Class 10 > Unit # 13: Geometrical Optics > Image Formation by Convex Lens

Talha's Physics Academy

Geometrical Optics - Lenses and Optical Instruments

Define convex and concave lens and describe power of lens.

A lens is a piece of transparent material bounded by two spherical surfaces (or one spherical and one plane surface) that can refract and focus or diverge a transmitted beam of light.

1. Converging or Convex Lenses

"A convex lens is thicker in the middle and thinner at the edges. It bends light rays towards its optical axis so that a beam of parallel rays converges at a focal point ($F$)."

Figure: Refraction of light through convex (converging) lenses.

For example, in bright sunlight, a convex lens can produce a bright, concentrated spot of light intense enough to ignite paper.

2. Diverging or Concave Lenses

"A concave lens is thinner in the middle and thicker at the edges. It bends light rays away from its optical axis so that a beam of parallel rays diverges in different directions."

Figure: Refraction of light through concave (diverging) lenses.

Power of a Lens

"The power of a lens is defined as the reciprocal of its focal length in meters."

Mathematically, power ($P$) is expressed as:

$P = \frac{1}{f \text{ (in meters)}}$

Unit of Power: The SI unit of power of a lens is the Dioptre (D). One Dioptre is defined as the power of a lens whose focal length is one meter ($f = 1\text{ m} \implies P = 1\text{ D}$).

Determine Position, Size, and Nature of Image When the Object is Placed in Front of Convex Lens

(i) Object Beyond $2F$

• Position of Image: Formed between $F$ and $2F$ on the other side.
• Size of Image: Smaller in size (diminished).
• Nature of Image: Real and inverted.

Figure: When object is beyond 2F.

(ii) Object At $2F$

• Position of Image: Formed at $2F$ on the other side.
• Size of Image: Equal to the size of the object.
• Nature of Image: Real and inverted.

Figure: When object is at 2F.

(iii) Object Between $F$ and $2F$

• Position of Image: Formed beyond $2F$.
• Size of Image: Larger in size (magnified).
• Nature of Image: Real and inverted.

Figure: When object is between F and 2F.

(iv) Object at $F$

• Position of Image: Formed at infinity.
• Size of Image: Highly enlarged.
• Nature of Image: Real and inverted.

Figure: When object is at F .

(v) Object Between Optical Center ($O$) and $F$

• Position of Image: Formed on the same side of the lens as the object.
• Size of Image: Magnified (larger than the object).
• Nature of Image: Virtual and erect.

Figure: When object is between O and F .

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