📌 Key Points
- Light is a form of energy that travels in straight lines (rectilinear propagation) at a speed of about 3,00,000 km/s.
- Objects that emit their own light are luminous (Sun, candle). Objects that reflect light are non-luminous (Moon, book).
- When light bounces off a surface, it is called reflection. The incoming ray is the incident ray; the bounced ray is the reflected ray.
- The normal is an imaginary line drawn perpendicular to the mirror surface at the point where the light strikes.
- First Law of Reflection: The angle of incidence (i) is always equal to the angle of reflection (r).
- Second Law of Reflection: The incident ray, reflected ray, and normal all lie in the same plane.
- Regular reflection occurs on smooth surfaces (mirror, still water) and produces clear images.
- Diffused (irregular) reflection occurs on rough surfaces (wall, paper) and does not produce clear images. It still follows the laws of reflection.
- A plane mirror forms an image that is virtual, erect, same size, at the same distance behind the mirror, and laterally inverted.
- Lateral inversion: left appears right and right appears left. AMBULANCE is written in reverse so it reads correctly in rear-view mirrors.
- Concave mirror (converging): reflecting surface curves inward. Used by dentists, in torches, shaving mirrors, and solar cookers.
- Convex mirror (diverging): reflecting surface curves outward. Always forms virtual, erect, and diminished images. Used as rear-view mirrors and at blind turns.
- Focal length (f) = Radius of curvature (R) / 2.
- Convex lens: thicker in middle, converging. Used in magnifying glass, cameras, and spectacles for hypermetropia (farsightedness).
- Concave lens: thinner in middle, diverging. Always forms virtual, erect, diminished images. Used in spectacles for myopia (nearsightedness).
- Dispersion: White light splits into 7 colours (VIBGYOR) when passed through a glass prism.
- VIBGYOR order: Violet, Indigo, Blue, Green, Yellow, Orange, Red. Violet bends most, Red bends least.
- Newton's disc: A disc with 7 spectrum colours appears white when spun rapidly, proving white light is a mixture of 7 colours.
- Rainbow forms when sunlight undergoes refraction, dispersion, and internal reflection in water droplets. Red is on the outer edge, violet on the inner edge.
- Real image: light rays actually meet; can be projected on screen; always inverted. Virtual image: rays appear to meet; cannot be on screen; always erect.
📘 Important Definitions
🔢 Formulas & Laws
Focal Length and Radius of Curvature
f = R / 2
Focal length is half the radius of curvature for spherical mirrors
Laws of Reflection
Angle of incidence (i) = Angle of reflection (r)
Both angles are measured from the normal, not from the mirror surface
Speed of Light
c = 3 x 10^8 m/s (approximately 3,00,000 km/s)
Light is the fastest known entity in the universe
VIBGYOR (Spectrum Order)
Violet - Indigo - Blue - Green - Yellow - Orange - Red
Violet bends most (shortest wavelength), Red bends least (longest wavelength)
⚠️ Common Mistakes
✗ Wrong: Measuring angle of incidence from the mirror surface.
✓ Correct: The angle of incidence (and reflection) is always measured from the normal, which is perpendicular to the mirror surface at the point of incidence.
✗ Wrong: Thinking diffused reflection does not follow the laws of reflection.
✓ Correct: Diffused reflection also follows the laws of reflection. Each individual ray obeys the laws, but the normals at different points of a rough surface point in different directions, causing scattering.
✗ Wrong: Confusing which mirror is used for rear-view vs dentist's mirror.
✓ Correct: Convex mirror is used as a rear-view mirror (wider field of view, diminished image). Concave mirror is used by dentists (enlarged image when object is close).
✗ Wrong: Thinking virtual images are invisible or imaginary.
✓ Correct: Virtual images can be clearly seen with our eyes (e.g. your image in a plane mirror). They simply cannot be projected onto a screen because the light rays do not actually converge.
✗ Wrong: Confusing which lens corrects myopia and hypermetropia.
✓ Correct: Concave lens corrects myopia (short-sightedness). Convex lens corrects hypermetropia (long-sightedness/farsightedness).
✗ Wrong: Writing the spectrum colours in wrong order.
✓ Correct: The correct order is VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red. Violet bends the most, Red bends the least.
📝 Exam Focus
These questions are frequently asked in CBSE exams:
Diagram to practice: Practice drawing: (1) Ray diagram showing incident ray, reflected ray, and normal with angles of incidence and reflection, (2) Concave mirror showing pole, focus, centre of curvature, and principal axis with parallel rays converging at focus, (3) Convex and concave lens shapes showing how they bend light rays, (4) Dispersion of white light through a glass prism showing VIBGYOR spectrum.
🎯 Last-Minute Recall
Close your eyes and try to recall: Key definitions, formulas, and 3 common mistakes. If you can recall 80% without looking, you're exam-ready!