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

Physics 10th - Question Answers

PROPAGATION AND REFLECTION OF LIGHT

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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.