NATURE OF LIGHT AND ELECTROMAGNETIC SPECTRUM CHAPTER # 15 Physics 10th - Question Answers

 Physics 10th - Question Answers

NATURE OF LIGHT AND ELECTROMAGNETIC SPECTRUM
CHAPTER # 15

Q.1: What is Newton’s corpuscular theory of light?

Ans:
This theory which was proposed by Newton is given briefly as under:

• Light consists of tiny (minute) particles called corpuscles.
• These corpuscles are emitted from a source of light (like the sun).
• These corpuscles are traveled in a straight line.
• When these particles enter the eyes, they create the sensation of sight.
• Velocity of light in a denser medium is greater.

Q.2: What is Huygens’s waves theory of light?

Ans:
Huygens proposed a theory known as the wave theory of light. According to this theory:

• Light is a form of energy, and it traveled in the form of waves.
• Medium was essential for the propagation of waves; therefore, it was assumed that all space was filled with a hypothetical medium called ether.
• Waves in different media propagate with different speeds.
• Waves of different wavelengths bend differently.
• The waves of light were so tiny that there had no visible distortion in typical mirrors and lenses and in the formation of shadows.
• Wave theory explained the rectilinear propagation of light, formation of shadows, reflection, refraction, and a few other properties of light.

Q.3: What is quantum theory and dual nature of light?

Ans:
In 1905, Max Planck concluded after an experiment that radiation was emitted in the form of small packets of energy.

• When the light falls on a metal surface, electrons are emitted from it.
• The physical nature of light was not that of a wave but of little packets of light energy called photons.
• Light has a dual nature. Sometimes it behaves as a particle and sometimes as a wave. It never exhibits both characteristics at the same time.
• It explains the existing phenomenon of light through empty space.

Q.4: Write down the difference between corpuscular and wave theory of light?

Corpuscular Theory	Wave Theory
According to this theory light consists of small particles called corpuscles.	According to wave theory, light is a form of energy because it travels in the form of waves.
Light particles travel with the velocity of light in a straight line.	Waves travel with the velocity of light but not in a straight line.
Light travels with high velocity in a denser medium.	Light travels with slow velocity in a denser medium.

Q.5: What do you understand by dispersion of light?

Ans:
When a beam of sunlight (white light) falls on a prism, it splits up into seven colors. This phenomenon is called the dispersion of light.

Q.6: Describe an experiment to show that light consists of seven colors?

Ans:
A beam of sunlight is allowed to enter a dark room through a hole. A prism is placed in the path of the beam of light. A band of various colors is produced on the wall. This phenomenon is called the "Dispersion of light."

We know that the amount of refraction of waves depends on their frequencies. This experiment shows that sunlight consists of waves of different frequencies. When these waves pass through the prism, the waves of higher frequency bend more than those of lower frequencies. Due to this, seven colors are obtained which are violet, indigo, blue, green, yellow, orange, and red. We can remember the names and their colors by the word (VIBGYOR).

• The shortest visible wavelength is violet, and its deviation is the greatest.
• The longest visible wavelength is red, and its deviation is the least.

Q.7: What is a rainbow? How does a rainbow form?

Ans:
Rainbow:
The rainbow is an arc of spectral colors formed across the sky during or after rainfall in the morning and evening.

Formation of Rainbow:
When rain falls, drops of water behave like prisms and white light entering the drops of water is split up into colors on refraction. The light enters the raindrop from one side and does not pass through. It is reflected from the opposite side and re-emerges through the side it entered but at an angle of about 42° to its original direction. We see an arc with red on the top and violet below. Sometimes the light is further reflected before emerging and this gives rise to a secondary rainbow seen above the first one.

Q.8: How does the emission of light occur by an atom?

Ans:
Besides sunlight, we obtain light from many sources, for example from solids and gases. Since light consists of electromagnetic waves and all substances are made up of atoms:

In atoms, electrons are moving in certain orbits. The energy of the electrons in each orbit is well defined. The orbit closer to the nucleus has less energy compared with the orbit away from it. When an electron in any orbit is excited, it jumps from the orbit at lower energy to an orbit at higher energy. However, it can’t remain in the excited state for long, so it jumps back to its parent orbit and radiates energy in the form of photons equal to the difference of those two energy levels. These photons cause light to produce and thus light is emitted by an atom.

