NCERT Science Notes - Class 10
Chapter 9 - Light and Reflection

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 10 | Science | Chapter - 9 | Light and Reflection. This chapter takes students on an illuminating journey through the properties and behaviors of light. It begins with the nature of light as an electromagnetic wave and explains how it travels in straight lines. The chapter dives into the laws of reflection, demonstrating how light interacts with different surfaces, forming images in mirrors. Students learn about plane and spherical mirrors, the concepts of focal length, and image formation through ray diagrams. This chapter lays the groundwork for understanding optical phenomena and the practical applications of reflection in everyday life and technology.

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1. Visibility of Objects:
   • We can see various objects around us.
   • In a dark room, we cannot see anything.
   • Lighting up the room makes things visible.
2. Cause of Visibility:
   • Sunlight during the day enables us to see objects.
   • Objects become visible because they reflect light that falls on them.
   • The reflected light, when it reaches our eyes, enables us to see.
3. Seeing Through Transparent Media:
   • We can see through transparent media because light passes through them.
4. Optical Phenomena Related to Light:
   • Some common phenomena associated with light include:
     • Image formation by mirrors
     • The twinkling of stars
     • The colors of a rainbow
     • Bending of light by a medium
5. Properties of Light:
   • Studying light’s properties helps us understand these phenomena.
6. Straight-Line Path of Light:
   • Observing optical phenomena suggests that light travels in straight lines.
   • Evidence: A small light source casts a sharp shadow of an opaque object.
   • This straight-line path of light is typically represented by a ray of light.

More to know

9.1 - Reflection of Light

1. Reflection from Polished Surfaces:
   • A highly polished surface (e.g., a mirror) reflects most of the light falling on it.
2. Laws of Reflection:
   • Law 1: The angle of incidence is equal to the angle of reflection.
   • Law 2: The incident ray, the normal to the mirror at the point of incidence, and the reflected ray all lie in the same plane.
3. Image Formation by Plane Mirrors:
   • Properties of the image formed by a plane mirror:
     • The image is virtual and erect.
     • The size of the image is equal to that of the object.
     • The image is located as far behind the mirror as the object is in front of it.
     • The image is laterally inverted.
4. Curved Mirrors:
   • Curved surfaces can also reflect light and form images.
   • The curved surface of a shining spoon can be viewed as a curved mirror.
   • The most commonly used type of curved mirror is the spherical mirror.
   • The reflecting surface of spherical mirrors is part of a sphere.

Activity 9.1

1. Using a Large Shining Spoon:
   • Observe your face in the curved surface of the spoon.
   • Note whether you see the image and whether it appears smaller or larger.
   • Move the spoon away from your face and observe how the image changes.
   • Reverse the spoon and repeat the activity to compare how the image looks.
2. Comparison of Images:
   • Compare the characteristics of images formed on the concave and convex surfaces of the spoon.

9.2 - Spherical Mirrors

1. Types of Spherical Mirrors:
   • A spherical mirror may have its reflecting surface curved inwards or outwards.
     • Concave Mirror: Reflecting surface is curved inwards, facing towards the center of the sphere.
     • Convex Mirror: Reflecting surface is curved outwards.
2. Schematic Representation:
   • Diagrams typically show the back of the mirror shaded to indicate the reflecting surface.
3. Approximation to Spoon Shapes:
   • The inward-curved surface of a spoon approximates a concave mirror.
   • The outward-bulged surface of a spoon approximates a convex mirror.
4. Important Terms:
   • Pole (P): The center of the reflecting surface of a spherical mirror, located on the surface of the mirror.
   • Centre of Curvature (C):
     • A point that is the center of the sphere of which the reflecting surface is a part.
     • It is represented by the letter C and is not part of the mirror; it lies outside the reflecting surface.
     • In a concave mirror, the center of curvature lies in front of the mirror.
     • In a convex mirror, the center of curvature lies behind the mirror.
   • Radius of Curvature (R): The radius of the sphere of which the reflecting surface forms a part, represented by the letter R.
     • The distance PC (from the pole to the center of curvature) is equal to the radius of curvature.
   • Principal Axis:
     • A straight line that passes through the pole and the centre of curvature of a spherical mirror.
     • The principal axis is normal to the mirror at its pole.

Activity - 9.2

Caution: Do not look at the Sun directly or into a mirror reflecting sunlight, as it may damage your eyes.

