Question 12 Marks
If the lens is of focal length 25 cm. Calculate the power of the lens.
Answer
View full question & answer→Focal length = 25 cm = 0.25 m
$P=\frac{1}{0.25}=4 D$
$P=\frac{1}{0.25}=4 D$
Questions
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26 questions · timed · auto-graded
Question 22 Marks
A convex lens is of focal length 20 cm. Find its power.
Answer
View full question & answer→Given: Focal length f = 20cm = 0.2 m
Power $=\frac{1}{\text { Focal length in } m }=\frac{1}{0.2}=5 D$
Power $=\frac{1}{\text { Focal length in } m }=\frac{1}{0.2}=5 D$
Question 32 Marks
What will be the behaviour of a lens, if it is placed in water instead of air?
Answer
View full question & answer→Focal length depends upon the refractive index of the material of the lens with respect to the medium surrounding it. Since the surrounding medium changes from air to water, the refractive index decreases and so the focal length of the lens increases.
Question 42 Marks
Out of the two lenses one concave and the other convex state which is a convergent or a divergent type of a lens. Give a reason for your answer.
Answer
View full question & answer→The convex lens is a convergent type of a lens since it converges the light rays to a point while the concave lens is a divergent type of lens since it spreads the light rays.
Question 52 Marks
State two applications a concave lens .
Answer
View full question & answer→The two applications of a concave lens are:-
(i) A person suffering from short-sightedness or myopia wears spectacles having a concave lens.
(ii) A concave lens is used as an eye lens in a Galilean telescope to obtain an erect final image of the object.
(i) A person suffering from short-sightedness or myopia wears spectacles having a concave lens.
(ii) A concave lens is used as an eye lens in a Galilean telescope to obtain an erect final image of the object.
Question 62 Marks
State two applications of a convex lens.
Answer
View full question & answer→The two applications of a convex lens are:-
(i) It is used as an objective lens in a telescope, camera, slide projector, etc.
(ii) With its short focal length, it is also used as a magnifying glass.
(i) It is used as an objective lens in a telescope, camera, slide projector, etc.
(ii) With its short focal length, it is also used as a magnifying glass.
Question 72 Marks
What is magnifying glass? State its two uses.
Answer
View full question & answer→Magnifying glass is a convex lens of short focal length. It is mounted in a lens holder for practical use.
It is used to see and read the small letters and figures. It is used by watch makers to see the small parts and screws of the watch.
It is used to see and read the small letters and figures. It is used by watch makers to see the small parts and screws of the watch.
Question 82 Marks
Define magnification produced by a lens. State its unit.
Answer
View full question & answer→The magnification produced by a lens is defined as the ratio of the size of the image to the size of the object, i.e.,
Magnification $(m)=\frac{\text { Size of the image }}{\text { Size of the object }}$
It has no unit.
Magnification $(m)=\frac{\text { Size of the image }}{\text { Size of the object }}$
It has no unit.
Question 92 Marks
Can a normal convex lens behave like a concave lens and vice-versa?
Answer
View full question & answer→Yes, If a lens is kept in a medium whose refractive index is more than that of the material of the lens, the normal convex lens starts behaving like a concave lens and vice-versa.
Question 102 Marks
When does a ray of light falling on a lens pass through it undeviated?
Answer
View full question & answer→A ray of light falling on the lens passes through it undeviated when it passes through the optical centre.
Question 112 Marks
An object is placed in front of a lens between its optical centre and focus. The formed image is virtual, erect, and diminished. Name the lens used.
Answer
View full question & answer→The lens used is a concave lens.
Question 122 Marks
Define the power of a lens.
Answer
View full question & answer→The power of a lens (P) is the ability of the lens to converge or diverge an incident beam falling on it. It is expressed as the reciprocal of focal length (f).
The power of a lens (P) is given by
$P=\frac{1}{f(\text { in metre })}$
The power of a lens (P) is given by
$P=\frac{1}{f(\text { in metre })}$
Question 132 Marks
State the position of object, position of image, nature of image when: Convex lens is used as objective lens of astronomical telescope.
Answer
View full question & answer→Object is at infinity.
Image is formed at $F_2$ on the other side of lens. Image is real; inverted and almost diminished to a point.