Q.9: What is a spectrum?

Ans:
A band of colors formed by a prism or by a rainbow is called a spectrum. It has seven colors as red, orange, yellow, green, blue, indigo, and violet (VIBGYOR).

Q.10: Define electromagnetic spectrum?

Ans:
Light is electromagnetic in nature, so the spectrum formed by electromagnetic radiations is known as the electromagnetic spectrum. It consists of radio waves, microwaves, etc.

Q.11: Which waves are there in the spectrum of electromagnetic radiation? And what are their frequencies?

Ans: Waves of Electromagnetic Spectrum:

1. Radio Waves:
   Radio waves are electromagnetic waves with a large range of wavelength from a few millimeters to several meters.

2. Microwaves:
   Microwaves are radio waves with shorter wavelengths between 1 mm and 300 mm. Microwaves are used in radars and microwave ovens.

3. Infrared Waves:
   Infrared waves are also called heat waves. These waves are radiated by hot bodies at different temperatures. The earth’s atmosphere, at a mean temperature of 250K, radiates infrared waves with a wavelength having a mean value of 10 μm.

4. Visible Waves:
   Visible waves have a wavelength range between 400 nm and 700 nm. The peak of solar radiation is at a wavelength of about 550 nm. The human eye is most sensitive to this wavelength.

5. Ultraviolet Waves:
   The wavelength of ultraviolet waves ranges from 380 nm down to 60 nm. These are emitted by hotter stars having a mean temperature greater than 25000°C.

6. Gamma Rays:
   The wavelength of gamma rays is less than $10^{-11}$ m. They are emitted by the nuclei of certain radioactive substances.

Q.12: What do you understand by greenhouse?

Ans: Greenhouse:
A greenhouse is made up of glass relatively opaque to infrared radiation. Heat is trapped in the greenhouse. The term greenhouse is used for the earth's atmosphere along with the ozone layer which behaves like a greenhouse.

Q.13: What is the greenhouse effect?

Ans:
Heating of the atmosphere due to the presence of water vapors and carbon dioxide is called the greenhouse effect.

Q.14: Write down hazards of greenhouse effect?

Ans:
Heavy industries and automobiles are polluting the atmosphere, damaging the ozone layer and producing greenhouse effect.

Due to this, temperature of the earth may rise to such an extent that it will melt the polar ice caps, raising the level of the ocean, drowning all the coasts.

Q.15: What is photon?

Ans:
A photon is a packet of energy and it behaves as a particle but actually it is not a particle.

Refraction of Light and Optical Instruments (Chapter # 14) Physics 10th Question Answers

Refraction of Light and Optical Instruments

Q.1: Define refraction of light and laws of refraction?

Answer:

Refraction: When a ray of light enters from one medium into another obliquely, it undergoes a change not only in direction but in velocity as well. This change of direction and velocity of light as it enters from one medium into another is known as refraction of light.

Laws of Refraction: Refraction of light takes place under two laws, known as the laws of refraction of light:

1. The incident ray, the normal, and the refracted ray at the point of incidence all lie in the same plane.
2. The ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is constant for all rays of light passing from one medium to another. This constant is called the refractive index.

$$\text{Refractive Index} = \frac{\text{sine of angle of incidence}}{\text{sine of angle of refraction}} = \frac{\sin i}{\sin r}$$

This is known as Snell’s law.

Q.2: What is refractive index or refractive constant?

Answer:

Refractive Index: The ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is constant for all rays of light passing from one medium to another. It is denoted by $n$.

$$n = \frac{\sin i}{\sin r}$$

OR

The ratio between the speed of light in the first medium to the speed of light in the second medium is called the refractive index of the medium.

$$n = \frac{\text{speed of light in rare medium}}{\text{speed of light in denser medium}}$$

PROPAGATION AND REFLECTION OF LIGHT (Chapter # 13) Physics 10th - Question Answers

Physics 10th - Question Answers

PROPAGATION AND REFLECTION OF LIGHT

---

Q.1: What do you understand by rectilinear propagation of light or path of light?

Ans:

• A ray of light travels in a straight path or straight line. This phenomenon is known as rectilinear propagation of light.

Q.2: Define the following terms:

• Light
• Ray of light
• Beam of light
• Medium
• Rarer medium
• Denser medium

Ans:

1. Light:

   • Light is a form of energy with which we can see all types of objects easily. It is called light.