1. Procedure:
   • Hold a concave mirror and direct its reflecting surface towards the Sun.
   • Direct the light reflected by the mirror onto a sheet of paper held close to the mirror.
   • Gradually move the sheet of paper back and forth until a bright, sharp spot of light is found on the paper.
   • Hold the mirror and the paper in the same position for a few minutes.
2. Observation:
   • The paper begins to burn, producing smoke, and may eventually catch fire.
3. Explanation:
   • The bright spot of light is the image of the Sun on the sheet of paper, formed by the concave mirror converging sunlight at a point.
   • This point is referred to as the focus of the concave mirror.
   • The heat produced from the concentrated sunlight ignites the paper.
   • The distance from the mirror to this image provides the approximate value of the focal length of the mirror.

Reflection of Light

1. Burning of Paper:
   • The paper begins to burn and produce smoke when exposed to the concentrated sunlight.
   • Reason: The light from the Sun is converged at a point (focus) by the concave mirror, creating a sharp, bright spot on the paper.
   • This point is the image of the Sun on the sheet of paper.
   • The heat generated by this concentrated light ignites the paper.
2. Focal Length:
   • The distance from the mirror to the focus provides the approximate value of the focal length of the mirror.

1. Concave Mirror:
   • Incident Rays: A number of rays parallel to the principal axis fall on a concave mirror.
   • Reflected Rays: The reflected rays converge at a point on the principal axis.
   • Principal Focus: This point is called the principal focus (F) of the concave mirror.
2. Convex Mirror:
   • Incident Rays: Rays parallel to the principal axis are reflected by a convex mirror.
   • Reflected Rays: The reflected rays appear to diverge from a point on the principal axis.
   • Principal Focus: This point is also referred to as the principal focus (F) of the convex mirror.

Key Terms

• Principal Focus (F): The point where rays parallel to the principal axis converge (for a concave mirror) or appear to diverge from (for a convex mirror).
• Focal Length (f): The distance between the pole (P) and the principal focus (F) of a spherical mirror, represented by the letter f.

Spherical Mirrors

1. Reflecting Surface:
   • The reflecting surface of a spherical mirror is spherical and has a circular outline.
2. Aperture:
   • The diameter of the reflecting surface is called its aperture.
   • In the diagram (Fig. 9.2), the distance MN represents the aperture.
3. Consideration:
   • The discussion will focus on spherical mirrors whose aperture is much smaller than its radius of curvature.
4. Relationship Between Radius of Curvature and Focal Length:
   • For spherical mirrors with small apertures, there is a relationship:
     • Formula: $R = 2f$
       
       • Where $R$R is the radius of curvature and $f$ is the focal length.
   • This implies that the principal focus (F) of a spherical mirror is located midway between the pole (P) and the center of curvature (C).

9.2.1 - Image Formation by Spherical Mirrors

1. Introduction:
   • The formation of images by spherical mirrors is different from that of plane mirrors.
   • The nature, position, and relative size of images formed by spherical mirrors depend on the object's position relative to points P (pole), F (focus), and C (center of curvature).
2. Nature of Images:
   • Images formed by concave mirrors can be:
     • Real: Formed when the object is beyond the center of curvature (C).
     • Virtual: Formed when the object is between the focus (F) and the pole (P).
3. Size of Images:
   • The size of the image can be classified as:
     • Magnified: The image is larger than the object.
     • Reduced: The image is smaller than the object.
     • Same Size: The image is equal to the object in size.
4. Summary of Observations:
   • The nature, position, and size of the image vary based on the object's location concerning points P, F, and C.
   • A table (Table 9.1) summarizes these observations for reference, detailing the conditions under which images are real, virtual, magnified, reduced, or of the same size.

9.2.2 - Representation of Images Formed by Spherical Mirrors Using Ray Diagrams

1. Introduction:
   • Ray diagrams help in studying the formation of images by spherical mirrors.
   • Each small portion of an extended object acts as a point source emitting an infinite number of rays.
   • For clarity, only two rays are used to construct ray diagrams.
2. Choosing Rays:
   • The intersection of at least two reflected rays indicates the position of the image of the point object.
   • The following rays can be used to locate the image:
   (i) Ray Parallel to the Principal Axis:
   • For a concave mirror: A ray parallel to the principal axis will pass through the principal focus (F) after reflection.
   • For a convex mirror: A ray parallel to the principal axis appears to diverge from the principal focus (F) after reflection.
   (ii) Ray Passing Through the Principal Focus:
   • For a concave mirror: A ray passing through the principal focus, after reflection, emerges parallel to the principal axis.
   • For a convex mirror: A ray directed towards the principal focus emerges parallel to the principal axis after reflection.
   (iii) Ray Passing Through the Center of Curvature:
   • For a concave mirror: A ray passing through the center of curvature (C) is reflected back along the same path.
   • For a convex mirror: A ray directed towards the center of curvature is also reflected back along the same path.
   • This occurs because the incident rays strike the mirror along the normal to the reflecting surface.
   (iv) Ray Incident Obliquely to the Principal Axis:
   • A ray incident obliquely towards the pole (P) of the mirror is reflected obliquely.
   • The incident and reflected rays follow the laws of reflection, making equal angles with the principal axis.
3. Laws of Reflection:
   • In all cases mentioned above, the laws of reflection apply:
     • The angle of incidence equals the angle of reflection at the point of incidence.
4. Ray Diagrams for Concave Mirror:
   • Figure 9.7 illustrates the ray diagrams for the formation of images by a concave mirror at various object positions.

