case /data -based (4 Marks)

Question 14 Marks

Mr. Rakesh was helping her daughter Aruna at home understanding about the basics of reflection of light. He found the simplest way to convey the role of curved mirrors using stainless steel teaspoon. Aruna got surprised to know that virtual image is that which can be seen but cannot be obtained on the screen. Mr. Rakesh then explained the formation of image using Ray diagrams.
(i) What do you mean by reflection of light?
(ii) The angle between incident ray and reflected ray is $60^{\circ}$. What is the value of angle of incidence?
(iii) A ray of light is incident on a plane mirror at an angle of $30^{\circ}$. what is the angle of reflection?
or
(iv) What happens to a light ray that is incident normally on a surface?

Answer

(i) When a ray of light approaches a smooth polished surface and the light ray bounces back, it is called the reflection of light.
(ii) $30^{\circ}$
(iii) The incident ray will have an angle of reflection of $30^{\circ}$ (made with a surface normal to the mirror surface). The reflected ray will make an angle of $60^{\circ}$ $\left(90^{\circ}-30^{\circ}\right.$ degrees $)$ with the mirror surface.
or
(iv) When a ray of light is incident normally on the surface it retraces its path after reflection.
Angle of incidence $=$ Angle of refraction $=90^{\circ}$

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Question 24 Marks

Study the following table for a convex lens for different positions of object and answer the following questions:

Position of object	Position of image	Relative size of image
At infinity	At focus F_(2)	Highly diminished point sized
Beyond 2F_(1)	Between F_(2) and 2F_(2)	Diminished
At 2F_(1)	At 2F_(2)	Same size
Between F_(1) and 2F_(1)	Beyond 2F_(2)	Enlarged
At focus F_(1)	At infinity	Infinitely large or highly enlarged
Between focus F_(1) and optical centre O	On the same side of the lens as the object	Enlarged

(i) What is the nature of the image, if an object is placed at infinity ?
(ii) Identify the nature of the image for which the object is between focus and optical centre.
(iii) What is position of image, when object is place at focus $\left(f_1\right)$ ?
or
(iv) What is the focal length of a lens for an object placed $50 cm$ from the lens producing virtual image at a distance of $10 cm$ in front of the lens?

Answer

(i) When the object is at infinity, nature of the image will be real, inverted, highly diminished, point sized.
(ii) Virtual and erect
(ii) Infinity
(iv) Given,
Object distance, $\quad u=-50 cm$
Image distance, $\quad v=-10 cm$
As we know that,
$\begin{aligned}\frac{1}{v}-\frac{1}{u} & =\frac{1}{f} \\-\frac{1}{10}-\frac{1}{-50} & \frac{1}{f} \\\frac{-5+1}{50} & =\frac{1}{f} \\f & =-\frac{50}{4} \\& =-12.5 cm\end{aligned}$

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Question 34 Marks

Is there a relationship between the radius of curvature $R$, and focal length $f$, of a spherical mirror ? For spherical mirrors of small apertures, the radius of curvature is found to be equal to twice the focal length. We put this as $R=2 f$. This implies that the principal focus of a spherical mirror lies midway between the pole and centre of curvature.
(i) Write relation between radius of curvature and focal length.
(ii) For which type of mirrors above relation is verified?
(iii) What should be size of the aperture?
or
(iv) Where is the principle focus of a spherical mirror lies?

Answer

(i) $\quad R=2 f$
(ii) Spherical
(iii) Small
or
(iv) Midway between the pole and centre of curvature

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Question 44 Marks

"Change in path of a light ray as it passes from one medium to another medium is called refraction of light."

When light travels from a rarer medium to a denser one, it bends towards the normal $(i>r)$ and when travels from a denser medium to a rarer one. it bends away from the normal $(i<r)$.
Where, $\quad i=$ Angle of incidence
and $\quad r=$ Angle of refraction
We can see refraction in our daily life, some of the examples are given below :
The bottom of a tank or pond containing water appears to be raised due to refraction of light which takes place when light rays pass from the pool of water into the air. The letters appear to be raised when viewed through a glass slab placed over the document because of refraction of light.
When a light ray enters in a glass slab, then the emergent ray is parallel to the incident ray but it is shifted sideward slightly.
In this case, refraction takes place twice, first when ray enters glass slab from air and second when exits from glass slab to air.
(i) What do you mean by optically rarer and denser medium ?
(ii) What is the cause of refraction?
(iii) Draw a ray diagram shotving refraction through a glass slab.
or
(iv) Give one example of refraction from our daily life experience other than the two examples given above.

