Grade 9
  1. Mechanics  1.1. Motion  1.1.1. Types of motion  1.1.2. Distance and displacement  1.1.3. Speed and Velocity  1.1.4. Acceleration  1.1.5. Graphical representation of motion  1.1.6. Equations of motion  1.1.7. Uniform and non-uniform motion  1.1.8. Free fall and acceleration due to gravity  1.1.9. Relative speed  1.1.10. Circular motion  1.2. Laws of force and motion  1.2.1. The concept of force  1.2.2. newton's first law of motion  1.2.3. Newton's second law of motion  1.2.4. Newton's third law of motion  1.2.5. Inertia and types of inertia  1.2.6. Motion  1.2.7. Conservation of momentum  1.2.8. Application of Newton's laws in daily life  1.2.9. Types of Forces (Contact and Non-Contact)  1.2.10. Friction and its effects  1.3. Gravitational force  1.3.1. Newton's law of universal gravitation  1.3.2. Importance of Gravitational Force  1.3.3. Acceleration due to gravity  1.3.4. Free fall and weightlessness  1.3.5. Mass and weight  1.3.6. Variation of g with height and depth  1.3.7. Kepler's laws of planetary motion  1.3.8. Satellites and their orbits  1.4. Work, Energy and Power  1.4.1. Concept of work  1.4.2. Positive and negative actions  1.4.3. Work done by constant and variable force  1.4.4. Energy and its forms  1.4.5. Kinetic energy and potential energy  1.4.6. Law of conservation of energy  1.4.8. Commercial unit of energy  1.4.9. Work–energy theorem  1.5. Simple machines  1.5.1. Types of simple machines  1.5.2. Mechanical advantage  1.5.3. Machine efficiency  1.5.4. Levers and their types  1.5.5. Pulleys and Inclined Planes  1.5.6. Gears and their uses
  2. Properties of matter  2.1. States of matter  2.1.1. Solids, liquids and gases  2.1.2. Properties of different states of matter  2.1.3. Change in state of matter  2.1.4. Plasma and Bose–Einstein condensate  2.2. Density and pressure  2.2.1. Concept of density and relative density  2.2.2. Pressure in solids, liquids and gases  2.2.3. Pascal's law and its applications  2.2.4. Atmospheric pressure and its variations  2.2.5. Surface tension and capillarity  2.3. Buoyancy and Archimedes' principle  2.3.1. Buoyant force  2.3.2. Archimedes principle  2.3.3. Applications of Archimedes' Principle  2.3.4. floating and sinking objects  2.3.5. Theory of Flotation and Archimedes' Principle
  3. Heat and Thermodynamics  3.1. Temperature and heat  3.1.1. Concept of heat and temperature  3.1.2. Temperature measurement  3.1.3. Temperature scales  3.2. Heat transfer  3.2.1. Conduction of heat  3.2.2. Convection of heat  3.2.3. heat radiation  3.2.4. Applications of heat transfer  3.2.5. Thermal conductivity  3.3. Thermal expansion  3.3.1. Expansion of solids, liquids and gases  3.3.2. Abnormal expansion of water  3.3.3. Applications of Thermal Expansion  3.4. Specific heat capacity and latent heat  3.4.1. Concept of specific heat capacity  3.4.2. Measurement and applications of specific heat  3.4.3. Latent heat of fusion and vaporization  3.4.4. Heat Engines and Efficiency
  4. Waves and sound  4.1. Waves and their types  4.1.1. Mechanical and electromagnetic waves  4.1.2. Longitudinal and Transverse Waves  4.1.3. Properties of Waves  4.1.4. Wavelength, frequency and speed of the wave  4.1.5. Superimposition of waves  4.2. Sound waves  4.2.1. Nature and transmission of sound  4.2.2. Speed of sound in different mediums  4.2.3. Reflection of sound and echo  4.2.4. Sonar and ultrasound  4.2.5. Resonance and pulsation  4.3. Sound characteristics  4.3.1. Intensity, loudness and quality of sound  4.3.2. Doppler effect in sound
  5. Lighting and Optics  5.1. Reflection of light  5.1.1. Laws of reflection  5.1.2. Reflection from a plane mirror  5.1.3. Reflection from a spherical mirror  5.1.4. Image formation by concave and convex mirrors  5.1.5. Applications of Reflection  5.2. Refraction of light  5.2.1. Laws of refraction  5.2.2. Refractive index  5.2.3. Refraction through a glass slab  5.2.4. Refraction in water and air  5.2.5. Lenses and their types  5.2.6. Image formation by convex and concave lenses  5.2.7. Total internal reflection and its applications  5.3. Dispersion and scattering of light  5.3.1. Spectrum of white light  5.3.2. Prisms and Dispersion  5.3.3. Tyndall effect and blue sky
  6. Electricity and Magnetism  6.1. Electric charge and static electricity  6.1.1. The concept of electric charge  6.1.2. Charging by friction, contact and induction  6.1.3. Electroscope and its working  6.2. Current Electricity  6.2.1. The concept of electric current  6.2.2. Ohm's Law and Resistance  6.2.3. Factors affecting resistance  6.2.4. Series and Parallel Circuits  6.2.5. Heating effect of electric current  6.2.6. Electric power and energy  6.3. Magnetism  6.3.1. Natural and artificial magnets  6.3.2. Properties of magnets  6.3.3. Earth's magnetism  6.3.4. Magnetic field and field lines  6.3.5. Electromagnets and their applications
  7. Modern Physics  7.1. structure of the atom  7.1.1. Atomic Structure and Subatomic Particles  7.1.2. Electrons, Protons, and Neutrons  7.1.3. Rutherford and Bohr model  7.2. Radioactivity  7.2.1. Types of radioactive emissions  7.2.2. Half-life and radioactive decay  7.2.3. Uses and Hazards of Radioactivity
  8. Space science and astronomy  8.1. The universe and its components  8.1.1. Stars and galaxies  8.1.2. Planets and Solar System  8.2. Satellites  8.2.1. Natural and artificial satellites  8.2.2. Use of satellites

