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 ↓


Refraction in water and air


When light passes from one medium to another, its speed changes. This phenomenon is known as refraction. Refraction occurs because light travels at different speeds in different mediums. For example, light travels faster in air than in water.

Understanding refraction

Refraction can be observed when a straw appears bent when placed in a glass of water. This happens because the rays of light bend as they pass from water to air. Let's use simple terms and examples to see how refraction works and why it happens.

Light bends at the interface of two different materials. This means that light changes its direction when it enters a material at an angle to another material. When light enters a denser medium from a less dense medium, such as from air to water, it bends toward the normal. Conversely, when light leaves a denser medium and enters a less dense medium, it bends away from the normal.

Consider a ray of light entering water from air:

Air   / 
/ (Light ray bending towards the normal)
|| Water

Snell's law

To tell how much light bends, we use Snell's law. This law is written as:

n1 * sin(θ1) = n2 * sin(θ2)

Here, n1 and n2 are the refractive indices of the medium, and θ1 and θ2 are the angles of the incident and refracted rays, respectively.

Refractive index

The refractive index is a measure of how much light will slow down in a medium. The refractive index of air is about 1, while the refractive index of water is about 1.33. This difference explains why light bends when moving between these two substances.

Example of calculating refraction using Snell's law:

Given:
n1 (air) = 1
n2 (water) = 1.33
θ1 (incident angle in air) = 30 degrees

Using Snell's Law:
1 * sin(30) = 1.33 * sin(θ2)

Calculate:
sin(θ2) = sin(30) / 1.33 ≈ 0.375
θ2 = sin^(-1)(0.375) ≈ 22 degrees

Visual representation of refraction

Air Water θ1 θ2

Real-life examples of refraction

Bent straw in glass of water

When you put a straw in a glass full of water, it appears bent on the surface of the water. This happens because the rays of light get refracted as they pass from water to air, causing the straw to change its position.

Mirage in the desert

Mirages usually occur over deserts or hot roads. They are caused by the refraction of sunlight. Warm air near the ground's surface bends rays of light upward, giving the illusion that there is water on the road or desert ahead.

Fisheye lens

Fisheye lenses use the principle of refraction to bend light and capture a highly detailed scene. They are often seen in photography to create wide panoramic shots or artistic distortions.

How refraction affects our vision

Refraction is important in the way we see the world. The lenses in our eyes refract light, allowing us to focus on objects at different distances. Wearing corrective glasses adjusts the refraction of light entering the eyes, improving vision for people with focus problems such as nearsightedness or farsightedness.

Conclusion

Refraction is a fundamental concept in physics that affects our daily lives in many ways, from the way we see objects through lenses to complex optical technologies. Understanding refraction not only explains common optical illusions but also underlies the technology we use in glasses, cameras and scientific instruments.


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Refraction in water and air

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