Answer

(i) $\quad \Lambda$ medium in which the speed of light is more, is known as optically rarer medium and the medium in which speed of light is lesser is known as optically denser medium.
(ii) Speed of light is different in different media. It is lesser in denser medium and higher in rarer medium. So, when light enters a denser medium, its speed reduces and it bends towards the normal and when it enters rarer medium, its speed increases and it bends away from the normal
(iv) A lemon kept in water in a glass tumbler appears to be bigger than its actual size, when viewed from the sides.

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Question 54 Marks

When light ray goes from one transparent medium to another transparent medium, it suffers a change in direction, into second medium. The extent of the change in direction suffered by the phenomenon of change in the path of light rays when going from one medium to another medium is known as refraction. Ray is a given pair of media can be expressed in terms of refractive index. The refractive index is related to an important physical quantity in the relative speed of light in different media.

(i) A ray of light enters into the glass from air. Does it bend towards normal ?
(ii) What is the unit of refractive index?
(iii) Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is $3 \times 10^3 ms ^{-1}$.
or
(iii) When light goes from one medium to another, which of the three parameters, frequency, wavelength, velocity change?

Answer

(i) Yes, it bends towards normal.
(ii) No unit.
$n_m=\frac{\text { Speed of light in vacuum }}{\text { Speed of light in the medium }}=\frac{c}{v}$
Speed of light in vacuum,
$c=3 \times 10^8 ms ^{-1}$
Refractive index of glass,
$n_s=1.50$
Speed of light in the glass,
$\begin{aligned}v & =\frac{c}{n_g} \\& =\frac{3 \times 10^8}{1.50}=2 \times 10^8 ms ^{-1}\end{aligned}$

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Question 64 Marks

A concave lens is thick at the edges and thin at the centre, while a convex lens is thick at the centre and thin at the edges. We can distinguish between a concave lens and a convex lens without touching them. For this keep a book close to a lens and observe the image of the text of the book through the lens. If the letters appear enlarged, then it is a convex lens and if the letters appear diminished then it is a concave lens.

Convex lens converges light rays and hence known as converging lens. Similarly, concave lens diverges light rays and is known as diverging lens. Linear magnification produced by a lens is equal to the ratio of the image distance to the object distance. Power of a lens is defined as the reciprocal of its focal length.
(i) What type of image is always made by a concave lens?
(ii) If magnification produced by a spherical lens is $+0.75$, then what is the nature of the lens ?
(iii) What is the power of a convex lens with focal length $80 cm$ ?
or
(iii) What kind of lens is present in human eye?

Answer

(i) Concave lens always forms virtual, erect and diminished image of an object.
(ii) Here, $m=0.75$ (less than 1) so the image formed is smaller than the object or diminished. A virtual, erect and diminished image can be formed only by a concave lens, so the nature of the lens is concave.
(iii) Power of the convex lens
$\begin{aligned}P & =\frac{1}{f}=\frac{1}{80} \times 100 \\& =1.25 D\end{aligned}$
or
(iii) Converging lens is present in human eye.

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Question 74 Marks

A concave mirror forms image of an object thrice in its size on a screen. Magnification of a mirror gives information about the size of the image relative to the object. It is defined as the ratio of size of image to the size of object. It is represented by $m$.
$
m=\frac{\text { Size of image }}{\text { Size of object }}
$
Sign of magnification by mirror gives the information about the nature of the image produce by it.
(i) Describe the nature of image formed.
(ii) If the object $x$ distance from the pole of mirror, then find image distance from the pole.
(iii) If the radius of curvature of mirror is $R$, then write the relation between object distance, image distance and focal length of the mirror.
or
(iv) Give one use of concave mirror.

Answer

Coming soon

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Question 84 Marks

Lenses are objects made of transparent materials such as glass or clear plastic that has curved surfaces. Diverging lenses are thicker at their edges than at their centres and makes light rays passing through them spread out. Converging lenses are thicker in their middle than at this edges and make light rays passing through them focus at a point. These are used in spectacles to help people with poor vision see better. The converging lenses magnify by bending the rays of light that pass through them to meet at a point called focus. Thicker the converging lens is at its centre, the more its magnifies and closer the focus is to the lens.
(i) Ravi uses two lenses $A$ and $B$ of same size and same material as shown. $P _1$ and $P _2$ are the powers of $A$ and B. An object is kept at the same distance from the lens between $F$ and $2 F$ of each lens on the principal axis in turn. Let $I_1$ and $I_2$ be the image formed by two lenses respectively. What is the relation of image distances of both lens?
(ii) Write down the relation between the power of lens of both lenses ?
(iii) Meenakshi uses above two lenses $A$ and $B$ along with another two lenses $C$ and $D$, as shown :

She is able to see the subject matter on the black board while sitting in the front row in the classroom but is unable to see the same matter while sitting in the last row.
Which of the above four lenses will she require to correct the defect in her vision? Why ?
or
(iv) Natasha places an object on the principal axis of above given lens A. One end of this object coincides with the focus $F$ and the other end with $2 F$. What will be the nature of the image formed by the lens on the other side?

