Light - Class 7 Science (CBSE)
Based on the 2025-26 NCERT syllabus for Class 7 Science. This chapter covers the behaviour of light including reflection, mirrors, lenses, and the formation of rainbows.
1. Why this chapter matters
Light enables us to see the world. Understanding how light behaves helps us design mirrors, lenses, cameras, telescopes, and microscopes. In CBSE exams, this chapter contributes 8-10 marks with diagram-based and application questions.
2. Reflection of light
Reflection is the bouncing back of light when it strikes a smooth, shiny surface.
Laws of reflection
- The angle of incidence is equal to the angle of reflection.
- The incident ray, reflected ray, and the normal all lie in the same plane.
Key terms
- Incident ray: The ray of light that strikes the surface.
- Reflected ray: The ray that bounces back.
- Normal: The perpendicular line at the point of incidence.
- Angle of incidence (i): Angle between incident ray and normal.
- Angle of reflection (r): Angle between reflected ray and normal.
3. Plane mirror and its properties
A plane mirror is a flat, smooth reflecting surface.
Image formed by a plane mirror
| Property | Description |
|---|---|
| Nature | Virtual (cannot be obtained on a screen) |
| Size | Same size as the object |
| Distance | Same distance behind the mirror as the object is in front |
| Orientation | Laterally inverted (left and right swapped) |
| Position | Directly opposite the object |
Lateral inversion
In a plane mirror, the left side of the object appears as the right side of the image. This is called lateral inversion. Example: The word AMBULANCE is written backwards on vehicles so that it appears correctly in rear-view mirrors.
4. Concave and convex mirrors
Spherical mirrors
Mirrors that have a curved reflecting surface.
Concave mirror
A spherical mirror with the reflecting surface on the inner (curved-in) side.
Features:
- Can form real or virtual images.
- Can magnify objects (when object is close).
- Used in: shaving mirrors, dentist mirrors, solar cookers, headlights.
Convex mirror
A spherical mirror with the reflecting surface on the outer (curved-out) side.
Features:
- Always forms a virtual, diminished, and upright image.
- Gives a wider field of view.
- Used in: rear-view mirrors in vehicles, shop security mirrors.
5. Comparison: concave vs convex mirror
| Feature | Concave mirror | Convex mirror |
|---|---|---|
| Shape | Curved inward | Curved outward |
| Image nature | Real (or virtual) | Always virtual |
| Image size | Can be magnified or diminished | Always diminished |
| Field of view | Narrower | Wider |
| Use | Shaving, dentist, solar cooker | Vehicle mirrors, security |
6. Lenses: convex and concave
A lens is a transparent material (usually glass) with curved surfaces that refracts light.
Convex lens (converging lens)
Thicker in the middle than at the edges. It converges (brings together) light rays.
Used in: magnifying glass, camera, spectacles for far-sightedness, telescope.
Concave lens (diverging lens)
Thinner in the middle than at the edges. It diverges (spreads out) light rays.
Used in: spectacles for near-sightedness, flashlights, door peepholes.
7. Comparison: convex vs concave lens
| Feature | Convex lens | Concave lens |
|---|---|---|
| Shape | Thicker in middle | Thinner in middle |
| Effect on light | Converges | Diverges |
| Image | Real or virtual | Always virtual, diminished |
| Use | Magnifying glass | Near-sightedness glasses |
| Focal length | Positive | Negative |
8. Prism and rainbow
Refraction through a prism
When white light passes through a triangular prism, it splits into its component colours. This is called dispersion of light.
The seven colours (VIBGYOR)
Violet, Indigo, Blue, Green, Yellow, Orange, Red.
The colours always appear in this order. Violet bends the most, red bends the least.
Rainbow
A rainbow is formed when sunlight passes through tiny water droplets in the atmosphere. Each droplet acts like a tiny prism, dispersing sunlight into its colours.
Rainbows appear when it is raining in one part of the sky and sunny in another, and the observer stands with the Sun behind them.
9. Worked examples
Example 1: A student stands 2 m in front of a plane mirror. How far is the image from the mirror? How far is the image from the student?
The image is 2 m behind the mirror. The total distance between student and image is 2 + 2 = 4 m.
Example 2: Why is a convex mirror used as a rear-view mirror in vehicles?
A convex mirror gives a wider field of view and always forms an upright, diminished image, allowing the driver to see more traffic behind.
Example 3: What type of mirror is used by a dentist to see a magnified image of teeth?
A concave mirror is used. When an object is placed close to a concave mirror, it produces a magnified, virtual image.
Example 4: What is the order of colours in a rainbow?
Violet at the inner edge, Red at the outer edge: VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).
10. Common mistakes and how to fix them
| Mistake | Fix |
|---|---|
| Thinking concave and convex mirror properties are same | Concave converges light; convex diverges light |
| Forgetting lateral inversion in plane mirror | Left appears right and right appears left |
| Saying rainbow has 5 or 8 colours | Rainbow has exactly 7 colours (VIBGYOR) |
| Confusing convex mirror with convex lens | Mirror reflects; lens refracts (transmits) light |
| Believing image in plane mirror is real | A plane mirror image is virtual (cannot be projected on a screen) |
11. CBSE exam focus
| Question type | Marks | Frequency |
|---|---|---|
| Laws of reflection | 2-3 marks | 1 question |
| Plane mirror image properties | 2 marks | 1 question |
| Concave/convex mirror uses | 2-3 marks | 1 question |
| Convex/concave lens identification | 2 marks | 1 question |
| Dispersion and rainbow | 3 marks | 1 question |
12. Self-test
- State the two laws of reflection.
- Why does the word AMBULANCE appear backwards on emergency vehicles?
- Differentiate between a concave and a convex mirror (any three points).
- Which type of lens is used to correct near-sightedness?
- What is dispersion of light? How is a rainbow formed?
- You look at your face in a plane mirror. If you move 1 m closer to the mirror, how far does your image move?
13. Answer key
- Angle of incidence = angle of reflection. Incident ray, reflected ray, and normal are in the same plane.
- So that drivers in front can read it correctly in their rear-view mirrors (lateral inversion corrects it).
- See comparison table in section 5.
- Concave lens (diverging lens) is used for near-sightedness.
- Dispersion is the splitting of white light into its colours. A rainbow forms when water droplets disperse sunlight.
- Your image moves 1 m closer to the mirror too (same distance change as object).
14. Quick revision
- Reflection: light bounces off surfaces.
- Laws of reflection: i = r; all in same plane.
- Plane mirror: virtual, same size, laterally inverted.
- Concave mirror: converges, used for magnification.
- Convex mirror: diverges, wider field of view.
- Convex lens: converges, magnifying glass.
- Concave lens: diverges, near-sighted glasses.
- Dispersion: white light splits into VIBGYOR.
- Rainbow: sunlight + water droplets = dispersion.
