Text Book Back Questions and Answers - Chapter 2 Optics 10th Science Guide Samacheer Kalvi Solutions - SaraNextGen [2024-2025]

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Chapter 2 - Optics - 10th Science Guide Samacheer Kalvi Solutions

Question $1 .$
Light rays travel from vacuum into a glass whose refractive index is $1.5$. If the angle of incidence is $30^{\circ}$, calculate the angle of refraction inside the glass.
Solution:
According to Snell's law,
$
\begin{aligned}
\frac{\sin i}{\sin r} &=\frac{\mu_{2}}{\mu_{1}} \\
\mu_{1} \sin i &=\mu_{2} \sin r \\
\text { Here } \mu_{1} &=1.0, \mu_{2}=1.5, i=30^{\circ} \\
(1.0) \sin 30^{\circ} &=1.5 \sin r \\
1 \times \frac{1}{2} &=1.5 \sin r \\
\sin r &=\frac{1}{2 \times 1.5}=\frac{1}{3}=(0.333) \\
r &=\sin ^{-1}(0.333) \\
r &=19.45^{\circ}
\end{aligned}
$

Question 2.
A beam of light passing through a diverging lens of focal length $0.3 \mathrm{~m}$ appears to be focused at a distance $0.2 \mathrm{~m}$ behind the lens. Find the position of the object.
Solution:
$
f=-0.3 \mathrm{~m}, \mathrm{v}=-0.2 \mathrm{~m}
$
$
\frac{1}{f}=\frac{1}{v}-\frac{1}{u}
$
$
\frac{1}{u}=\frac{1}{v}-\frac{1}{f}
$
$
\begin{aligned}
\frac{1}{u} &=\frac{1}{-0.2}-\frac{1}{-0.3} \\
&=\frac{-10}{6} \\
u &=\frac{-6}{10}=-0.6 \mathrm{~m}
\end{aligned}
$

Question $3 .$
A person with myopia can see objects placed at a distance of $4 \mathrm{~m}$. If he wants to see objects at a distance of $20 \mathrm{~m}$, what should be the focal length and power of the concave lens he must wear?
Solution:
Given that $\mathrm{x}=4 \mathrm{~m}$ and $\mathrm{y}=20 \mathrm{~m}$.
Focal length of the correction lens is, $f=\frac{x y}{x-y}$
$
\begin{aligned}
f &=\frac{4 \times 20}{4-20} \\
&=\frac{80}{-16}=-5 \mathrm{~m}
\end{aligned}
$
Power of the correction lens $=\frac{1}{f}=-\frac{1}{5}=-0.2 \mathrm{D}$.
 

Question $4 .$
For a person with hypermetropia, the near point has moved to $1.5 \mathrm{~m}$. Calculate the focal length of the correction lens in order to make his eyes normal.
Solution:
Given that, $\mathrm{d}=1.5 \mathrm{~m} ; \mathrm{D}=25 \mathrm{~cm}=0.25 \mathrm{~m}$ (For a normal eye).
The focal length of the correction lens is,
$
\begin{aligned}
f &=\frac{d \times \mathrm{D}}{d-\mathrm{D}} \\
&=\frac{1.5 \times 0.25}{1.5-0.25} \\
&=\frac{0.375}{1.25}=0.3 \mathrm{~m}
\end{aligned}
$

I. Choose the correct answer.
Question $1 .$
The refractive index of four substances $\mathrm{A}, \mathrm{B}, \mathrm{C}$ and $\mathrm{D}$ are $1.31,1.43,1.33,2.4$ respectively. The speed of light is maximum in
(a) $A$
(b) $\mathrm{B}$
(c) $\mathrm{C}$
(d) D.
Answer:
(a) $\mathrm{A}$

Question $2 .$
A small bulb is placed at the principal focus of a convex lens. When the bulb is switched on, the lens will produce:
(a) a convergent beam of light
(b) a divergent beam of light
(c) a parallel beam of light
(d) a coloured beam of light
Answer:
(a) a convergent beam of light
 

