Image Formation by Concave and Convex Lenses

On Aug 31, 2016

Image Formation By Convex Lens in Different Cases

Case 1: Object at Infinity

A point object lying on the principal axis:

Rays come parallel to the principal axis and after refraction from the lens, actually meet at the second principal focus F₂.

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Fig. Convex lens point object at infinity, image at focus.

The image is formed at focus F₂. It is real and point sized.

A big size object with its foot on the principal axis:

Parallel rays come inclined to the principal axis. Image of foot is formed at the focus.

Image is formed at the second principal focus F₂. It is real inverted and diminished (smaller in size than the object). (Fig.)

Parallel rays from infinity

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Fig. Convex lens : big size object at infinity, image at focus

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Case 2: Object at distance more than twice the Focal Length

Real object AB has its image A’B’ formed between distance f and 2f.

The image is real inverted and diminished (smaller in size than the object)

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Fig. Convex lens : object beyond 2f, image between f and 2f.

Case 3: Object at distance twice the Focal Lengths

Real object AB has its image A’B’ formed at distance 2f.

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Fig. Convex lens : object at distance 2f, image at distance 2f.

The image is real, inverted and has same size as the object.

Case 4: Object at distance more than Focal Length and less than twice is Focal Length

Real object AB has its image A’B’ formed beyond distance 2f.

Image-Formation-By-Concave-And-Convex-Lenses-5

Fig. Convex lens : object at distance between f and 2f, image beyond 2f.

The image is real inverted and enlarged (bigger in size than the object).

Case 5: Object at Focus

Real object AB has its image formed at infinity.

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Fig. Convex lens : object at focus, image at infinity.

The image is imaginary inverted (refracted rays to downward) and must have very large size.

Case 6: Object between Focus and Optical Centre

Real object AB has its image A’B’ formed in front of the lens.

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Image Formation By Concave Lens in Different Cases

Case 1: Object at infinity

A point object lying on the principal axis:

Rays come parallel to the principal axis and after refraction from the lens, appears to come from the second principal focus F₂.

Image-Formation-By-Concave-And-Convex-Lenses-9

Fig. Concave lens point object at infinity, image at focus.

The image is formed at focus F2. It is virtual and point sized (fig.)

A big size object with its foot on the principal axis:

Parallel rays come inclined to the principal axis. Image of foot is formed at focus.

The image is formed at the second principal focus F₂.

It is virtual–erect and diminished (fig.)

Image-Formation-By-Concave-And-Convex-Lenses-10

Fig. Concave lens : big size object at infinity image at focus.

Case 2: Object at a Finite Distance

Real object AB has its image A’B’ formed between second principal focus F₂ optical centre C.

The image is virtual–erect and diminished.