Answer

(i) Distance of image $I _2$ will be less than distance of $I _1$ from the lens.
(ii) $P _1< P _2$
(iii) She will require lens C. Because, she is suffering from myopia and in myopia concave lens is required to correct it.
or
(iv) The nature of the image formed will be infinite in size.

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Question 94 Marks

The picture shows the path of light as it travels from one medium to another.

9. What is medium 1 and medium 2?

	Medium 1	Medium 2
A.	Water	Kerosene
B.	Kerosene	Diamond
C.	Flint glass	Water
D.	Kerosene	Flint glass

10. The power of a lens (P) is calculated by the formula,
$P=\frac{1}{f}$
where f is the focal length of the lens.
A lens has a focal length of – 0.25 m.
Is it a convex lens or a concave lens? Explain your answer.

Answer

9. C. Flint glass/water
10. Mentions that a lens with a negative focal length has a negative power by the mentioned formula. Concave lenses have a negative power.
● The lens is concave. A lens with a negative focal length has a negative power. Concave lenses have negative power.

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Question 104 Marks

The table below shows the refractive index of different materials.

	Water	Kerosene	Flint glass	Diamond
Refractive index of the material	1.33	1.44	1.65	2.42

The formula for calculating the refractive index (nm) of a material is,
$n_m=\frac{\text { Speef of light in } X}{\text { Speef of light in the medium }}$
8. What does X stand for?

Answer

8. Mentions that ‘X’ represents air.
● Air

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Question 114 Marks

The pictures show the ray diagrams of images formed by convex mirrors.
P is the pole or centre of the relecting surface of the mirror.
C is the centre of curvature of the mirror.
F is the focus of the mirror.
AB is the object and A’B’ is the image of the object.

5. Which statement is supported by the two diagrams?
Circle ‘Yes’ or ‘No’ for each statement.

Is the statement supported by the two diagrams?	Yes or No
Convex mirrors produce virtual images	Yes/No
Convex mirrors produce erect images	Yes/No
Convex mirrors have their focus behind the mirrors	Yes/No

6. Which of these is a convex mirror?
A. Shaving mirror
B. Dentist’s mirror
C. Headlight mirror of a bike
D. Rear-view mirror of a car
7. A mirror magniies the image of an object by minus 1.5 times.
Which of the following is true about the image produced by the mirror
A. The image is real and larger than the object.
B. The image is real and smaller than the object.
C. The image is virtual and larger than the object.
D. The image is virtual and smaller than the object.

Answer

5. Yes
Yes
Yes
6. D. Rear-view mirror of a car
7. A. The image is real and larger than the object.

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Question 124 Marks

The pictures show four ray diagrams of images formed by concave mirrors.
P is the pole or centre of the relecting surface of the mirror.
F is the focus of the mirror.
C is the centre of curvature of the mirror.
AB is the object and A’B’ is the image of the object.

1. Which of these can be concluded from diagram 1?
A. Image is formed at the focus.
B. Size of the image is equal to the size of the object.
C. Distance between pole and centre of curvature is twice the focal length.
D. Distance between the image and focus is half the distance between the object and focus.
2. Which diagram shows a real, inverted and enlarged image formed by the mirror?
A. Diagram 1
B. Diagram 2
C. Diagram 3
D. Diagram 4
3. In which condition does a concave mirror produce a virtual image?
A. When object is located within the focal length
B. When object is located at the centre of curvature
C. When object is located in between ininity and the centre of curvature
D. When object is located in between the centre of curvature and the focus
4. Solar cookers contain a concave mirror.
How does the concave mirror help in heating the food?
Circle ‘Yes’ or ‘No’ for the correct response.

How does the concave mirror heat the food?	Yes or No
Sun’s rays are absorbed by the mirror.	Yes/No
Sun’s rays relected by the mirror converge at a point.	Yes/No
Sun’s rays diverge out when relected by the mirror.	Yes/No

Answer

1. C. Distance between pole and centre of curvature is twice the focal length.
2. C. Diagram 3
3. A. When object is located within the focal length
4. No
Yes
No

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Science case /data -based (4 Marks)

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