Light — Reflection, Mirrors, Lenses, and Colour
"'Light travels in STRAIGHT LINES. When it hits a surface, it can BOUNCE BACK (reflection) or PASS THROUGH (refraction). These properties let us SEE the world.'"
1. What Is Light?
Light is a form of ENERGY that enables us to SEE things.
Properties of Light
- Travels in STRAIGHT LINES (rectilinear propagation).
- Speed of light in vacuum = 300,000,000 m/s (3 × 10⁸ m/s).
- Can travel through vacuum (unlike sound).
- Light travels FASTER than sound. 'This is why you see LIGHTNING before you hear THUNDER.'
Luminous vs Non-Luminous Objects
| Type | Definition | Examples |
|---|---|---|
| Luminous | Objects that PRODUCE their own light | Sun, stars, bulb, candle flame, fireflies |
| Non-luminous | Objects that REFLECT light from other sources | Moon, table, book, your face |
'The Moon is NOT a source of light — it REFLECTS the Sun's light. This is why we see only the part facing the Sun.'
2. Reflection of Light
Reflection: When light bounces BACK from a surface.
Laws of Reflection
- The INCIDENT ray, the REFLECTED ray, and the NORMAL (perpendicular) 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. This is the MOST important law of reflection. It holds for ALL reflecting surfaces — plane, concave, convex.'
Regular vs Diffused Reflection
| Type | Surface | Rays | Image formed? | Example |
|---|---|---|---|---|
| Regular (specular) | SMOOTH surface | Parallel rays remain parallel | CLEAR image | Plane mirror, still water |
| Diffused (irregular) | ROUGH surface | Rays scatter in DIFFERENT directions | NO clear image | Wall, paper, cloth |
3. Plane Mirrors — Image Formation
Plane mirror: A FLAT, smooth reflecting surface.
Image Properties in a Plane Mirror
| Property | Characteristic |
|---|---|
| Type | VIRTUAL (cannot be projected 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-right reversed) |
| Shape | ERECT (not upside down) |
Lateral inversion: 'When you raise your RIGHT hand in front of a mirror, the image raises its LEFT hand. This is LATERAL INVERSION — left and right are swapped.'
Uses of Plane Mirrors
- Looking glass / dressing mirror
- PERISCOPE (two plane mirrors at 45°)
- Kaleidoscope (multiple reflections create patterns)
- Reflection in shops and showrooms
4. Spherical Mirrors
Spherical mirrors: Mirrors that are parts of a HOLLOW SPHERE.
| Type | Shape | Focus | Uses |
|---|---|---|---|
| Concave mirror | Curves INWARD (like a cave) | REAL focus (light converges) | Torch, headlight, shaving mirror, dentist's mirror |
| Convex mirror | Curves OUTWARD (bulging out) | VIRTUAL focus (light diverges) | Rear-view mirror in vehicles, shop security mirror |
Concave Mirror
'Concave mirrors CONVERGE parallel light rays to a SINGLE point (the FOCUS).'
Image formed depends on object distance:
| Object Position | Image Position | Image Nature |
|---|---|---|
| Far beyond centre | Between focus and centre | REAL, INVERTED, SMALLER |
| At centre | At centre | REAL, INVERTED, SAME SIZE |
| Between focus and centre | Beyond centre | REAL, INVERTED, LARGER |
| Between focus and mirror | Behind mirror | VIRTUAL, ERECT, LARGER |
'When you use a concave shaving mirror, you keep your face BETWEEN the focus and the mirror — you see an ENLARGED, upright image.'
Convex Mirror
'Convex mirrors DIVERGE parallel light rays — they appear to come from a point behind the mirror (virtual focus).'
Properties: Convex mirrors ALWAYS form:
- VIRTUAL image
- ERECT image
- DIMINISHED (smaller) image
'Convex mirrors are used as REAR-VIEW MIRRORS in cars because they show a WIDER field of view, even though objects appear SMALLER.'
5. Lenses
Lens: A transparent material (usually glass) with CURVED surfaces that REFRACTS (bends) light.
| Type | Shape | Effect on Light | Image | Uses |
|---|---|---|---|---|
| Convex (converging) | Thicker in the MIDDLE | CONVERGES light rays | Can form real or virtual | Magnifying glass, camera, eye lens |
| Concave (diverging) | Thinner in the MIDDLE | DIVERGES light rays | ALWAYS virtual and smaller | Spectacles for myopia (nearsightedness) |
Convex Lens
'Convex lens CONVERGES light to a FOCAL POINT. It can form both REAL images (on a screen) and VIRTUAL images (magnified).'