Question 3 .
A small bulb is placed at the principal focus of a convex lens. When the bulb is switched on, the lens will produce
(a) a convergent beam of light
(b) a divergent beam of light
(c) a parallel beam of light
(d) a coloured beam of light.
Answer:
(c) a parallel beam of light
 

Question $4 .$
Magnification of a convex lens is:
(a) Positive
(b) negative
(c) either positive or negative
(d) zero
Answer:
(a) Positive
 

Question $5 .$
A convex lens forms a real, diminished point sized image at the focus. Then the position of the
object is at
(a) focus
(b) infinity
(c) at $2 \mathrm{f}$
(d) between $f$ and $2 f$.
Answer:
(b) infinity

Question 6 .
Power of a lens is $-4 D$, then its focal length is
(a) $4 \mathrm{~m}$
(b) $-40 \mathrm{~m}$
(c) $-0.25 \mathrm{~m}$
(d) $-2.5 \mathrm{~m}$.
Answer:
(c) $-0.25 \mathrm{~m}$
 

Question 7.
In a myopic eye, the image of the object is formed:
(a) behind the retina
(b) on the retina
(c) in front of the retina
(d) on the blind spot
Answer:
(c) in front of the retina
 

Question $8 .$
The eye defect 'presbyopia' can be corrected by
(a) convex lens
(b) concave lens
(c) convex mirror
(d) bifocal lenses.
Answer:
(d) bifocal lenses.

Question $9 .$
Which of the following lens would you prefer to use while reading small letters found in a dictionary?
(a) A convex lens of focal length $5 \mathrm{~cm}$
(b) A concave lens of focal length $5 \mathrm{~cm}$
(c) A convex lens of focal length $10 \mathrm{~cm}$
(d) A concave lens of focal length $10 \mathrm{~cm}$.
Answer:
(c) A convex lens of focal length $10 \mathrm{~cm}$
 

Question $10 .$
If $\mathrm{V}_{\mathrm{B}}, \mathrm{V}_{\mathrm{G}}, \mathrm{V}_{\mathrm{R}}$ be the velocity of blue, green and red light respectively in a glass prism, then which of the following statement gives the correct relation?
(a) $V_{B}=V_{G}=V_{R}$
(b) $\mathrm{V}_{\mathrm{B}}>\mathrm{V}_{\mathrm{G}}>\mathrm{V}_{\mathrm{R}}$
(c) $V_{B} (d) $V_{B}V_{R}$
Answer:
(c) $\mathrm{V}_{\mathrm{B}}<\mathrm{V}_{\mathrm{G}}<\mathrm{V}_{\mathrm{R}}$

II. Fill in the Blanks.
Question 1.
The path of the light is called as
Answer:
ray of light.
 

Question $2 .$
The refractive index of a transparent medium is always greater than
Answer:
one.
 

Question $3 .$
If the energy of the incident beam and the scattered beam are the same, then the scattering of light is called scattering.
Answer:
elastic.
 

Question $4 .$
According to Rayleigh's scattering law, the amount of scattering of light is inversely proportional to the fourth power of its
Answer:
Wavelength.
 

Question 5 .
Amount of light entering into the eye is controlled by
Answer:
Iris.
 

III. True or False. If False Correct it.
Question $1 .$
The velocity of light is greater in a denser medium than in a rarer medium?
Answer:
False.
Correct Statement: Velocity of light is lesser in a denser medium than in rarer medium
 

Question $2 .$
The power of the lens depends on the focal length of the lens?
Answer:
True.
 

Question $3 .$
Increase in the converging power of eye lens cause 'hypermetropia'.
Answer:
True.
 

Question $4 .$
The convex lens always gives a small virtual image.
Answer:
False.
Correct Statement: The convex lens give enlarged. Virtual image when it is placed between pole and principal focus.
 

IV. Match the Following.
Question $1 .$

Answer:
1. (d) Screen of the eye
2. (a) Pathway of light
3. (e) Power of accommodation
4. (b) Far point comes closer
5. (c) Near point moves away
 

V. Assertion and Reasoning Type Questions
Mark the correct choice as?
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but the reason is true.
 

