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Light - Images Formed by Lenses

Grade 7CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A lens is a piece of transparent material bound by two surfaces, of which at least one is spherical. Lenses form images by the refraction of light.

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A Convex Lens is thicker in the middle than at the edges. It converges (bends inward) a parallel beam of light falling on it; therefore, it is called a converging lens.

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A Concave Lens is thinner in the middle than at the edges. It diverges (bends outward) the light rays passing through it; therefore, it is called a diverging lens.

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The nature of the image formed by a convex lens depends on the distance of the object from the lens. It can be real and inverted or virtual, erect, and magnified (when the object is very close, i.e., within the focal length ff).

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A concave lens always forms an image that is virtual, erect, and diminished (smaller in size than the object), regardless of the object's position.

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The distance from the optical center of the lens to the principal focus is known as the focal length, denoted by ff.

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A Real Image is an image that can be obtained on a screen, while a Virtual Image is an image that cannot be obtained on a screen.

📐Formulae

Lens Formula: 1f=1v−1u\text{Lens Formula: } \frac{1}{f} = \frac{1}{v} - \frac{1}{u}

Magnification (m)=hiho=vu\text{Magnification (m)} = \frac{h_{i}}{h_{o}} = \frac{v}{u}

Power of a Lens (P)=1f (in meters)\text{Power of a Lens (P)} = \frac{1}{f \text{ (in meters)}}

💡Examples

Problem 1:

An object is placed at a distance of 20 cm20\text{ cm} from a convex lens. If the image formed is real, inverted, and of the same size as the object, what is the focal length ff of the lens?

Solution:

When a real image is the same size as the object in a convex lens, the object is placed at 2u=2f2u = 2f. Here, distance u=20 cmu = 20\text{ cm}. Therefore, 2f=20 cm  ⟹  f=202=10 cm2f = 20\text{ cm} \implies f = \frac{20}{2} = 10\text{ cm}.

Explanation:

For a convex lens, the image is of the same size as the object only when the object is placed at twice the focal length (2f2f).

Problem 2:

Why is a convex lens used as a magnifying glass?

Solution:

When an object is placed very close to a convex lens (at a distance u<fu < f), it forms a virtual, erect, and highly enlarged image.

Explanation:

The magnifying glass utilizes the property of the convex lens to produce a magnified image of small objects like letters or insects when held close to them.

Problem 3:

A student observes an image through a lens and finds that the image is always smaller than the object and erect, no matter how far the object is moved. Identify the type of lens.

Solution:

The lens is a Concave Lens.

Explanation:

A concave lens is a diverging lens that always produces a virtual, erect, and diminished image (hi<hoh_{i} < h_{o}) regardless of the object's position.

Images Formed by Lenses - Revision Notes & Key Formulas | CBSE Class 7 Science