Light - Class 7 Science - New St. Mery English School

Light is a form of energy that enables us to see the beautiful world around us. Without light, everything would appear dark and invisible. The Sun is the main natural source of light for the Earth, while electric bulbs, candles, and tube lights are artificial sources of light.

Light travels in a straight line and helps us understand important concepts like reflection, shadows, and images formed by mirrors. When light falls on an object, it may be reflected, absorbed, or pass through it. This is why some objects are transparent, some are translucent, and others are opaque.

In this chapter, we will learn about sources of light, reflection of light, mirrors, and how light helps us see objects clearly. Let us explore the amazing properties of light and understand how it plays an important role in our daily life.

Light - Class 7 Science - New St. Mery English School

What is Light ?

Ans:- Light is the form of electromagnetic energy that makes everything visible. It can detected by human eyes.

What is rectilinear propagation of light?

Ans:- When light moves from one place to another, it follows a straight path as long as it travels through the same medium. This property of light is known as rectilinear propagation of light.

Ex:- When you switch on a torch in a dark room, the beam of light goes straight. You can see a straight line of light — that shows rectilinear propagation.

🌑 Shadow Formation

Shadows are formed because light travels in straight lines.
If an object comes in the path of light, it blocks the light and a shadow forms behind it.

Rectilinear propagation of light happens only when light travels in a uniform medium. When it changes medium (like air to water), it bends. That bending is called refraction.

Write an experiment that show that light travel in straight path

🎯 Aim

To prove that light travels in a straight path.

🧰 Materials Required

• 3 pieces of cardboard

• A candle

• Matchbox

• A table

🛠 Procedure

1. Take three cardboards of the same size.

2. Make a small hole at the center of each cardboard.

3. Fix the three cardboards vertically on a table in such a way that the holes are at the same height.

4. Place the cardboards in a straight line, one behind the other.

5. Put a lighted candle behind the first cardboard.

6. Look through the hole of the third cardboard from the other side.

👀 Observation

• When all three holes are in one straight line, the flame of the candle is clearly visible.

• If you move any one cardboard slightly to the side, the candle flame cannot be seen.

🧠 Conclusion

This experiment shows that light travels in a straight line. When the holes are aligned, light passes straight through them. When the path is disturbed, light cannot bend around the cardboard to reach the eye.

Reflection of light

When light hits an object like a mirror, it does not pass through. Instead, it returns back into the same medium. This is called reflection of light.

Types of Reflection

There are two types of reflection of light

1- Regular Reflection

2- Irregular Reflection

Regular Reflection

Regular reflection happens when light rays fall on a smooth, polished surface and reflect in an orderly, parallel way. In regular reflection parallel rays of light hit a smooth surface and the reflected rays also remain parallel.

Ex:- A plane mirror gives regular reflection. That’s why you can see a clear image of yourself in it.

Irregular Reflection

Irregular reflection happens when light falls on a rough surface and reflects in many different directions. In irregular reflection parallel rays of light hit a rough surface and the reflected rays scatter in different directions.

Ex:- When light strike on a wall it gives Irregular reflection, Reflection of light in water

ray of light?
Ans:- A ray of light is a narrow stream (प्रबहित होना ) of light that represents the path along which light energy travels in a medium is called ray of light.  it is represented by a straight line and an arrow marked on it. the arrow represents the direction of ray of light

beam of light?
Ans:- A group of light rays is called beam of light.

 types of beam of light?
Ans:- There are three types of beam of light.
           1- Parallel beam of light
           2- Convergent beam of light
           3 - Divergent beam of light

Parallel beam of light?
Ans:- When the rays of light travel parallel to each other then the collection of such rays is called parallel beam of light .

convergent beam of light?
Ans:- When the rays of light coming from the different directions and meat at a specific point then the collection of such rays is called convergent beam of light.

divergent beam of light?

Ans:- When the rays of light emerge from a point source of light and travel in different direction , such beam of light is called divergent beam of light.