Question $1 .$
Assertion: If the refractive index of the medium is high (denser medium) the velocity of the light in that medium will be small
Reason: Refractive index of the medium is inversely proportional to the velocity of the light Answer:
(a) If both assertion and reason are true and reason is the correct explanation of assertion.

Question $2 .$
Assertion: Myopia is due to the increase in the converging power of the eye lens.
Reason: Myopia can be corrected with the help of the concave lens.
Answer:
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
 

VI. Answer Briefly.
Question $1 .$
What is refractive index?
Answer:
The ratio of the speed of light in a vacuum to the speed of light in a medium is defined as the refractive index ' $\mu$ ' of that medium.
 

Question $2 .$
State Snell's law.
Answer:
The ratio of the sine of the angle of incidence and sine of the angle of refraction is equal to the ratio of refractive indices of the two media. This law is also known as Snell's law.
$\frac{\sin i}{\sin x}=\frac{\mu_{2}}{\mu_{1}}$
 

Question $3 .$
Draw a ray diagram to show the image formed by a convex lens when the object is placed between
$\mathrm{F}$ and $2 \mathrm{~F}$.
Answer:
Ray diagram for an object placed between $\mathrm{F}$ and $2 \mathrm{~F}$

Question $4 .$
Define dispersion of light.
Answer:
When a beam of white light or composite light is refracted through any transparent media such as glass or water, it is split into its component colours. This phenomenon is called as 'dispersion of light'.
 

Question $5 .$
State Rayleigh's law of scattering.
Answer:
Rayleigh's scattering law states that "The amount of scattering of light is inversely proportional to the fourth power of its wavelength".
Amount of scattering $\mathrm{s} \propto \frac{1}{\lambda^{4}}$

Question $6 .$
Differentiate convex lens and concave lens.
Answer:

Question $7 .$
What is power of accommodation of eye?
Answer:
The ability of the eye lens to focus nearby as well as the distant objects is called power of accommodation of the eye.
 

Question $8 .$
What are the causes of 'Myopia'?
Answer:
Myopia, also known as short-sightedness, occurs due to the lengthening of the eyeball. With this defect, nearby objects can be seen clearly but distant objects cannot be seen clearly.
 

Question $9 .$
Why does the sky appear in blue colour?
Answer:
When sunlight passes through the atmosphere, the blue colour (shorter wavelength) is scattered to a greater extent than the red colour (longer wavelength). This scattering causes the sky to appear in blue colour.
 

Question $10 .$
Why are traffic signals red in colour?
Answer:

1. The wavelength of red colour is more than other colours.
2. Red colour will travel longer distance without scattering.
3. Red colour gets least scattered and reaches people.
 

VII. Give the Answer in Detail.
Question $1 .$
List any five properties of light.
Answer:
1. Light is a form of energy.
2. Light always travels along a straight line.
3. Light does not need any medium for its propagation. It can even travel through vacuum.
4. The speed of light in vacuum or air is, $\mathrm{c}=3 \times 10^{8} \mathrm{~ms}^{-1}$.
5. Since, light is in the form of waves, it is characterized by a wavelength ( $k)$ and a frequency (v), which are related by the following equation: $c=v \lambda$ (c - velocity of light).
6. Different coloured light has different wavelength and frequency.
7. Among the visible light, violet light has the lowest wavelength and red light has the highest wavelength.
8. When light is incident on the interface between two media, it is partly reflected and partly refracted.
 

Question $2 .$
Explain the rules for obtaining images formed In a convex lens with the help of a ray diagram.
Answer:
Rule 1: When a ray of light strikes the convex or concave lens obliquely at its optical centre, it continues to follow its path without any deviation.

Rule 2 : When rays parallel to the principal axis strikes a convex or concave lens, the refracted rays are converged to (convex lens) or appear to diverge from (concave lens) the principal focus.

Rule 3: When a ray passing through (convex lens) or directed towards (concave lens) the principal focus strikes a convex or concave lens, the refracted ray will be parallel to the principal axis.