2. Ray of Light:

   • The path on which light energy travels is called a ray of light.

3. Beam of Light:

   • Collection of rays is called a beam of light.

4. Medium:

   • A transparent medium from which light can be passed is called a medium. There are two types of mediums:
     • Rarer medium
     • Denser medium

5. Rarer Medium:

• A medium from which light can pass easily is called a rarer medium. In a rarer medium, the distance between the molecules is greater, and due to this, light can pass through the medium easily.

6. Denser Medium:

• A medium in which light energy cannot pass easily is called a denser medium. In a denser medium, the distance between the molecules is smaller or closely packed, so light cannot pass easily.

Q.3: What is a pinhole camera?

Ans:

• The rectangular box having a black coating and containing a very small hole on one side and a frosted glass screen on the opposite side is known as a pinhole camera. The image formed by a simple pinhole camera is smaller than the object and is inverted and real.

Q.4: Write down the construction and working of the pinhole camera?

Ans:

Construction:

• A pinhole camera is one of the practical applications of rectilinear propagation of light. It was invented in the sixteenth century to observe eclipses of the sun without damaging the eye sight. It is also known as a camera obscura (dark room).
• It consists of a rectangular box containing a very small hole on one side. When a narrow pencil of light rays passes through the pinhole, an inverted and real image of the object is formed on the back of the camera. For observing a clear image, external light is excluded by covering the box with a dark cloth.

Working or Principle:

• To understand the image formation in the pinhole camera, we consider the following diagram:

Image by a Pinhole Camera:

• A narrow pencil of rays starting from point $A$ passes through the pinhole $O$ and illuminates a small area as $A'$. Similarly, a narrow pencil of rays starting from $C$ illuminates a small area at $C'$. In this way, points lying between $A$ and $C$ illuminate corresponding points between $A'$ and $C'$, and a real and inverted image $A'C'$ of the object $AC$ is formed on the back of the camera.

Q.5: Define reflection of light? Write down its laws.

Ans: Reflection of Light:

• The reflection of light can be defined as, if a light is traveling through a medium and it strikes the surface of an opaque body, some part of the light reflects back in the same medium. This is known as the reflection of light.

Laws of Reflection: There are two laws of reflection:

1. The incident ray, reflected ray, and the normal lie in the same medium.
2. The angle of incidence is always equal to the angle of reflection. $\angle i = \angle r$

Q.6: How many kinds of reflection do you know?

Ans:

• There are two kinds of reflection:
  1. Regular reflection
  2. Irregular reflection

Regular Reflection:

• When some parallel beams of light strike an ideal smooth and polished surface, they reflect back in a particular direction and remain parallel to each other. This reflection is known as regular reflection.

Irregular Reflection:

• When some parallel beams of light strike a rough surface, they reflect back in different directions. This type of reflection is known as irregular reflection.

Q.7: Write down the difference between regular and irregular reflections?

Regular Reflections	Irregular Reflections
It occurs when parallel rays of light strike with an ideal smooth plane surface.	It occurs when parallel rays of light strike with an irregular non-smooth surface.
Parallel rays of light remain parallel after reflection.	Parallel rays of light do not remain parallel after reflection.
It occurs with plane mirrors.	It occurs by irregular surfaces such as a painted wall.

Q.8: What is the importance of irregular reflection?

Ans: Importance of Irregular Reflection:

• Due to this reflection, the sunlight reaches us before sunrise and persists for times even after the sunset.
• Due to this reflection, we get sufficient light in our rooms and other places where sunlight does not reach directly.
• Due to this reflection, we can see non-luminous objects.
• Due to this reflection, the sunlight reaches each leaf of a tree, and photosynthesis takes place on a large scale.

Q.9: Define the following terms:

• Mirror
• Plane mirror
• Incident ray
• Angle of incidence
• Normal
• Reflected ray
• Angle of reflection
• Point of incidence

Ans: Mirror: Any substance or object which can reflect light is called a mirror.

PLANE MIRROR: A flat smooth reflecting surface which shows regular reflection is known as a plane mirror.

INCIDENT RAY: A ray which strikes the mirror is called an incident ray.

REFLECTED RAY: A ray which returns back after reflection is called a reflected ray.