What is incident ray?
Ans:- A ray of light that is coming from source of light towards the reflecting surface is called incident ray of light.

What is reflecting ray?
Ans:- A ray of light bouncing away from the reflecting surface after reflection is called reflecting ray.

What is point of incident?
Ans:- The point on a reflecting surface where incident ray, reflecting ray and normal meet together is called point of incident.

What is normal?
Ans:- A line drawn perpendicular to the reflecting surface is called normal.

What is incidence angle?
Ans:- An angle which is formed between incident ray and normal is called incidence angle. It is represented by ' i ' 

Incidence angle = ∠i

What is reflected angle?
Ans:- An angle which is formed between normal and reflected ray is called reflected angle. It is represented by ' r '.

Reflected angle = ∠r 

What are the 2 Laws of reflection of light?  

Ans:- The two laws of reflection are

1- The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.

2-The angle of incidence is equal to the angle of reflection. , ∠i  = ∠r .

Image

An image is the picture of an object formed when light rays coming from the object reflect from a mirror or refract through a lens and reach our eyes.

Real image

The image that can be obtained on a screen is called real image

Virtual image

The image that can not be obtained on a screen is called virtual image

What is mirror?

Ans:- Any smooth polished surface which can turn the rays of light back into the same medium is called mirror.

Ex:- looking glass, highly polished metal, still water, still oil etc

How many types of mirror?

Ans:- There are two types of mirror.

1- Plane Mirror

2- Spherical Mirror

What is Plane Mirror?

Ans:- A plane mirror is a flat and smooth surface that reflects light to form an image. A plane mirror forms an image that is virtual, erect, Laterally inverted and Same size of the object.

electric current and Its Effects - Class 7 Science - New St. Mery English School

Electricity plays an important role in our daily life. From lighting our homes to running fans, televisions, and mobile phones, everything depends on the flow of electricity. This flow of electric charge is called electric current. Without electric current, most of the modern machines we use today would not work.

In this chapter, Electric Current, students will learn what electric current is, how it flows through a circuit, and how it helps electrical devices to function. We will also study electric cells, batteries, switches, and conductors, which are necessary for the flow of electric current. Understanding this topic will help students know how electricity is produced, controlled, and used safely in our everyday life.

This lesson builds a strong foundation for learning more about electricity in higher classes and helps students become more aware of energy and its uses.

electric current and Its Effects - Class 7 Science - New St. Mery English School

Electric current is the flow of electric charges (usually electrons) through a conductor like a wire.

👉 Electric current is the movement of electricity from one place to another.

It is represented by 'I'

Example:

When you switch on a bulb, electric current flows through the wire to the bulb and makes it glow 💡.

Unit:

Electric current is measured in ampere (A).

Electric Current = Electric Charge / time

I = Q / t

Where:

I = Electric current (in ampere, A)

Q = Electric charge (in coulomb, C)

t = Time (in seconds, s)

The SI unit of electric current is Ampere because electric current cannot be measured directly, but its magnetic effect can be measured very accurately.

When electric current flows through a wire, it produces a magnetic field around it. Scientists found that this magnetic effect can be measured very precisely. So, they decided to define electric current using this effect.

Scientific idea:

One ampere is the amount of current which, when flowing through two parallel wires, produces a fixed magnetic force between them.

In simple words:

👉 Ampere is used as the SI unit because it allows electric current to be measured accurately and reliably.

Who was André-Marie Ampère?

André-Marie Ampère (1775–1836) was a French physicist and mathematician.

He is known as the “Father of Electrodynamics.”

Why is the unit named Ampere?

He discovered the relationship between electricity and magnetism.

His work showed that electric current produces magnetic effects, which is the basis for motors, generators, and many electrical devices.

To honor his contribution: 👉 The SI unit of electric current was named “Ampere (A)” after him.

Numerical 

Q1. If 10 coulomb of charge flows in 5 seconds, find the electric current.

Q2. A charge of 30 C flows through a wire in 10 s. Find the current.