Question $3 .$
Differentiate the eye defects: Myopia and Hypermetropia?
Answer:

Question $4 .$
Explain the construction and working of a 'Compound Microscope'.
Answer:
Construction:
- A compound microscope consists of two convex lenses. The lens with the shorter focal length is placed near the object, and is called an 'objective lens' or 'objective piece'.
- The lens with larger focal length and larger aperture placed near the 'observer's eye is called as 'eye lens' or 'eyepiece'.
- Both the lenses are fixed in a narrow tube with adjustable provision.

Working:
- The object $(A B)$ is placed at a distance slightly greater than the focal length of the objective lens $\left(u>f_{0}\right)$.
- A real, inverted and magnified image $\left(A^{\prime} B^{\prime}\right)$ is formed at the other side of the objective lens.
- This image behaves like the object for the eye lens. The position of the eye lens is adjusted in such a way, that the image (A'B') falls within the principal focus of the eyepiece.
- This eyepiece forms a virtual, enlarged and erect image $\left(A^{\prime \prime} B^{\prime \prime}\right)$ on the same side of the object.
- A compound microscope has 50 to 200 times more magnification power than the simple microscope.
 

VIII. Numerical Problems:
Question $1 .$
An object is placed at a distance $20 \mathrm{~cm}$ from a convex lens of focal length $10 \mathrm{~cm}$. Find the image distance and nature of the image.
Solution:

\begin{aligned}
&u=-20 \mathrm{~cm} \\
&f=10 \mathrm{~cm} \\
&\frac{1}{f}=\frac{1}{v}-\frac{1}{u} \\
&\frac{1}{v}=\frac{1}{f}+\frac{1}{u} \\
&\frac{1}{v}=\frac{1}{10}+\frac{1}{(-20)} \\
&\frac{1}{v}=\frac{1}{10}-\frac{1}{20} \\
&v=\frac{1}{20} \Rightarrow v=20 \mathrm{~cm}
\end{aligned}

Nature of the image real enlarged and inverted image.
 

Question 2.
An object of height $3 \mathrm{~cm}$ is placed at $10 \mathrm{~cm}$ from a concave lens of focal length $15 \mathrm{~cm}$. Find the size of the image.
Solution:
$\mathrm{u}=-10 \mathrm{~cm}$ [left side of lens]
$\mathrm{f}=-15 \mathrm{~cm}$ [left side of lens]
$\mathrm{h}=3 \mathrm{~cm}$

$\begin{aligned} m &=\frac{v}{u}=\frac{-6}{-10}=0.6 \\ m &=\frac{h^{\prime}}{h} \Rightarrow h^{\prime}=m \times h \\ \frac{1}{f} &=\frac{1}{v}-\frac{1}{u} \\ \frac{1}{v} &=\frac{1}{f}+\frac{1}{u} \\ &=\frac{1}{-15}+\frac{1}{-10} \\ \frac{1}{v} &=-\frac{1}{15}-\frac{1}{10}=\frac{-10-15}{15}=\frac{-25}{150} \\ \frac{1}{v} &=-\frac{1}{6} \\ \frac{v}{v} &=-6 \mathrm{~cm} \end{aligned}$

Nature of image: When an object is placed at $10 \mathrm{~cm}$ from the left side of the lens, a virtual image is formed between the optical centre and focus of a concave lens. The size of the image is smaller than that of the object.

IX. Higher-Order Thinking (HOT) Questions:

Question $1 .$
While doing an experiment for the determination of the focal length of a convex lens, Raja Suddenly dropped the lens. It got broken into two halves along the axis. If he continues his experiment with the same lens,
1. Can he get the image?
2. Is there any change in the focal length?
Answer:
1. Yes, he can get the image, because the lens is broken into two halves along the axis.
2. No, focal length remains the same for the lens if it's broken (or) not broken.
 

Question $2 .$
The eyes of the nocturnal birds like owl are having a large cornea and a large pupil. How does it help them?
Answer:
1. Owl has large pupil so that it provides a larger pathway for light to flow towards the retina.
2. In addition it has large comer so that a bigger image of the insects would be formed on the retina.
3. In this way the eyes of the nocturnal birds help them to identify the elements in the surroundings.

