Class 10 Science Notes – Chapter 7: Lenses (Maharashtra Board)

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12 Jun, 2025
Class 10 Science Notes – Chapter 7: Lenses (Maharashtra Board)

Class 10 Science Notes – Chapter 7: Lenses (Maharashtra Board)

A lens is a transparent medium bounded by two surfaces, at least one of which is curved. Lenses are used to converge or diverge light and are essential in devices like spectacles, microscopes, and cameras.

🔵 Types of Lenses

  • Convex Lens (Converging Lens): Thicker in the middle and thinner at the edges.
  • Concave Lens (Diverging Lens): Thinner in the middle and thicker at the edges.

📌 Terms Related to Lenses

  • Principal Axis: The line passing through the optical center and the center of curvature of both surfaces.
  • Optical Center (O): The central point of a lens.
  • Principal Focus (F): Point on the principal axis where rays parallel to the axis converge (convex) or appear to diverge from (concave).
  • Focal Length (f): Distance between the optical center and principal focus.

🌀 Refraction Through a Lens

Refraction of light through a lens depends on the shape of the lens and the material used. Rays of light change direction as they pass through the lens due to the difference in optical densities.

📷 Ray Diagrams for Convex Lens

  • Ray parallel to the axis passes through the principal focus after refraction.
  • Ray passing through optical center continues without deviation.
  • Ray passing through focus becomes parallel after refraction.

📷 Ray Diagrams for Concave Lens

  • Ray parallel to axis appears to diverge from the principal focus.
  • Ray directed towards focus emerges parallel to axis.
  • Ray through optical center goes undeviated.

📊 Image Formation Table

Object Position Convex Lens Image Concave Lens Image
At infinity Point image at focus, real and inverted Point image at focus, virtual and erect
Beyond 2F Between F and 2F, real, inverted, diminished Virtual, erect, diminished
At 2F At 2F, real, inverted, same size Virtual, erect, diminished
Between F and 2F Beyond 2F, real, inverted, magnified Virtual, erect, diminished
At F At infinity, real, inverted, highly magnified Virtual, erect, diminished
Between lens and F Virtual, erect, magnified Virtual, erect, diminished

🧠 Lens Formula and Magnification

Lens formula: 1/f = 1/v - 1/u

Magnification (M): M = h₂/h₁ = v/u

  • h₂ = height of image
  • h₁ = height of object
  • v = image distance
  • u = object distance

💡 Sign Convention

Distances measured to the left of the lens are negative; to the right are positive. Heights above the principal axis are positive, and below are negative.

⚙️ Uses of Lenses

  • Spectacles for correcting vision
  • Magnifying glasses
  • Cameras
  • Microscopes and telescopes

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