Chapter 15: Light & Optics
प्रकाश (समतल, अवतल व उत्तल दर्पण, लेंस एवं प्रकाश का विक्षेपण)
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- Rectilinear propagation: Light travels strictly along straight paths. Demonstrated through looking at a burning candle through a straight pipe vs a bent pipe.
- Plane Mirror Characteristics: The image formed is erect, virtual (cannot be captured on a screen), of the exact same size as the object, and lies at the same distance behind the mirror as the object is in front of it. Exhibits Lateral Inversion (left appears right, right appears left; word 'AMBULANCE' written laterally reversed).
- Concave Mirror (अवतल दर्पण): Inner curved reflecting surface. Can form real, inverted images (magnified or diminished). When object is placed very close to mirror, forms an erect, magnified, virtual image. Uses: Used by dentists to see enlarged teeth images; headlights of cars, torches, solar cookers.
- Convex Mirror (उत्तल दर्पण): Outer curved reflecting surface. Always forms an erect, virtual, and diminished (smaller) image. Has a wide field of view. Uses: Side rear-view mirrors in cars/motorcycles.
- Convex Lens (उत्तल लेंस): Thicker at the center, thinner at edges. Converging lens. Can form real, inverted images; when object is held close, acts as a Magnifying Glass forming an erect, enlarged virtual image.
- Concave Lens (अवतल लेंस): Thinner at the center, thicker at edges. Diverging lens. ALWAYS forms an erect, virtual, and smaller (diminished) image regardless of distance.
- Dispersion of Light & Rainbow: Sunlight consists of 7 colors (VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red). A triangular glass prism separates white sunlight into a spectrum of 7 colors. Newton's disc: Circular disc painted with the 7 colors in equal sectors appears almost white when rotated rapidly.
- Trap: Which lens ALWAYS forms a virtual, erect, and diminished image? Correct: CONCAVE LENS (अवतल लेंस).
- Trap: Which mirror is used by dentists to view enlarged teeth? Correct: CONCAVE MIRROR (अवतल दर्पण).
- Trap: Why are side mirrors of cars convex? Correct: Because CONVEX MIRRORS form erect, smaller images and provide a much WIDER field of view.
NCERT Core Facts: Mirrors, Lenses & Dispersion of Sunlight
In CTET Paper 2 Science, optics questions test spherical mirror applications (dentists vs car rear-view), real vs virtual image properties, and Newton's disc.
1. Plane Mirror & Lateral Inversion
- Image Properties:
- a.Erect (upright).
- b.Virtual (cannot be obtained on a physical screen).
- c.Same size as the object.
- d.Formed at the same distance behind the mirror as the object is in front.
- Lateral Inversion (पार्श्व परिवर्तन):
- The left side of the object appears on the right side of the image, and vice-versa.
- Application: The word "AMBULANCE" is written in laterally reversed letters so drivers ahead see it correctly in their rear-view mirrors.
2. Spherical Mirrors Comparison
| Feature | Concave Mirror (अवतल दर्पण) | Convex Mirror (उत्तल दर्पण) |
|---|---|---|
| Curvature | Reflecting surface curves Inwards | Reflecting surface curves Outwards |
| Image Type | Can form real, inverted (diminished or magnified) OR virtual, erect, magnified (when held close). | Always forms virtual, erect, and diminished (smaller) images. |
| Field of View | Narrow | Wide field of view |
| NCERT Applications | Dentist examination mirrors, reflectors of vehicle headlights, searchlights, shaving mirrors. | Side rear-view mirrors of automobiles to view trailing traffic. |
3. Spherical Lenses Comparison
| Lens Type | Shape Profile | Optical Nature | Image Formation |
|---|---|---|---|
| Convex Lens (उत्तल लेंस) | Thicker at middle, thinner at edges | Converging (bends light rays inward to a focal point) | Forms real & inverted images; forms virtual, erect, magnified image when object is close → Magnifying Glass. |
| Concave Lens (अवतल लेंस) | Thinner at middle, thicker at edges | Diverging (spreads light rays outward) | Always forms virtual, erect, and diminished images, no matter where the object is placed. |
4. Sunlight: A Blend of Seven Colors (Dispersion)
- Prism Dispersion: Passing a narrow beam of sunlight through a glass prism splits the white light into seven spectral colors:
- Rainbow Formation: Sunlight is refracted and dispersed by microscopic floating water droplets in the atmosphere after a rain shower. A rainbow is always visible in the direction opposite to the sun.
- Newton's Disc: Isaac Newton painted a circular cardboard disc with the seven spectral colors in equal sectors. When spun rapidly, the colors blend together due to persistence of vision, making the disc appear whitish.
Test Your Mastery: Light & Optics
Authentic CBSE exam questions tested directly from this NCERT chapter.
A doctor uses a spherical mirror to examine the teeth and throat of a patient, while a driver uses another mirror to view traffic behind. Which pair of mirrors is used?
एक डॉक्टर मरीज के दाँत और गले की जाँच के लिए एक गोलीय दर्पण का उपयोग करता है, जबकि एक चालक पीछे के ट्रैफिक को देखने के लिए दूसरे दर्पण का उपयोग करता है। दर्पणों का सही युग्म कौन-सा है?
Which of the following optical components ALWAYS forms a virtual, erect, and diminished image of an object regardless of distance?
निम्नलिखित में से कौन-सा प्रकाशिक घटक वस्तु की दूरी पर ध्यान दिए बिना हमेशा एक आभासी, सीधा और छोटा प्रतिबिंब बनाता है?
When a circular disc painted with the seven rainbow colors (Newton's disc) is rotated rapidly in daylight, it appears:
जब इंद्रधनुष के सातों रंगों से पुती एक वृत्ताकार चकती (न्यूटन की चकती) को दिन के प्रकाश में तेजी से घुमाया जाता है, तो वह दिखाई देती है:
Which mirror is used by dentists to inspect teeth?
A concave mirror is used by dentists because it produces an enlarged, upright image of teeth when held close.
Which lens always forms an erect, virtual, and diminished image?
A concave lens always forms an erect, virtual, and diminished image.
What happens when Newton's seven-color disc is rotated rapidly?
The seven colors blend together due to persistence of vision, making the disc appear white.