I. Choose the correct answer.
Question $1 .$
The speed of light in vacuum or air is
(a) $3 \times 10^{-8} \mathrm{~ms}^{-1}$
(b) $3 \times 10^{8} \mathrm{~ms}^{-1}$
(c) $3 \times 10^{8} \mathrm{~m}^{-1} \mathrm{~s}$
(d) $3 \times 10^{-18} \mathrm{~ms}^{-1}$.
Answer:
(b) $3 \times 10^{8} \mathrm{~ms}^{-1}$
 

Question $2 .$
If the energy of the incident beam of light and the scattered beam of light are same, then it is known as:
(a) Inelastic scattering
(b) Elastic scattering
(c) Raman scattering
(d) Mie scattering
Answer:
(b) Elastic scattering

Question $3 .$
Which is the following is not an example of colloid
(a) milk
(b) ice - cream
(c) pure water
(d) smoke.
Answer:
(c) pure water
 

Question $4 .$
The band of colours is termed as
(a) monochromatic source
(b) composite light
(c) spectrum
(d) dispersion of light.
Answer:
(c) spectrum
 

Question $5 .$
Water droplets, pollen and dust cause scattering.
(a) Elastic
(b) Inelastic
(c) Mie
(d) Rayleigh
Answer:
(c) Mie
 

Question $6 .$
__________scattering takes place when the diameter of the Scatterer is similar to or larger than the wavelength of the incident light.
(a) Rayleigh
(b) Tyndall
(c) Mie
(d) Raman.
Answer:
(c) Mie

Question $7 .$
The colour of the sun is red at sunrise and sunset. This occurs due to _________ scattering.
(a) Rayleigh
(b) Mie
(c) Tyndall
(d) Raman.
Answer:
(a) Rayleigh
 

Question $8 .$
A lens may be considered to be made up of:
(a) lenses
(b) prisms
(c) mirrors
(d) transparent medium
Answer:
(b) prisms
 

Question $9 .$
If one of the faces of a bi-convex lens is plane, it is known as a
(a) convex lens
(b) Plano-convex lens
(c) concave lens
(d) Plano concave lens.
Answer:
(b) Plano-convex lens

Question $10 .$
The spectral lines having a frequency equal to the incident ray frequency is called
(a) Spectral lines
(b) Raman lines
(c) Colour lines
(d) Rayleigh lines.
Answer:
(d) Rayleigh lines.
 

Question $11 .$
The lens that is thinner in the middle than at the edges is called:
(a) concave lens
(b) bifocal lens
(c) cylindrical lens
(d) convex lens
Answer:
(a) concave lens

Question $12 .$
Magnification of a concave lens is
(a) positive
(b) negative
(c) either positive or negative
(d) zero.
Answer:
(b) negative
 

Question 13.
Where should an object be placed so that a real image is formed at infinity is obtained by a convex lens
(a) $2 \overline{\mathrm{F}}$
(b) 0
(c) $\mathrm{F}$
(d) $>2 \mathrm{~F}$.
Answer:
(c) $\mathrm{F}$
 

Question $14 .$
The focal length of a lens is the distance between:
(a) optic centre and principal focus
(b) optic centre and centre of curvature
(c) principal focus and centre of curvature
(d) none
Answer:
(a) optic centre and principal focus

Question $15 .$
Lens Maker's formula is
(a) $m=\frac{h^{\prime}}{h}$
(b) $m=\frac{v}{u}$
(c) $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
(d) $\frac{1}{f}=(\mu-1)\left[\frac{1}{\mathrm{R}_{1}}-\frac{1}{\mathrm{R}_{2}}\right]$

Answer:
(d) $\frac{1}{f}=(\mu-1)\left[\frac{1}{R_{1}}-\frac{1}{R_{2}}\right]$
 

Question $16 .$
The power of a lens is numerically defined as the reciprocal of its
(a) wavelength
(b) frequency
(c) focal length
(d) refractive index.
Answer:
(c) focal length
 