Grade 9 → Lighting and Optics → Refraction of light ↓


Image formation by convex and concave lenses


Lenses are pieces of transparent matter, such as glass or plastic, that refract or bend light rays in predictable ways. In grade 9 physics, the study of lenses focuses on two main types: convex lenses and concave lenses. These lenses are essential for understanding how light behaves and how images are formed. This explanation will take a closer look at these lenses and how they are used to form images. We will discuss some simple examples, provide visual aids, and also dive into some basic physics formulas to provide a clear understanding.

Convex lens

Convex lenses, also called converging lenses, are thicker at the center than at the edges. When parallel rays of light pass through a convex lens, they bend inward and converge at a point known as the focal point. This feature makes convex lenses very useful in situations where you want to focus light into a single point.

Focal point and focal length

The focal point of a convex lens is the point where parallel rays of light converge after passing through the lens. The distance between the center of the lens and the focal point is called the focal length. This distance is important in determining how the lens will affect image formation.

The simple formula for finding the focal length (f) is as follows:

1/f = 1/v + 1/u

Where v is the image distance and u is the object distance.

Image formation by a convex lens

The type of image formed by a convex lens depends on the position of the object relative to the lens. Let's explore different scenarios:

Objects beyond 2F

When an object is placed at a distance twice the focal length, the image formed is real, inverted and diminished. This means the image is smaller than the object and appears on the opposite side of the lens.

F 2F

Object at 2F

When the object is at twice the focal length, the image is real, inverted, and the same size as the object. This image is also formed on the opposite side of the lens.

2F 2F

Object between F and 2F

If the object is located between the focal point and twice the focal length, the image is real, inverted and magnified. It appears larger than the object located on the other side of the lens.

F 2F

Object at focal point F

When the object is placed exactly at the focal point, no image is formed because the refracted rays are parallel to each other and never meet.

F

The object between the lens and the focal point

If the object is between the lens and the focal point, the image formed is virtual, erect and enlarged. This image appears on the same side as the object and is larger than the object.

F

Concave lens

Concave lenses, also called diverging lenses, are thinner at the center than at the edges. When parallel rays of light pass through a concave lens, they spread outward and appear to diverge from a point known as the principal focus. Concave lenses are used in situations where dispersion of light is required.

Focal point and focal length of a concave lens

In a concave lens, the focal point is the point from which parallel rays of light appear to diverge after passing through the lens. Here the focal length is also the distance between this focal point and the centre of the lens.

The focal length can still be described using the same lens formula:

1/f = 1/v + 1/u

In this case, the focal length (f) is negative because the focal point is virtual.

Image formation by a concave lens

Unlike convex lenses, concave lenses always form virtual, erect and smaller images, regardless of the position of the object. Let's look at this process:

Any state of the object

For any position of the object, the image formed by a concave lens is virtual, erect and smaller than the object. The image appears the same size as the object.

F

Applications of lenses

Lenses have many applications in everyday life and technology. Here are some common examples:

  • Glasses: Both convex and concave lenses are used in glasses to correct vision. Convex lenses help people with farsightedness, while concave lenses help people with nearsightedness.
  • Camera: A camera uses a lens to focus light onto film or an image sensor, producing a sharp, detailed picture.
  • Microscope: The convex lens in a microscope is used to magnify small objects for observation.
  • Telescopes: Telescopes use convex lenses (and sometimes concave lenses) to gather and focus light coming from distant objects, such as stars and planets.

Conclusion

In conclusion, the study of image formation by convex and concave lenses is an essential part of understanding optics. Understanding how these lenses work allows us to use them effectively in various practical applications. Convex lenses converge light and can form real or virtual images depending on the position of the object. On the other hand, concave lenses diverge light and always form virtual, smaller images. This knowledge lays the groundwork for exploring more complex optical systems in higher grades.


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Image formation by convex and concave lenses

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