| Distance | Image is AS FAR BEHIND the mirror as the object is in FRONT |
| Position | Laterally inverted, erect |
Worked Example: If an object is placed 15 cm in front of a plane mirror, where is the image formed?
The image is 15 cm BEHIND the mirror (virtual).
4. Spherical Mirrors
| Term | Meaning |
|---|---|
| Pole (P) | Centre of the mirror surface |
| Centre of curvature (C) | Centre of the SPHERE of which the mirror is a part |
| Principal axis | Line joining P and C |
| Focus (F) | Point where PARALLEL rays CONVERGE (concave) or APPEAR TO DIVERGE from (convex) |
| Focal length (f) | Distance from P to F. f = R/2 |
Concave Mirror (Converging)
- Reflecting surface CURVES INWARDS
- Parallel rays CONVERGE at focus
- f is POSITIVE
Convex Mirror (Diverging)
- Reflecting surface BULGES OUTWARDS
- Parallel rays DIVERGE — APPEAR to come from focus behind mirror
- f is NEGATIVE
5. Ray Diagrams for Concave Mirror
Object at INFINITY
Image: At F (point-sized, real, inverted)
Object BEYOND C
Image: Between C and F (real, INVERTED, DIMINISHED)
Object AT C
Image: At C (real, inverted, SAME size)
Object BETWEEN C and F
Image: Beyond C (real, inverted, ENLARGED)
Object AT F
Image: At INFINITY (highly enlarged)
Object BETWEEN F and P
Image: BEHIND the mirror (virtual, ERECT, ENLARGED)
6. Ray Diagrams for Convex Mirror
ALWAYS produces: Virtual, ERECT, DIMINISHED image, regardless of object position.
Image lies between P and F (behind the mirror).
7. Uses of Spherical Mirrors
| Concave Mirror | Convex Mirror |
|---|---|
| TORCH and headlight reflectors | REAR-VIEW mirrors in vehicles |
| SHAVING mirrors (enlarged image) | Security mirrors in shops |
| DENTIST's mirror | Street light reflectors |
| SOLAR cookers and furnaces | |
| MAKEUP mirrors |
8. Real vs Virtual Images
| Aspect | Real Image | Virtual Image |
|---|---|---|
| Can be projected on a screen? | YES | NO |
| Formed by | CONVERGENCE of actual rays | APPARENT convergence (rays appear to diverge) |
| Nature | Always INVERTED | Always ERECT |
| Example | Image on a cinema screen | Image in a plane mirror |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Image in a plane mirror is real' | Plane mirror images are VIRTUAL — they cannot be projected on a screen |
| 'Convex mirrors are converging' | Concave = CONVERGING. Convex = DIVERGING |
| 'Concave mirrors always give real images' | When the object is BETWEEN F and P, the image is VIRTUAL and ENLARGED |
| 'f = R for spherical mirrors' | f = R/2. Focal length is HALF the radius of curvature |
ICSE Exam Focus (6–8 marks)
- 2-mark questions: Laws of reflection, define focus, centre of curvature
- 3-mark questions: Distinguish real vs virtual images
- 4-mark questions: Ray diagrams for concave mirror (two positions)
- 6-mark questions: Applications of spherical mirrors with reasons
Self-Test
Q1. State the two laws of reflection. A1. (1) Incident ray, reflected ray, and normal lie in the SAME plane. (2) Angle of incidence = Angle of reflection.
Q2. What type of mirror is used as a rear-view mirror in vehicles? Why? A2. CONVEX mirror. It gives an ERECT, DIMINISHED image and provides a WIDER field of view.
Q3. Where should an object be placed in front of a concave mirror to get a real, inverted, and same-sized image? A3. At the CENTRE of curvature (C).
Q4. What is the difference between a real and a virtual image? A4. A real image can be PROJECTED on a screen (formed by convergence of actual rays). A virtual image CANNOT be projected (rays only appear to diverge from it).
Q5. The focal length of a concave mirror is 10 cm. Find its radius of curvature. A5. R = 2f = 2 × 10 = 20 cm.
Q6. Why does a concave mirror produce an enlarged image when the object is close to it? A6. When the object is between F (focus) and P (pole), the rays diverge after reflection. They APPEAR to come from behind the mirror, forming a VIRTUAL, ERECT, and ENLARGED image.