Electric charge 

Electric Charge is a property of matter that causes it to experience electrical force.

In simple words:

👉 Electric charge is the reason why objects attract or repel each other electrically.

It is represented by 'Q'

Types of electric charge

There are two types:

Positive charge

Negative charge

Example:

When you rub a balloon on your hair, it gets electric charge and can stick to a wall 🎈. This happens because of electric charge.

The SI unit of electric charge is Coulomb.

Symbol - C

👉 1 coulomb (C) is the amount of charge that flows when 1 ampere of current flows for 1 second.

Electric Charge = Electric Current × Time 

Who was Charles-Augustin de Coulomb?

Charles-Augustin de Coulomb (1736–1806) was a French physicist.

He studied the forces between electric charges and magnetic poles.

Why is the unit named Coulomb?

He discovered a law called Coulomb’s Law, which explains: 👉 How electric charges attract or repel each other.

To honor his work, the SI unit of electric charge was named “Coulomb (C)” after him.

Numerical 

Q1. How much charge flows if a current of 5 A flows for 4 seconds?

Q2. A current of 2 A flows for 15 seconds. Find the charge.

How many types of substances on the basis of electric current 

Ans:- There are two types of substances on the basis of electric current.

1- Conductors 

2- Insulators 

What are Conductors?

Ans:- Substances which allow electric current to flow through them are called conductors.

Ex:- Metals like aluminium, iron, copper, etc, alkali solutions, acid solutions, etc. are good conductors of electricity

What are Insulators?

Substances which do not allow electric current to flow through them are called insulators.

Ex:- Wood, plastic, paper, rubber, air, etc.

Electric Circuit 

What is electric Circuit?

Ans:- The path along which electric current flows is called electric circuit. An electric circuit consists of a number of components like cell or battery, connecting wires, switch, bulb etc. These components are called the elements of an electric circuit. The elements of a circuit are connected between the two terminal of a cell that is( +ve and -ve ). The current starts from the positive terminal of a cell, passes through the various conducting elements connected to it and ends at the negative terminal of the cell

How many types of electric Circuit?

There are two types of electric Circuit 

1- Closed electric Circuit 

2- Open electric Circuit 

What is Closed electric Circuit?

Ans- The circuit in which the various components are connected between the cells without any break is called a closed electric circuit.

Electric current flows through a closed circuit.

What is Open electric Circuit?

Ans- The circuit in which the various components connected between the cells have a break at some point is called an open electric circuit.

No current flows through an open circuit.

Elements of Circuit 

Cell  - : Cell is the source of electric current. It has two terminals, one positive and the other negative. The most commonly used cell in the laboratory is the dry cell.

Battery - the combination of two or more cells to produce electric current is called battery. In a battery the individual cells are connected in series or parallel.

How many types of battery? 

There are two types of battery. 

1- battery with cells in series 

2- battery with cells in parallel 

Battery with cells in series

In this kind of battery, the individual cells are connected in such a way that positive terminal of one cell is connected to the negative terminal of the other cell.

Battery with cells in parallel 

In this kind of battery, the individual cells are connected in such a way that their positive terminals are connected to each other and the negative terminals connected to each other with one free positive and one free negative terminal.

Switch 

Switch is a simple device used to turn an electric circuit ON or OFF.

Switch is another important element of the

circuit. It helps us to close or open the circuit. When the switch is in 'ON' position, the current flows through the circuit. The circuit is now said to be closed. On the other hand, when the switch is in 'OFF' position, no current flows through the circuit. The circuit is now said to be open.

A switch does not have terminals and can be connected anywhere in the circuit.

Bulb

An electric bulb is a device that produces light when electric current passes through it.

Inside the bulb there is a very thin wire called a filament. When electricity flows through the filament, it becomes very hot and glows, giving us light.

The bulb has a thin wire, called the filament that glows when current passes through it. If the filament is broken, the bulb will not glow. It is then said to be fused. A fan uses electric current to rotate to blow air.