NORMAL: A line segment which makes an angle of 90° at the point of incidence is called normal.

ANGLE OF INCIDENCE (∠i): An angle which is formed by an incident ray and normal is called angle of incidence. It is denoted by ∠i.

ANGLE OF REFLECTION (∠r): An angle which is formed by a reflected ray and normal is called angle of reflection. It is denoted by ∠r.

POINT OF INCIDENCE: The point on which the incident ray strikes on the mirror is called point of incidence.

Q.10: Explain briefly the image formation by a plane mirror?

Ans: Suppose MM’ be a plane mirror as shown in the figure. Consider a single point P lying on the tip of an object. From point P, rays are travelling in all directions. Some of these rays strike the mirror and reflect back, obeying the laws of reflection. If these reflected rays are produced backward, they appear to come from point “P”. Hence point “P” is the image of P. Similarly, the infinite points lying on the object produce infinite points. Infinite images of points then form the complete image of an object.

Q.11: Write the characteristics of image formed by plane mirror?

Ans: Characteristics Of Image Formed By Plane Mirror:

• The image is laterally inverted.
• The size of the image is the same as that of the object.
• The image is erect and virtual.
• The image is as far behind the mirror as the object is in front of the mirror.

Q.12: Define spherical mirror? Write down its types?

Ans: Spherical Mirror: A spherical mirror can be defined as a mirror whose reflecting surface is a part of a sphere, called a spherical mirror.

Types Of Spherical Mirror:

• Concave mirror
• Convex mirror

Q.13: What are concave mirror and convex mirror?

Ans:

Concave Mirror: It is a spherical mirror whose reflecting surface is towards the center of the sphere. It is also called a converging mirror because it converges all the parallel rays of light after reflection at a fixed point. This fixed point is called the focus.

Convex Mirror: It is a spherical mirror whose reflecting surface is on the opposite side of the center of the sphere. It is also called a diverging mirror because it diverges all the parallel rays of light after reflection.

Q.14: Define the following terms:

• Center of curvature
• Radius of curvature
• Pole or vertex
• Principal axis
• Focal length
• Principal focus or focus
• Aperture

Ans:

Center Of Curvature: It is the center of the hollow sphere from which the curved mirror is obtained.

Radius Of Curvature (R):

A straight line drawn from the center of curvature to the reflecting curved surface is called radius of curvature (R). OR The distance between the center of curvature and the pole of the mirror is called radius of curvature.

Pole or Vertex: The geometric center of the spherical mirror is known as pole (P) or vertex.

Principal Focus or Focus (F): In the case of a concave mirror, the rays coming parallel to the principal axis after reflection converge to a point. That point is called principal focus and it is denoted by F.

Focal Length (F): The distance between the principal focus and the pole is called focal length.

Principal Axis: The straight line that passes through the center of curvature and pole is called the principal axis.

Aperture: The circular diameter of a spherical mirror is called aperture.

Q.15: Write down the differences between concave mirror and convex mirror?

Concave Mirror	Convex Mirror
Its reflecting surface lies towards the center of curvature.	Its reflecting surface lies away from the center of curvature.
It forms real and inverted images.	It forms virtual and erect images.
Its focal length is positive.	Its focal length is negative.
It is a converging mirror.	It is a diverging mirror.

Q.16: Explain the reflection and formation of images by a concave mirror?

Ans: Rays reflected from a concave mirror follow the following rules:

Rule 1: A ray that is parallel to the principal axis is reflected through the focus (F).

Rule 2: A ray that passes through the principal focus is reflected parallel to the principal axis.

Rule 3: A ray that is coming through the center of curvature is reflected back along the same path.

Rule 4: A ray incident on the pole at an angle θ with the principal axis is reflected back at the same angle.

Q.17: Find the position, nature, and size of the image with the help of a ray diagram when an object is placed on the following positions in front of a concave mirror:

• At infinity
• Beyond the center of curvature (C)
• At the center of curvature (C)
• Between F (focus) and C
• At focus (F)
• Between F and P (pole)

Ans:

Object at Infinity:

• Position of Object: Object lying at infinity.
• Position of Image: Image is formed at focus (F).
• Nature of Image: Real and inverted.
• Size of Image: Highly diminished.

Object Beyond C:

• Position of Object: Object lying beyond C.

• Position of Image:Image formed in between F and C.

• Nature of Image:Image is real and inverted.

• Size of Image:Small in size.

Object at C:

• Position of Object: Object lying at C.
• Position of Image: Image is formed at C.
• Nature of Image: Image is real and inverted.
• Size of Image: Equal in size.

Object in Between C and F:

• Position of Object: Object lying between C and F.
• Position of Image: Image formed beyond C.
• Nature of Image: Image is real and inverted.
• Size of Image: Magnified.

Object at F:

• Position of Object: Object lying at F.
• Position of Image: Image formed at infinity.
• Nature of Image: Real and inverted.

Size of Image:
Highly magnified.

Object in Between P and F:

• Position of Object: Object lying in between F and P.
• Position of Image: Image formed behind the mirror.
• Nature of Image: Image will be virtual and erect.
• Size of Image: Image will be magnified.

Q.18: Write down the characteristics of image formed by convex mirror with the help of ray diagram?

Ans: Characteristics:

• Image formed by convex mirror is always virtual and erect.
• Image forms behind the mirror.
• Image is smaller in size than the object.
• Image is always formed in between pole P and principal focus F.

(Diagram of location of image formed by a convex mirror)

Q.19: Write down the characteristics of the image formed by concave mirror?

Ans: Characteristics:

• Image formed by concave mirror is always real and inverted.
• Image will be formed in front of the reflecting surface.
• Size of the image depends on object distance.

(Diagram of image formation by concave mirror)

Q.20: Derive the mirror equation or mirror formula?
$\frac{1}{f} = \frac{1}{p} + \frac{1}{q}$

Ans:
In order to derive a formula for the spherical mirrors, we consider the formation of an image by a concave mirror when an object is placed between the principal focus and center of curvature of the mirror.

AB is an object placed before the mirror between the focus and the center of curvature. Two rays AP and AD are incident on the mirror.

Ray AP is reflected with the same angle along the direction PA obeying the law of reflection.

(Diagram showing focal length, object distance, image distance, and object and image height)

As triangles $\triangle A'PB'$ and $\triangle APB$ are similar:
Therefore,
$\frac{AB}{A'B'} = \frac{PB}{PB'}$

Or
$\frac{h_0}{h_1} = \frac{p}{q}$

Ray AD which passes through F (focus) becomes parallel to the principal axis PB.

As triangle $\triangle ABF$ and $\triangle APD$ are similar:
Therefore,
$\frac{AB}{DB} = \frac{BF}{FP}$

As:

• $AB = h_0$
• $DP = A'B'$
• $A'B' = h_i$
• $FP = f$
• $BF = p - f$
• h1​h0​​=fp−f​

• Dividing both sides by $p$, we get
  $\frac{p}{pq} = \frac{p - f}{pf}$

• Or
  $\frac{p}{pq} = \frac{p}{pf} - \frac{f}{pf}$

  $\frac{1}{q} = \frac{1}{f} - \frac{1}{p}$

• The above equation is known as the mirror equation or mirror formula.

Q.21: Define magnification?

Ans:
Magnification:
The change in the size of image after reflection or refraction is called magnification.
OR
It is the ratio between the size of image and size of the object. Magnification is written as:
$M = \frac{h_i}{h_0} = \frac{q}{p}$

Q.22: What are the uses of spherical mirrors?

Ans:
Uses of Spherical Mirrors:

• A concave mirror produces erect and magnified images of an object when placed within the focal length of the mirror; for this reason, they are used as shaving mirrors.
• Doctors use concave mirrors for the examination of ears, nose, throat, and eyes of the patients.
• Concave mirrors are used to throw light on slides in microscopes for clearer viewing.
• Concave mirrors are used in searchlights and automobiles.
• Concave mirrors are used as objectives in big telescopes.

Q.23: Differentiate between real and virtual images?

Real Images	Virtual Images
It is always inverted.	It is always erect.
It can be seen on screen.	It cannot be seen on screen.
It exists physically.	It doesn’t exist physically.
It always forms in front of reflecting surface.	It always forms behind the mirror.

Q.24: How a concave mirror is used in headlights and searchlights to throw light at a long distance?

Ans:
As we know, if an object is placed on the principal focus of a concave mirror, it produces the image at infinity.

Hence, the bulb in the searchlight is placed at the principal focus of the concave mirror, allowing it to throw light at a large distance.