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medium importancemedium difficulty5 min

Image By Convex Lens

A convex lens can form real or virtual images depending on where the object is placed.

In one line: Beyond focus, image on the other side; within focus, image on the same side.

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Understand it

If the object is beyond the focus, the lens usually forms a real image on the other side. If the object is between the focus and the lens, the image is virtual, erect, and enlarged on the same side. This chapter concept is often tested through ray diagrams and image-formation tables.

Example

An object placed beyond 2F gives a real inverted diminished image between F and 2F.

Formula and rule

A ray parallel to the principal axis passes through the focus, and a ray through the optical centre goes undeviated.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

If an object is beyond 2F, the image forms between F and 2F on the other side, and it is real, inverted, and smaller than the object.

Method you can reuse

  1. 1Locate the object relative to F and 2F.
  2. 2Use the standard rays.
  3. 3Predict image position, nature, and size.
  4. 4Check whether the image is real or virtual.

Sketch it in your answer

Draw the principal axis, optical centre, focal points F and 2F, then trace the two standard rays from the object.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

When an object is placed between the focus and a convex lens, what kind of image is formed?

Think first, then show answer
A virtual, erect, and enlarged image is formed on the same side of the lens as the object.

Common wrong answer

Saying a virtual image forms on the opposite side of the lens. For a convex lens, a virtual image forms on the same side as the object.

Don't mix this up

Students often think every convex lens image is real. A convex lens can also form a virtual image when the object is within the focal length.

Exam tip

Remember the standard image table for convex lenses; it saves time in both theory and ray-diagram questions.

Image By Convex Lens diagram

Open full screen to read every label clearly.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions ask you to predict image nature from object position or to draw the ray diagram for a convex lens.
  1. 1

    Enough for 1 mark

    A convex lens can form real or virtual images depending on where the object is placed.

  2. 2

    Add for 2 marks

    Example: An object placed beyond 2F gives a real inverted diminished image between F and 2F.

  3. 3

    Add for 3 marks

    If the object is beyond the focus, the lens usually forms a real image on the other side.

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