Connecting wire

It is used to connect various components in an electric circuit. It is made of copper or aluminium coated with insulating material like plastic.

Voltmeter 

Ammeter 

Why are symbols for some electric components used instead of components?

Symbols for electric components are used in a circuit diagram instead of drawing the real components because:

1. They make diagrams simple and clear

Real components are big and complicated to draw. Symbols are small and easy, so the diagram looks neat.

2. They save time and space

Using symbols is much faster than drawing actual bulbs, cells, wires, etc.

3. They are understood all over the world

Electric symbols are standard. Scientists, engineers, teachers, and students everywhere understand the same symbols.

4. They help us understand the circuit easily

Symbols show how components are connected, so we can quickly follow the flow of electric current.

5. They avoid confusion

Real drawings may look different, but symbols are fixed, so there is no misunderstanding.

Some commonly used diagram of components are shown below 

Volt

Volt is the force that pushes electric charges. Volt is the unit that tells us how strong the push of electricity is. 

Volt (V) is the unit of electric potential difference. 

One volt is the potential difference when 1 joule of energy is used to move 1 coulomb of charge.

1 volt = 1 Joule / 1 Coulomb 

📌 Example

A 1.5 V battery gives small push (used in toys)

A 12 V battery gives stronger push (used in vehicles)

220 V is used in homes

Motion and Time - Class 7 Science - New st. Mery English School

This chapter explains different types of motion such as uniform and non-uniform motion, measurement of time, and the use of simple devices like sundials, sand clocks, and water clocks. Students will also learn how to calculate speed, draw distance–time graphs, and observe motion in daily life with simple examples. Perfect for CBSE Class 7 students, teachers, and parents looking for clear and well-structured science notes.

Motion and Time - Class 7 Science - New st. Mery English School

 What is Motion?

Ans:- Motion is the change in position of an object with respect to its surroundings in a given interval of time.

Ex:- A moving car, A flying bird, A rolling ball, A moving train

How many types of motion?

Ans:- There are four types of motion.

1- Rectilinear Motion

2- Circular Motion

3- Periodic Motion

4- Rotational Motion

What is Rectilinear Motion?

Ans:- Motion along a straight line is known as Rectilinear Motion. It is also known as linear motion or Motion in a straight line.

Ex:- A car moving on a straight road, A train moving on a straight track, A laser beam travelling in a straight line, A ball rolling straight on the floor

Key points of Rectilinear Motion

1- In Rectilinear Motion, object does not change its direction.

2- In Rectilinear Motion, The path is straight.

3- In Rectilinear Motion, Speed may be uniform or non-uniform, but the path remains straight.

What is Circular Motion?

Ans:- The motion of an object in a circular path along the circumference of a circle is known as Circular Motion.   

Ex:- The movement of The hands of a clock, A stone tied to a string and swung in a circle, The Earth revolving around the Sun

Key points of Circular motion

1- In circular motion, The object keeps changing its direction, but stays on a circular path.

2- In circular motion, There is always a fixed centre around which the motion takes place.

What is Periodic Motion?

Ans:- A motion that repeats itself after equal intervals of time is known as Periodic Motion.

Ex:- Swinging pendulum of a clock, Heartbeat, Revolution of the Earth around the Sun, Day and night cycle

Key points of Periodic Motion

1- The Periodic motion happens in a pattern.

2- Each cycle of Periodic motion takes the same amount of time, known as the time period.

What is Rotational Motion?

Ans:- When an object spins or turns about a fixed axis, the motion is called rotational motion.

Ex:- The Earth rotating on its axis, Spinning top, Wheels of a bicycle or car

Key points of Rotational Motion

1- In Rotational Motion, The object does not move from one place to another, it spins in its own position.

2- In Rotational Motion, The line about which it rotates is called the axis of rotation.

Slow Motion and Fast Motion

An object that takes longer time to cover a certain distance is said to be exhibit slow motion but An object that takes less time to cover the same distance is said to be exhibit fast motion. 