Image is formed at $F_2$ on the other side of lens. Image is real; inverted and almost diminished to a point.
Question 142 Marks
What are conjugate foci?
Answer
View full question & answer→Two points lying on the principal axis such that the image of an object kept at such point is formed on the other point, are called conjugate foci.
Question 152 Marks
Where should an object be placed in front of a convex lens so as to form an inverted and enlarged image? Will the image be real or virtual? Draw a ray diagram to illustrate your answer.
Answer
View full question & answer→To form an inverted and enlarged image by a convex lens, the object should be placed between F and 2F in front of the lens.
The image will be real.
The ray diagram is shown in the figure. For the object AB, the images are A’B’.
The image will be real.
The ray diagram is shown in the figure. For the object AB, the images are A’B’.
Question 162 Marks
Complete the ray diagram to show how the rays pass into and out of the lens?
Answer
View full question & answer→The path of the ray is shown in the diagram.
Question 172 Marks
Show, using a ray diagram, how a converging lens can be used to form a virtual image.
Answer
View full question & answer→The ray diagram is given below.
Question 182 Marks
Show with the help of a ray diagram, the path of the ray when incident normally on the first surface of the glass block, through the block and the liquid.
Answer
View full question & answer→We are assuming that the glass block is bounded by parallel plane refracting surfaces.
Question 192 Marks
In the figure given below L is a convex lens, M is a plane mirror and S is a point source of light. Rays of light from the source S return to their point of origin. Complete the ray diagram to show this. What is the point S called?
Answer
View full question & answer→The complete ray diagram is shown below:
The point S is called the focus of the lens.
The point S is called the focus of the lens.
Question 202 Marks
Copy the following figure and complete it to show the formation of the image of the object AB. Name the lens used in the figure.
Question 212 Marks
Draw a diagram to show the convergent action of a convex lens by treating it as a combination of glass block and two triangular glass prisms, with the aid of two parallel incident rays.
Answer
View full question & answer→The diagram is shown below:
Question 222 Marks
Complete the following diagram and state what happens to the ray of light after refraction through the lens.
Answer
View full question & answer→A ray of light that passes through the principal focus or appears to pass through the principal focus, goes parallel to its principal axis.
Question 232 Marks
Complete the following diagram and state what happens to the ray of light after refraction through the lens.
Answer
View full question & answer→A ray of light goes undeviated through the optical centre of a lens, whether convex or concave.
Question 242 Marks
The diagram showed a lens as a combination of one glass block and two prisms. Complete the ray diagram and show the part of the incident ray AB after passing through the lens.
(i) Name of the lens formed by the combination.
(ii) What is the line XX’ called?
(iii) Mark the focus F.
(i) Name of the lens formed by the combination.
(ii) What is the line XX’ called?
(iii) Mark the focus F.
Answer
View full question & answer→A ray of light parallel to the principal axis, after refraction by the lens, either passes through its principal focus or appear to pass through its principal focus.
Question 252 Marks
An erect, magnified, and a virtual image is formed when an object is placed between the optical centre and principal focus of a lens.
(i) Name the lens.
(ii) Draw a ray diagram to show the formation of the image with the above-stated characteristics.
(i) Name the lens.
(ii) Draw a ray diagram to show the formation of the image with the above-stated characteristics.
Answer
View full question & answer→(i) The lens is convex.
(ii)
(ii)
Question 262 Marks
Diagram shows an object $AB$ placed on the principal axis $B$ of a convex lens placed in air. $F_1$ and $F_2$ are the two foci of the lens.
(i) Copy the diagram:
Draw a ray of light starting from $B$ and passing through $O$. Show the same ray after refraction by the lens. Draw another ray from $B$ which passes through $F_2$ after refraction by the lens. Locate the final image
(ii) Is the image real or virtual?
(i) Copy the diagram:
Draw a ray of light starting from $B$ and passing through $O$. Show the same ray after refraction by the lens. Draw another ray from $B$ which passes through $F_2$ after refraction by the lens. Locate the final image
(ii) Is the image real or virtual?
Answer
View full question & answer→(i)
(ii) Image is virtual.
(ii) Image is virtual.