Chapter 15 Class 7 Light Notes

Class 7 Light Notes

Here is the list of topics we will discuss or study in this chapter 15 Light. These are the best Class 7 Light Notes we have prepared for you.

Best Revision Science Chapter 15 Class 7 Light Notes

Definition of Light

• Light is a form of energy that enables us to see objects around us.
• Light travels from one place to another in a straight line.

What is an Image?

The image of an object can be defined as the impression of the object created by the light on the mirror.

Types of Image

• Real – The images that can be formed on a screen or called real images.
• Virtual – The images that cannot be formed on the screen are called virtual images.

Erect Image: An image is said to be erect if the image is formed on the same side up as that of the object.

Inverted Image: The image will be called Inverted if it is formed upside-down compared to the object.

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Reflection of Light

This phenomenon in which light falling on the surface of the object is sent back by the surface of the object is called the reflection of light.

Incident Ray – Rays that fall on the reflecting surface.

Reflected Rays – rays that get reflected back from the reflecting surface

Types of Reflection

1. Regular Reflection –

• In regular reflection, the reflected rays reflect or bounced back in a particular direction.
• The reflected rays are always parallel to each other.
• Regular Reflection is due to a smooth reflecting surface.

2. Irregular or Diffused Reflection:

• In this type, the reflected rays bounce back in different or irregular directions.
• It is due to the rough reflecting surface.

Plane Mirror

Characteristics of the image formed by a plane mirror:

• The image formed by a plane mirror is always virtual and erect.
• The image undergoes lateral inversion.
• The size of the image formed is the same as that of the object.
• The distance of the object from the mirror is equal to the distance of the image from the mirror.

Spherical Mirrors

• A spherical mirror, as the name suggests, has a sphere-like shape. It appears as if it is part of a sphere.

There are two types of spherical mirrors:

Concave Mirror

• It is a spherical mirror whose reflecting surface is curved inwards.
• It is also known as a converging mirror, as a parallel beam of light after reflection from a concave mirror converges at a point.
• Concave Mirrors form real images:
• The image formed by a concave mirror can be a real image captured on a screen. The image is also inverted.
• Concave Mirrors also form virtual images when a candle is close to the mirror a virtual image is formed.

Uses of Concave Mirrors

• Concave mirrors of large focal lens are used as shaving mirrors.
• Concave mirrors are used as reflectors in headlights of vehicles, searchlights, etc.
• Concave and convex mirrors are used in reflection telescopes.
• Concave mirrors are used in devices such as solar cookers.

Convex Mirror

• It is a spherical mirror whose reflecting surface is curved outwards.
• It is also known as a diverging mirror, as the parallel beam of light after reflection from a convex mirror diverges.
• A convex mirror does not form a real image, it only forms virtual images.

Uses of Convex Mirrors

• Convex mirrors are used in buses to help the driver see the whole bus.
• Convex mirrors are used as rear view Mirrors of vehicles.
• Convex mirrors are used as field view Mirrors in supermarkets as they cover a wider range of vision.

Why convex mirror is used in a rearview mirror?

• forms a highly diminished image size
• gives a wider view of the vehicles behind us.

Lateral Inversion

What is Lateral Inversion?

It is the phenomenon due to which the left side of an object appears to be right in the mirror and the right side appears to be the left side in the mirror.

Why Ambulance is written in reverse order?

An ambulance is an emergency vehicle, so when it appears in the rearview mirror of the vehicle in front of the ambulance, the word “AMBULANCE” appears correctly due to lateral inversion.
The driver would read the letters correctly and hence give way to the Ambulance.

Lenses

• A lens is a transparent material such as glass.
• It has two surfaces out of which at least one is spherical.
• Since a lens has two spherical surfaces, it has two centers of curvature, one on each side of the lens.

Principal Axis – An imaginary straight line passing through the center of the lens and the two centers of curvatures of the lens.

There are two types of lenses

1. Convex lens
2. Concave lens

Convex lenses

A convex lens is curved outward.

Convex lenses are also called converging lenses since they converge a given parallel beam of light passing through them.

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They are thicker in the middle and thinner at the edges.

A convex lens forms a real, inverted, and diminished image. It also forms a virtual image when the object is very near to the lens. The virtual image formed is erect and magnified.

Uses of Convex Lens

• Convex lenses are used as magnifying glasses (Simple Microscope).
• A convex lens is used in spectacles for correcting eye defects (hypermetropia)
• A convex lens is used for cameras and projectors.
• A convex lens is used in Binoculars, telescopes, and microscopes.

Concave Lens

• The concave Lens is curved inward.
• Concave lenses are also called diverging lenses since they diverge a parallel beam of light passing through them.
• They are thinner in the middle and thicker at the edges.
• An image formed by a concave lens is always virtually erect and diminished.

Uses of Concave Lens

• Concave lens is used as a peephole.
• Concave lens is used in spectacles for correcting eye defects (myopia)
• Concave lens is used for cameras and projectors.
• Concave lens is used in Binoculars, telescopes, and microscopes.

Dispersion of light

The phenomenon of white light breaking or splitting into its component colors is called the Dispersion of light.

• When white light passes through a prism it breaks into its component colors.
• This splitting is due to the refraction of light.

Component Colors – VIBGYOR

• R- Red
• O – Orange
• Y – