Define the Term Principal Foci of a Concave Lens and Show Them with the Help of Proper Diagrams. - Physics


Define the term principal foci of a concave lens and show them with the help of proper diagrams.




A light ray can pass through a lens from either direction. Therefore, a lens has two principal foci.
For a concave lens, the first focal point is a point F1 on the principal axis of the lens such that the incident rays of light appearing to meet at it, after refraction from the lens become parallel to the principal axis of the lens.

The second focal point for a concave lens is a point F2 on the principal axis of the lens such that the rays of light incident parallel to the principal axis, after refraction from the lens, appear to be diverging from this point.

Concept: Concave Lens
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Chapter 5: Refraction through a Lens - Exercise 5 (A) 1 [Page 109]


Selina Concise Physics Class 10 ICSE
Chapter 5 Refraction through a Lens
Exercise 5 (A) 1 | Q 14 | Page 109


Due to gradual weakening of ciliary muscles and diminishing flexibility of the eye lens a certain defect of vision arises. Write the name of this defect. Name the type of lens required by such persons to improve the vision. Explain the structure and function of such a lens.

Explain with the help of a diagram, why the concave lens is also called a diverging lens.

Where must the object be placed for the image formed by a converging lens to be: 

 real, inverted and larger than the object? 

An object is placed 10 cm from a lens of focal length 5 cm. Draw the ray diagrams to show the formation of image if the lens is  converging, 

A beam of light is incident through the holes on one side of a box and emerges out through the holes on its opposite side as shown in the following figure:

The box contains:
(a) a glass prism
(b) a concave lens
(c) a convex lens
(d) a parallel-sided glass slab

When an object is placed 10 cm in front of lens A, the image is real, inverted, magnified and formed at a great distance. When the same object is placed 10 cm in front of lens B, the image formed is real, inverted and same size as the object. 

 What is the nature of lens A?

An object is placed at a distance of 4 cm from a concave lens of focal length 12 cm. Fine the position and nature of the image. 

What kind of lens is used to correct 


How does the action of a convex lens differ from that of a concave lens on a parallel beam of light incident on them? Draw diagram to illustrate your answer.

Study the following diagram :

Name the lens LL’

An object is placed on the axis of a lens. An image is formed by refraction in the lens. For all positions of the object on the axis of the lens, the positions of the image are always always between the lens and the object.State three characteristics of the image.

a) name the lens which always forms an erect and virtual image.
(b) state whether the image in part (a) is magnified or diminished.

State whether the following statement are ‘true’ or ‘false’ by writing T/F against them.

A concave lens, if kept at a proper distance from an object, can form its real image.

State whether the following statement are ‘true’ or ‘false’ by writing T/F against them.

A concave lens forms a magnified or diminished image depending on the distance of object from it.

At what distance from a concave lens of focal length 20 cm a 6 cm tall object be placed so as to obtain its image at 15 cm from the lens? Also calculate the size of the image formed.
Draw a ray diagram to justify your answer  for the above situation and label it.

Explain the working of an astronomical telescope using refraction of light.


A student obtained clear image of window grills on the screen. But the teacher told him to get the image of a tree far away, instead of window. To get a clear image,  the lens must be ............................

Write the SI unit of focal length of a concave lens.

Can a concave lens form an image of size two times that of the object? Give reason?

A concave lens forms the image of an object kept at a distance 20 cm in front of it, at a distance 10 cm on the side of the object.

What is the nature of the image?

Find the focal length of the lens.

Choose the correct alternative and rewrite the following: 

Inside water, an air bubble behaves ....................

In figure give below of thin concave lens, F1 and F2 are its foci, complete the path of the given ray of light after it emerges out of the lens.

Convex lens : converging : : concave lens : _______

Convex magnifying glass is called divergent magnifying glass and concave magnifying glass is called converging magnifying glass.

If the power of a lens is -4.0 D, then it means that the lens is a ______.

Distinguish between Concave lens and Convex Lens.

Distinguish between:

Concave Lens and Convex Lens


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