Activity 9.4: Ray Diagrams and Image Formation by Concave Mirrors

Instructions:

1. Draw neat ray diagrams for each position of the object as indicated in Table 9.1.
2. Use any two of the rays mentioned previously for locating the image.
3. Compare your diagrams with those given in Figure 9.7.
4. Describe the nature, position, and relative size of the image formed in each case.
5. Tabulate the results in a convenient format.
   
   

• Nature of Image: Indicates whether the image is real or virtual, and if it is upright or inverted.
• Position of Image: Refers to where the image is located concerning the mirror.
• Relative Size of Image: Indicates if the image is diminished, same size, or enlarged compared to the object.

• Make sure to clearly label the principal axis, pole (P), focus (F), center of curvature (C), and the rays in your diagrams. Use solid lines for incident rays and dashed lines for reflected rays.

Activity 9.5: Observing Image Formation by a Convex Mirror

1. Materials Needed:
   • Convex mirror
   • Pencil (held upright)
2. Procedure:
   • Hold the convex mirror in one hand.
   • Hold the pencil upright in the other hand.
   • Observe the image of the pencil in the mirror.
   • Answer the following questions based on your observations:
     • Is the image erect or inverted?
     • Is the image diminished or enlarged?
   • Slowly move the pencil away from the mirror and note the changes:
     • Does the image become smaller or larger?
   • Repeat the activity carefully and state:
     • Does the image move closer to or farther away from the focus as the object is moved away from the mirror?

Image Formation by a Convex Mirror

Positions of the Object

1. Object at Infinity:
   • Ray Diagram: As illustrated in Fig. 9.8 (a).
   • Nature of Image: Virtual and erect.
   • Position of Image: Behind the mirror.
   • Relative Size of Image: Diminished.
2. Object at a Finite Distance:
   • Ray Diagram: As illustrated in Fig. 9.8 (b).
   • Nature of Image: Virtual and erect.
   • Position of Image: Behind the mirror.
   • Relative Size of Image: Diminished.

• The image formed by a convex mirror is always virtual, erect, and diminished, regardless of the object's position relative to the mirror.
• As the object moves away from the mirror, the image remains diminished and continues to appear behind the mirror, moving slightly closer to the focus.

Activity 9.6: Observing Full-Length Images in Different Mirrors

1. Objective: To explore which type of mirror (plane, concave, or convex) can provide a full image of a large object.
2. Materials Needed:
   • Plane mirrors of various sizes
   • Concave mirror
   • Convex mirror
3. Procedure:
   • Step 1: Observe the image of a distant object (e.g., a distant tree) in a plane mirror.
     • Question: Can you see a full-length image?
     • Step 2: Try using plane mirrors of different sizes.
       • Question: Did you see the entire object in the image?
   • Step 3: Repeat the observation with a concave mirror.
     • Question: Does the concave mirror show a full-length image of the object?
   • Step 4: Now try using a convex mirror.
     • Question: Did you succeed in seeing the full-length image? Explain your observations.
4. Observation and Explanation:
   • Plane Mirror: The image size depends on the mirror size; smaller mirrors may not show the full object.
   • Concave Mirror: Typically shows a magnified image, but the full object may not fit depending on the distance.
   • Convex Mirror: A convex mirror can show a full-length image of tall objects like trees or buildings, even if the mirror is small.

• You can see a full-length image of a tall building or tree in a small convex mirror, as demonstrated by mirrors in places like Agra Fort facing the Taj Mahal. For clear visibility, position yourself suitably at the terrace adjoining the wall.

1. Rear-View Mirrors:
   • Commonly used as rear-view (wing) mirrors in vehicles.
   • Fitted on the sides of vehicles, allowing drivers to see traffic behind them.
2. Advantages:
   • Erect Images: Convex mirrors provide erect, albeit diminished, images.
   • Wider Field of View: Their outward curvature allows for a larger viewing area compared to plane mirrors.
3. Safety:
   • Facilitates safe driving by providing a broader perspective of surrounding traffic, enhancing visibility for drivers.

9.2.3 - Sign Convention for Reflection by Spherical Mirrors

When studying the reflection of light by spherical mirrors, we use the New Cartesian Sign Convention. This convention standardizes how we measure and interpret distances in relation to the mirror.

