Simply CBSE

Light — Class 8 Science

Quick revision notes for exam preparation.

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📌 Key Points

  • Light is a form of energy that enables us to see objects
  • Light travels in straight lines (rectilinear propagation)
  • Speed of light = 3 × 10⁸ m/s (3 lakh km/second)
  • Angle of incidence (∠i) = Angle of reflection (∠r)
  • Incident ray, reflected ray, and normal lie in the same plane
  • Plane mirror forms virtual, erect, same-size, laterally inverted image
  • Image distance = Object distance (in plane mirror)
  • Concave mirror: curves inward, converges light, used by dentists
  • Convex mirror: curves outward, diverges light, used as rear-view mirror
  • Focal length (f) = Radius of curvature (R) / 2
  • Convex lens: thicker in middle, converges light
  • Concave lens: thinner in middle, diverges light
  • Dispersion: splitting of white light into 7 colours (VIBGYOR)
  • Violet bends most, Red bends least
  • Rainbow: Red on outer edge, Violet on inner edge

📘 Important Definitions

Reflection of Light
The bouncing back of light when it strikes a surface is called reflection of light.
Normal
An imaginary line perpendicular to the reflecting surface at the point of incidence.
Lateral Inversion
The left-right reversal of an image as seen in a plane mirror.
Real Image
An image that can be obtained on a screen. Light rays actually meet to form it.
Virtual Image
An image that cannot be obtained on a screen. Light rays only appear to meet.
Dispersion
The splitting of white light into its seven component colours when passed through a prism.
Spectrum
The band of seven colours (VIBGYOR) obtained after dispersion of white light.

🔢 Formulas & Laws

Laws of Reflection

∠i = ∠r

Angle of incidence equals angle of reflection

Focal Length Formula

f = R / 2

Focal length is half of radius of curvature

Speed of Light

c = 3 × 10⁸ m/s

Fastest speed in the universe

⚠️ Common Mistakes

✗ Wrong: Thinking diffused reflection doesn't follow laws of reflection

✓ Correct: Diffused reflection DOES follow laws of reflection, but surface irregularities cause scattering

✗ Wrong: Confusing 'erect' with 'inverted' images

✓ Correct: Erect = upright (standing). Inverted = upside down

✗ Wrong: Mixing up concave and convex mirrors

✓ Correct: conCAVE = caves inward. conVEX = bulges outward

✗ Wrong: Confusing rainbow colour order with prism spectrum

✓ Correct: Rainbow: Red outside, Violet inside. Prism: Violet bends more than Red

✗ Wrong: Thinking virtual images are 'not real'

✓ Correct: Virtual images exist and can be seen, just cannot be projected on a screen

📝 Exam Focus

These questions are frequently asked in CBSE exams:

State the laws of reflection
2m★★★
Differentiate between real and virtual images
3m★★★
Why are convex mirrors used as rear-view mirrors?
2m★★★
What is lateral inversion? Give example
2m★★
Explain the formation of rainbow
5m★★
Define dispersion. Name the colours in spectrum
2m★★★
Compare concave and convex mirrors (uses)
3m★★
What is the relation between f and R?
1m★★★

Diagram to practice: Ray diagram for reflection, concave mirror image formation, dispersion through prism

🎯 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!