As a magnifying glass: 'Keep the object BETWEEN the lens and its focus — you see a MAGNIFIED, upright, virtual image.'
Concave Lens
'Concave lens SPREADS OUT light rays. It ALWAYS forms a VIRTUAL, ERECT, and SMALLER image.'
6. Prism and Dispersion of Light
Prism: A triangular glass block that BENDS and SEPARATES white light.
Dispersion
Dispersion: The SPLITTING of white light into its component COLOURS when it passes through a prism.
Newton's experiment: 'Isaac Newton showed that white light is a MIXTURE of seven colours. A prism SEPARATES them because each colour bends by a DIFFERENT amount.'
The Seven Colours of the Spectrum (VIBGYOR)
| Colour | Wavelength (increases down) |
|---|---|
| V — Violet | Shortest wavelength (bends MOST) |
| I — Indigo | |
| B — Blue | |
| G — Green | |
| Y — Yellow | |
| O — Orange | |
| R — Red | Longest wavelength (bends LEAST) |
'Red light bends the LEAST — it travels FASTEST through glass. Violet bends the MOST — it travels SLOWEST through glass.'
The Rainbow — Nature's Spectrum
Rainbow: A natural spectrum of light formed when sunlight is DISPERSED by WATER DROPLETS in the atmosphere.
Conditions for a rainbow:
- Sunlight AND rain at the SAME time.
- Observer standing with the Sun BEHIND.
- Water droplets act as TINY PRISMS.
'Rainbows form when sunlight enters a raindrop, REFRACTS, reflects off the inside back surface, and REFRACTS AGAIN as it exits — splitting into colours.'
AP Context: 'Rainbows are commonly seen in AP during the MONSOON season (June-December) when rain showers alternate with sunshine.'
7. Common Mistakes and Fixes
| Mistake | Why It Is Wrong | Correct Approach |
|---|---|---|
| In a plane mirror, left and right are NOT swapped | They ARE swapped — lateral inversion | The image's left hand is the object's right hand |
| Concave mirrors always form real images | When the object is between focus and mirror, the image is VIRTUAL | Check object position relative to focus |
| Convex mirrors always form enlarged images | Convex mirrors ALWAYS form SMALLER images | Convex = smaller field of view, diminished image |
| White light is colourless | White light is a MIXTURE of ALL seven colours | Prism reveals the colours hidden in white light |
| A concave lens can form real images | Concave lenses ALWAYS form virtual images | Concave = diverging = always virtual and diminished |
8. AP SSC Exam Focus
| Topic | Marks | Question Type |
|---|---|---|
| Laws of reflection | 2-3 | State and explain |
| Plane mirror — image properties | 3-4 | List and explain |
| Spherical mirrors — concave and convex | 3-4 | Compare, uses |
| Lenses — convex and concave | 2-3 | Compare, uses |
| Dispersion by prism (VIBGYOR) | 2-3 | Explain, diagram |
| Rainbow formation | 1-2 | Explain |
Quick Self-Test
Q1. State the two laws of reflection. A1. 1) The incident ray, reflected ray, and normal at the point of incidence lie in the same plane. 2) The angle of incidence equals the angle of reflection (∠i = ∠r).
Q2. What type of image is formed by a plane mirror? A2. VIRTUAL, ERECT (upright), SAME SIZE as the object, and laterally inverted. The image is as far behind the mirror as the object is in front.
Q3. Why are convex mirrors used as rear-view mirrors in vehicles? A3. Convex mirrors provide a WIDER field of view, allowing drivers to see more of the road behind them, even though objects appear smaller.
Q4. What is dispersion of light? A4. The splitting of white light into its seven component colours when it passes through a prism.
Q5. Name the seven colours of the spectrum in order. A5. Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR).
Q6. What is the difference between a concave and convex lens? A6. A convex lens is thicker in the middle and CONVERGES light (can form real or virtual images). A concave lens is thinner in the middle and DIVERGES light (always forms virtual, diminished images).
Q7. How is a rainbow formed? A7. Sunlight is dispersed by water droplets in the atmosphere. Each droplet acts as a tiny prism, splitting sunlight into its component colours.