Question $17 .$
When the object is placed at the principal focus of concave lens, the image will be formed at:
(a) $2 \mathrm{~F}$
(b) $\mathrm{F}$
(c) infinity
(d) between $F$ and $2 F$
Answer:
(c) infinity
 

Question $18 .$
Astigmatism can be corrected by using lens.
(a) convex
(b) concave
(c) cylindrical
(d) bifocal.
Answer:
(c) cylindrical

Question $19 .$
The least distance of distinct vision for the normal human eye is
(a) $30 \mathrm{~cm}$
(b) $25 \mathrm{~cm}$
(c) $35 \mathrm{~cm}$
(d) infinity.
Answer:
(b) $25 \mathrm{~cm}$
 

Question 20
To get a real and diminished image using convex lens, the object must be placed:
(a) at $\mathrm{F}$
(b) beyond $2 \mathrm{~F}$
(c) at $2 \mathrm{~F}$
(d) beyond $\mathrm{F}$
Answer:
(b) beyond $2 \mathrm{~F}$

II. Fill in the Blanks.
Question $1 .$
The interacting particle of the medium is called ________
Answer:
Scatterer.
 

Question 2.
When a beam of light is refracted through any transparent media, it split into its component colours. This is called as________
Answer:
dispersion of light.
 

Question $3 .$
The speed of light in a medium is high if the refractive index of the medium is________ $?$
Answer:
low.
 

Question 4 .
The spectral lines which are having frequencies other than the incident ray frequency are called________

Answer:
Raman lines.
 

Question $5 .$
________lenses are used to correct the defect of vision called hypermetropia.
Answer:
Convex.
 

Question $6 .$
________lenses are used as the eye lens of the Galilean Telescope.
Answer:
Concave.
 

Question 7.
By convention, the power of a convex lens is taken as________ whereas the power of a concave lens is taken as________

Answer:
positive, negative.
 

Question $8 .$
The diameter of a human eye is_______
Answer:
$2.3 \mathrm{~cm}$.
 

Question $9 .$
_______is achieved by changing the focal length of the eye lens with the help of ciliary muscles.
Answer:
Power of accommodation.
 

Question $10 .$
_______is the coloured part of an eye.
Answer:
Iris.
 

Question $11 .$
_______is used to observe parts of flower, insects and fingerprints in the field of forensic science.
Answer:
Simple microscope.
 

Question $12 .$
A_______ works based on the principle of the vernier.
Answer:
travelling microscope.
 

Question $13 .$
A _______telescope is used to view heavenly bodies like stars, planets galaxies and satellites.
Answer:
astronomical.
 

Question $14 .$
_______is an optical instrument used to see distant objects clearly.
Answer:
Telescope.
 

Question $15 .$
_______is the most sensitive part of the human eye.
Answer:
Retina.

Question $16 .$
A convex lens is_______ in the middle than at edges.
Answer:
Thicker.
 

Question $17 .$
_______lens produces mostly real images.
Answer:
Convex.
 

Question 18.
The SI unit of power of a lens is_______
Answer:
dioptre.
 

Question $19 .$
The amount of scattering of light is_______ proportional to the fourth power of its wavelength.
Answer:
inversely.
 

Question $20 .$
Refractive index of the medium is dependent on the_______ of the light.
Answer:
wavelength.
 

III. True or False. If False Correct it.
Question $1 .$
Light always travels along a straight line.
Answer:
True.
 

Question 2.
The incident ray, the refracted ray of light and the normal to the refracting surface all lie in the different plane.
Answer:
False.
Correct Statement: The incident ray, the refracted ray of light and the normal to the refracting surface all lie in the same plane.

Question $3 .$
The angle of refraction is the same for different colours.
Answer:
False.
Correct Statement: The angle of refraction is different for different colours.
 

Question $4 .$
The scattering of light rays by the colloidal particles in the colloidal solution is called the Tyndall effect.
Answer:
True.
 

Question $5 .$
The convex lens is also called a diverging lens.
Answer:
False.
Correct Statement: Concave lens is also called the diverging lens.
 

Question $6 .$
Convex lenses are used in making microscope, telescope and slide projectors.
Answer:
True.
 

