2 marksShort AnswerSection A

To construct a ray diagram we use two light rays which are so chosen that it is easy to know their directions after refraction from the lens. List these two rays and state the path of these rays after refraction. Use these two rays to locate the image of an object placed between ‘f’ and ‘2f’ of a convex lens.

Light — Reflection and Refraction

Refraction by Spherical Lenses

Official Answer

To construct a ray diagram for a lens, we commonly use the following two rays:

Ray parallel to the principal axis: A ray of light parallel to the principal axis, after refraction through a convex lens, passes through the principal focus ( $F_2$ ) on the other side of the lens.
Ray passing through the optical centre: A ray of light passing through the optical centre ( $O$ ) of a convex lens emerges without suffering any deviation.

Image Location for Object between $F_1$ and $2F_1$ :

When an object is placed between $F_1$ and $2F_1$ of a convex lens, the two rays intersect on the other side of the lens beyond $2F_2$ .

Position of Image: Beyond $2F_2$
Nature of Image: Real and inverted
Size of Image: Magnified (larger than the object)

parallel to principal axispasses through principal focusoptical centrewithout deviationbeyond 2F2real and invertedmagnified

Marking Scheme

1Listing the first ray and its path after refraction: 0.5 Mark
2Listing the second ray and its path after refraction: 0.5 Mark
3Stating the correct position, nature, and size of the image: 0.5 Mark
4Correct ray diagram showing the object between F and 2F and image beyond 2F: 0.5 Mark

Hint

Recall that a convex lens is a converging lens. A ray parallel to the principal axis converges at the focus on the other side, while a ray through the center goes straight.

Quick Oral Answer

If an object is placed between $F$ and $2F$ of a convex lens, where is the image formed and what is its nature? Answer: The image is formed beyond $2F$ on the other side of the lens. It is real, inverted, and magnified.

Analysis & Explanation

The intersection of at least two refracted rays determines the position of the image. By choosing a ray parallel to the principal axis (which must pass through the focus $F_2$ due to the converging nature of the convex lens) and a ray passing through the optical centre $O$ (which remains undeviated because the central region behaves like a thin parallel-sided glass slab), we can easily locate the real intersection point beyond $2F_2$ .

Common Mistakes

1Drawing the refracted ray parallel to the axis instead of passing through the focus.
2Forgetting to mark arrows indicating the direction of light propagation.
3Confusing the focal points $F_1$ (object side) and $F_2$ (image side).

Interesting Facts

The optical centre of a thin lens is a point where light passes through with negligible lateral displacement, which is why we treat it as completely undeviated in Class 10 ray diagrams.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2012?

This question carries 2 marks in the CBSE Class 10 Science 2012 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Light — Reflection and Refraction" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Refraction by Spherical Lenses" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2012 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2012 paper.