Key Features of the New Cartesian Sign Convention:

1. Origin:
   • The pole (P) of the mirror is taken as the origin of the coordinate system.
2. Principal Axis:
   • The principal axis of the mirror is considered as the x-axis (X’X).
3. Object Placement:
   • The object is always placed to the left of the mirror. This indicates that light from the object falls on the mirror from the left side.
4. Distance Measurements:
   • Parallel to the Principal Axis:
     • Distances measured to the right of the origin (along the + x-axis) are considered positive.
     • Distances measured to the left of the origin (along the - x-axis) are considered negative.
   • Perpendicular to the Principal Axis:
     • Distances measured perpendicular to and above the principal axis (along the + y-axis) are taken as positive.
     • Distances measured perpendicular to and below the principal axis (along the - y-axis) are taken as negative.

• The New Cartesian Sign Convention is visually represented in Figure 9.9, showing how to apply these rules to distances in relation to the spherical mirror.

• This sign convention is crucial for deriving the mirror formula and solving numerical problems related to spherical mirrors, ensuring consistency in calculations and interpretations.

9.2.4 - Mirror Formula and Magnification

Magnification

Magnification produced by a spherical mirror indicates how much larger or smaller the image of an object appears compared to the actual size of the object. It is represented by the letter m and is calculated using the following definitions:

Definitions:

1. Height of the Object (h):
   • The actual height of the object.
2. Height of the Image (h'):
   • The height of the image formed by the mirror.

Magnification Formula

The magnification m is given by the ratio of the height of the image to the height of the object:

Relationship with Object and Image Distances

The magnification can also be expressed in terms of the object distance (u) and image distance (v) as follows:

• The height of the object $h$ is considered positive since it is typically above the principal axis.
• The height of the image $h'$ is taken as:
  • Positive for virtual images (erect).
  • Negative for real images (inverted).
• A negative value of magnification indicates that the image is real (inverted).
• A positive value of magnification indicates that the image is virtual (erect).

Example - 9.3

A concave lens has focal length of 15 cm. At what distance should the object from the lens be placed so that it forms an image at 10 cm from the lens? Also, find the magnification produced by the lens. 

Example - 9.4

A 2.0 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 10 cm. The distance of the object from the lens is 15 cm. Find the nature, position and size of the image. Also find its magnification.

9.3.8 - Power of a Lens

• The power of a lens describes its ability to converge (convex lens) or diverge (concave lens) light rays, depending on the lens’s focal length.
• Definition of Power:
  The power (P) of a lens is defined as the reciprocal of its focal length (f), represented as: $$P=\frac{1}{f}$$
• SI Unit:
  The SI unit of lens power is the dioptre (D).
  • 1 dioptre is the power of a lens with a focal length of 1 metre.
  • Thus, $1 \, \text{D} = 1 \, \text{m}^{-1}$.
• Power of Convex and Concave Lenses:
  • Convex Lens: Has positive power as it converges light rays.
  • Concave Lens: Has negative power as it diverges light rays.
• Example of Lens Power in Use:
  • If a lens prescribed by an optician has a power of +2.0 D, it indicates:
    • It is a convex lens.
    • Its focal length $f$ is $+0.50 \, \text{m}$ (since $f = \frac{1}{P} = \frac{1}{2.0}$​).
  • For a lens with –2.5 D, it implies:
    • It is a concave lens.
    • Its focal length $f$ is $-0.40 \, \text{m}$ (since $f = \frac{1}{-2.5}$).

More to Know !

• Multiple Lenses in Optical Instruments:
  Optical instruments often use several lenses to improve image magnification and sharpness. When lenses are placed in contact, their net power (P) can be calculated by adding the individual powers of each lens.
• Formula for Net Power:
  If the powers of lenses in contact are $P_1$​, $P_2$​, $P_3$​, etc., then the combined power $P$ is: $$P=P_1+P_2+P_3+\dots$$
• Convenience in Eye-Testing:
  Opticians use lens power (rather than focal length) as it simplifies calculations. During eye tests, they combine lenses of different known powers in the testing spectacles' frame to determine the required correction by simple algebraic addition.
  • For example, a combination of lenses with powers $+2.0 \, D$ and $+0.25 \, D$ equals a single lens of power $+2.25 \, D$.
• Lens Systems in Cameras, Microscopes, and Telescopes:
  This additive property of lens power is crucial for designing lens systems in devices like cameras, microscopes, and telescopes. Such systems can correct certain image defects that a single lens might produce, enhancing image quality.

NCERT Science Notes Class 10 | Science | Chapter - 9 | Light and Reflection

NCERT Science Notes Class 10 | Science | Chapter - 9 | Light and Reflection