Question $7 .$
The distances measured against the direction of incident light are taken as negative.
Answer:
True.

Question 8.
Converging lenses are used in wide-angle spy hole indoors.
Answer:
False.
Correct Statement: Diverging lenses or concave lenses are used in wide-angle spy hole indoors.
 

Question $9 .$
If the magnification is less than 1 , then we get a diminished image.
Answer:
True.
 

Question $10 .$
A compound microscope has 50 to 200 times more magnification power than the simple microscope.
Answer:
True.
 

IV. Match the following.
Question 1.

Answer:
1. (h) $\frac{\sin i}{\sin r}=\frac{\mu_{2}}{\mu_{1}}$
2. (g) The sky to appear in blue colour
3. (i) White appearance of the clouds
4. (c) converging lens
5. (b) Diverging lens
6. (d) Front surface of an eyeball
7. (a) $25 \mathrm{~cm}$
8. (f) Infinity
9. (k) Bifocal lenses
10. (j) Torrid lenses
 

V. Assertion and Reasoning Type Questions
Mark the correct choice as?
(a) If both assertion and reason are true and the reason is the correct explanation of assertion.
(b) If both assertion and reason are true but the reason is not the correct explanation of assertion.
(c) Assertion is true but the reason is false.
(d) Assertion is false but the reason is true.
 

Question $1 .$
Assertion: The refractive index of the medium is different for different coloured lights. Reason: The refractive index of a medium is dependent on the wavelength of the light.
Answer:
(a) If both assertion and reason are true and reason is the correct explanation of assertion.

Question 2 .
Assertion: A parallel beam of light passing through the concave lens, is diverged or spread out.
Reason: A convex lens is also called a diverging lens.
Answer:
(c) Assertion is true but the reason is false.
 

Question $3 .$
Assertion: Magnification power of microscopes can be decreased by increasing the focal length of the lens used.

Reason: Due to constructional limitations, the focal length of the lens cannot be decreased beyond certain unit.
Answer:
(d) The assertion is false but the reason is true.
 

VI. Answer Briefly.
Question $1 .$
Why does the cloud appear in white colour?
Answer:
- Mie scattering is responsible for the white appearance of the clouds.
- When white light falls on the water drop, all the colours are equally scattered which together form the white light.
 

Question $2 .$
Define Raman Scattering.
Answer:
Raman Scattering is defined as "The interaction of light ray with the particles of pure liquids or transparent solids, which leads to a change in wavelength or frequency."
 

Question $3 .$
Differentiate stokes line and anti-stokes lines.
Answer:
1. Stokes lines: The lines having frequencies lower than the incident frequency is called stokes lines.
2. Antistokes lines: The lines having frequencies higher than the incident frequency are called Antistokes lines.

Question $4 .$
What is meant by colloid?
Answer:
A colloid is a microscopically small substance that is equally dispersed throughout another material.
Example: Milk, Ice cream, muddy water, smoke.
 

Question $5 .$
What are the applications of the convex lens?
Answer:
1. Convex lenses are used as camera lenses.
2. They are used as magnifying lenses.
3. They are used in making microscope, telescope and slide projectors.
4. They are used to correct the defect of vision called hypermetropia.

Question $6 .$
Give the lens formula.
Answer:
The lens formula gives the relationship among distance of the object (u), the distance of the image (v) and the focal length (f) of the lens. It is expressed as
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
It is applicable to both convex and concave lenses.
 

Question 7.
What are the applications of a concave lens?
Answer:
1. Concave lenses are used as eye lens of 'Galilean Telescope'
2. They are used in wide-angle spy hole indoors.
3. They are being used to correct the defect of vision called 'myopia'
 

Question 8.
What are the uses of a simple microscope?
Answer:
Simple microscopes are used
1. By watch repairers and jewellers.
2. To read small letters clearly.
3. To observe parts of flower, insects etc.
4. To observe Anger prints in the field of forensic science.