When an object covers more distance in a short time, it is said to be in fast motion.

Ex:- A car moving at high speed.

        A cheetah running.

        A train moving quickly.

        A fan spinning at high speed.

When an object covers less distance in a long time, it is said to be in slow motion.

Ex:- A tortoise walking.

        A person walking slowly.

       The movement of a snail.

       The hands of a clock (hour hand).

How to Compare Fast and Slow Motion?

We compare  Fast and Slow Motion using speed. 

If two objects cover different distances in the same time

The one covering more distance → faster

The one covering less distance → slower

Ex:- A bicycle covers 10 km in 1 hour and A scooter covers 30 km in 1 hour. Here the scooter is faster than cycle because it covers more distance in the same time.

Speed

The distance travelled by an object in per unit of time is called Speed.

SI unit of distance is Metre. SI unit of time is Second.

So The SI unit of speed is Metre / Second (m / s ).

Formula of Speed:

$$\text{Speed} = \frac{\text{Distance}}{\text{Time}}$$

Example:

If a car travels 100 km in 2 hours. Find its speed.

Solution:- Distance = 100 km

                  Time = 2 hour

                 

$$\text{Speed} = \frac{100}{2} = 50 \text{ km/h}$$Uniform and Non-Uniform Motion

Uniform Motion

When an object covers equal distances in equal intervals of time, the motion is called uniform motion.

Example:- A car moving at constant speed, like 40 km/h.

                A train moving without stopping and without changing speed.

               If a car covers 10 km every 10 minutes, its motion is uniform.

Non-uniform Motion

When an object covers unequal distances in equal intervals of time, the motion is called non-uniform motion.

Example:- A car moving in traffic (sometimes slow, sometimes fast).

                 A child running in a playground, changing speed.

Distance - time graph

Distance - Time graph is a line graph which shows the relation between distance travelled and time taken to travel that distance by a moving body. In the distance - time graph , distance is plotted on the Y-axis and time is plotted on the X-axis.

Steps to draw Distance-time graph

Step - 1

Take a graph sheet and make two mutually perpendicular axis on it. Label the horizontal  as X-axis and the vertical as Y-axis.

Step - 2

Put marking on the X-axis and Y-axis as per the data given in the table with appropriate multiple units. The scale of the graph should be mentioned on the top right side of the graph.

Step - 3

Finally, the points are put on the graph corresponding to data given on the X-axis and Y-axis

Step - 4

All the encircled points are joined together, It results in the formation of distance-time graph.

Precautions while making Distance-Time Graph

Time

Time is the continued progress of events from the past, through the present, and into the future.

Time is the way we measure how long things take or when events happen.

Ancient methods of measuring time

The time interval between one sunrise and the next is a unit of time called day.

The time period from one sunset to sunrise is called night.

Time interval between one moon and the next is called a month.

Sundial

A sundial is a device that tells the time of the day by using the shadow made by the sun.

It has two main parts:

1. Dial Plate – A flat surface with hour markings.

2. Gnomon – A stick or triangular plate that casts a shadow.

When sunlight hits the gnomon, it creates a shadow on the dial plate. The position of this shadow tells the time.

How Does a Sundial Work?

• When the sun rises in the east, the shadow falls to the west.

• At noon (12 PM), the shadow is the shortest.

• In the evening, the shadow becomes long and falls toward the east.

The sundial uses this movement of the shadow to show the time.

Uses of a Sundial

• To tell time during the daytime

• To study the movement of the sun

• Used in ancient times before clocks were invented

Limitations of a Sundial

• It does not work at night

• It cannot work on cloudy or rainy days

• Time is not very accurate compared to clocks

1- The graph should be plotted using a sharp pencil and a ruler.

2- The scale of the graph should be taken appropriately according to the minimum and maximum observations.

Note:-

1- A straight line distance-time graph shows uniform motion.

2- When this graph is not a straight line, than it shows that body is executing non-uniform motion.