Where Must a Point Source of Light Be Placed in Front of a Convex Lens So as to Obtain a Parallel Beam of Light? - Physics

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Where must a point source of light be placed in front of a convex lens so as to obtain a parallel beam of light?



If the point source of light is placed at the 'first focal point' (i.e. the focal point on the left of the optical centreof the convex lens), then the rays of light after being refracted through the length obtain a parallel beam of light.

Concept: Convex Lens
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Chapter 5: Refraction through a Lens - Exercise 5 (B) 1 [Page 119]


Selina Concise Physics Class 10 ICSE
Chapter 5 Refraction through a Lens
Exercise 5 (B) 1 | Q 3 | Page 119


A student is using a convex lens of focal length 18 cm to study the image formation by it for the various positions of the object. He observes that when he places the object at 27 cm, the location of the image is at 54 cm on the other side of the lens. Identify from the following diagram the three rays that are obeying the laws of refraction and may be used to draw the corresponding ray diagram.

(A) 1, 2 and 4

(B) 1, 3 and 5

(C) 2, 4 and 5

(D) 2, 3 and 4

(a) Draw a ray diagram to show the formation of image by a concave lens when an object is placed in front of it.

(b) In the above diagram mark the object distance (u) and the image distance (v) with their proper signs (+ve or –ve as per the new Cartesian sign convention) and state how these distances are related to the focal length (f) of the concave lens in the case.

(c) Find the nature and power of a lens which forms a real and inverted image of magnification –1 at a distance of 40 cm from the optical centre.

If you focus the image of a distant object, whose shape is given below, on a screen using a convex lens, the shape of the image of this object on the screen would be:





Draw a ray diagram to show the formation of the image of an object be placed so that a real and inverted image of the same size as the object is obtained using a convex lens

A lens forms an erect, magnified and virtual image of an object.

(i) Name the lens.
(ii) Draw a labelled ray diagram to show the image formation. 

Complete the following table:

Instrument Number of Convex
Simple Microscope .............. ..............
Compound Microscope .............. ..............
Telescope .............. ..............

f the image formed by a convex lens is of the same size as that of the object, what is the position of the image with respect to the lens? 

For what position of an object a real, diminished image is formed by a convex lens?

Define principal axis, principal focus and focal length of a convex lens.

What kind of lens can form: 

 am erect diminished image? 

An image formed on a screen is three times the size of the object. The object and screen are 80 cm apart when the image is sharply focussed. 

 State which type of lens is used. 

A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. The focal length of this combination of lenses will be:
(a) +10 cm
(b) +20 cm
(c) −10 cm
(d) −20 cm

Complete the following sentence:

A convex lens is placed in water. Its focal length will ……………………….

Study the following diagram :

Name a device in which this action of lens is used.

A converging lens forms the image of an object placed in front of it, beyond 2F2 of the lens.

Draw a ray diagram to show the formation of image.

A lens forms an inverted image of an object what is the nature of the image real or virtual?

A lens forms an upright and magnified image of an object  name the lens

Distinguish between:

Concave lens and Convex lens

i. Which type of microscope has the arrangement of lenses shown in the  adjoining figure?
ii. Label the figure correctly.
iii. Write the working of this microscope.
iv. Where does this microscope used?
v. Suggest a way to increase the efficiency of this microscope.

The image obtained while finding the focal length of convex lens is ....................

Observe the following figure and answer the questions.

a) Which optical instrument shows arrangement of lenses as shown in the figure?
b) Write in brief the working of this optical instrument.
c) How can we get different magnifications in this optical instrument?
d) Draw the figure again and labelled it properly

Yesh find out F1 and F2 of symmetric convex lens experimentally then which conclusion is true.

Observe the following figure and complete the table:

Points Answer
(i) Position of the object  
(ii) Position of the image  
(iii) Size of the image   
(iv) Nature of the image  

Draw neat diagram to show the
Convergent action of a convex lens,

A pin 2 cm long is placed 12 cm away from a convex lens at right angles to the principal axis. If the focal length of the lens is 20 cm, by scale drawing find the size of the image and its magnification.

At what position a candle of length 3 cm be placed in front of a convex lens so that its image of length 6 cm be obtained on a screen placed at distance 30 cm behind the lens?

What is the focal length of lens in part (a)?

If an object is placed in front of a convex lens beyond 2F1, then what will be the position, relative size, and nature of an image which is formed? Explain with a ray diagram.

Rewrite the following statement by selecting the correct option:
The power of a convex lens is 2.5 dioptres. Its focal length is ______________.

For a specific glass lens f = 0.5 m. This is the only information given to the student. Which type of lens is given to him and what is its power?

_______ times larger images can be obtained by using a simple microscope.

_______ is a combination of two convex lenses with small focal length.

Object at 2F1 of a convex lens : Image at 2F2 : : Object at F1 : _______

Simple microscope : Number of convex lens one : : compound microscope : _______

: Object near the lens : : ______ :

Find the odd one out and give its explanation.

Find the odd one out and give its explanation.

Write the name.

The lens used in simple microscope.

Write scientific reason.

Adults need bifocal lens spectacle.

Observe the given figure and answer the following questions.

  1. Where is the above type of lens construction used?
  2. What type of image is formed by an objective lens?
  3. What happens instead of placing at Fo if the object is placed in between O and Fo?

Differentiate convex lens and concave lens.

  1. In which type of microscope do you find the lens arrangement as shown in the following diagram?
  2. Write about the working and the use of this microscope.

The above images are that of a specialized slide projector. Slides are small transparencies mounted in sturdy frames ideally suited to magnification and projection since they have a very high resolution and a high image quality. There is a tray where the slides are to be put into a particular orientation so that the viewers can see the enlarged erect images of the transparent slides. This means that the slides will have to be inserted upside down in the projector tray.

To show her students the images of insects that she investigated in the lab, Mrs. Iyer brought a slide projector. Her slide projector produced 500 times enlarged and inverted image of a slide on a screen 10 m away.

a. Based on the text and data given in the above paragraph, what kind of lens must the slide projector have?

b. If v is the symbol used for image distance and u for object distance then with one reason state what will be the sign for `"𝑣"/"𝑢"` in the given case?

c. A slide projector has a convex lens with a focal length of 20 cm. The slide is placed upside down 21 cm from the lens. How far away should the screen be placed from the slide projector’s lens so that the slide is in focus?


c. When a slide is placed 15 cm behind the lens in the projector, an image is formed 3 m in front of the lens. If the focal length of the lens is 14 cm, draw a ray diagram to show image formation. (not to scale)

Distinguish between Concave lens and Convex lens.

Distinguish between:

Concave lens and Convex Lens

Distinguish between Concave lens and Convex Lens.

Distinguish between:

Concave lens and Convex Lens

Distinguish between:

Concave lens and Convex Lens


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