Question $9 .$
Define power of a lens.
Answer:
1. The ability of a lens to converge (convex lens) or diverge (concave lens) is called its power.
2. The power of a lens can be defined as the deep of convergence or divergence of light rays.
3. Power of a lens is numerically defined as the reciprocal of its focal length.
$\mathrm{P}=\frac{1}{f}$
The SI unit of power of a lens is dioptre.

Question $10 .$
Define magnification of a lens.
Answer:
It is defined as the ratio of the height of the image to the height of an object. Magnification is denoted by the letter ' $m$ '.If the height of the object is ' $h$ ' and the height of the image is h', the magnification produced by a lens is,
$m=\frac{\text { height of the image }}{\text { height of the object }}=\frac{h^{\prime}}{h}$
Also, it is related to the distance of the object (w) and the distance of the image (v) as follows:
$
m=\frac{\text { distance of the image }}{\text { distance of the object }}=\frac{v}{u}
$
 

Question $11 .$
Give the lens maker's formula.
Answer:
The lens maker's formula is one such equation. It is given as
$\frac{1}{f}=(\mu-1)\left(\frac{1}{\mathrm{R}_{1}}-\frac{1}{\mathrm{R}_{2}}\right)$
where $\mu$ is the refractive index of the material of the lens; R1 and R2 are the radii of curvature of the two faces of the lens; $f$ is the focal length of the lens.
 

Question $12 .$
Based on the initial and final energy of the light beam, classify the scattering.
Answer:
Based on the initial and final energy of the light beam, scattering can be classified as
- Elastic scattering
- Inelastic scattering

Question $13 .$
Based on the nature and size of the scatter. Classify the scattering.
Answer:
The nature and size of the scatterer result in different types of scattering. They are
- Rayleigh scattering
- Mie scattering
- Tyndall scattering
- Raman scattering
 

Question $14 .$
Why does the sky appear in red colour at sunrise and sunset?
Answer:
1. At sunrise and sunset, the light rays from the Sun have to travel a larger distance in the atmosphere than at noon.
2. Most of the blue lights are scattered away and only the red light which gets least scattered reaches us. Therefore, the colour of the Sun is red at sunrise and sunset.
 

Question $15 .$
Discuss the advantages and disadvantages of the telescope.
Answer:
Advantages:
- Elaborate view of the Galaxies, Planets, stars and other heavenly bodies is possible.
- The camera can be attached for taking photographs for the celestial objects.
- The telescope can be viewed even with the low intensity of light.

Disadvantages:
- Frequent maintenances needed.
- It is not easily portable one.
 

VII. Give the Answer in Detail.
Question $1 .$
What is scattering? Explain two types of scattering.
Answer:
When a beam of light, interacts with a constituent particle of the medium, it undergoes many kinds of scattering. Based on initial and final energy of the light beam,scattering can be classified as,
(i) Elastic scattering: If the energy of the incident beam of light and the scattered beam of light are same, then it is called as 'elastic scattering'.
(ii) Inelastic scattering: If the energy of the incident beam of light and the scattered beam of light are not same, then it is called as 'inelastic scattering'.
 

Question $2 .$
Explain the types of Scattering.
Answer:
Types of Scattering.
When a beam of light, interacts with a constituent particle of the medium, it undergoes many kinds of scattering.
Based on the initial and final energy of the light beam, scattering can be classified as 

1. Elastic scattering: If the energy of the incident beam of light and the scattered beam of light are same, then it is called as 'elastic scattering'.
2. Inelastic scattering: If the energy of the incident beam of light and the scattered beam of light are not same, then it is called as 'inelastic scattering'.
The nature and size of the Scatterer result in different types of scattering. They are
- Rayleigh scattering
- Mie scattering
- Tyndall scattering
- Raman scattering
 

Question $3 .$
Explain the Cartesian sign convention.
Answer:
According to Cartesian sign convention,
1. The object is always placed on the left side of the lens.
2. All the distances are measured from the optical centre of the lens.
3. The distances measured in the same direction as that of incident light are taken as positive.
4. The distances measured against the direction of incident light are taken as negative.
5. The distances measured upward and perpendicular to the principal axis is taken as positive.
6. The distances measured downward and perpendicular to the principal axis is taken as negative.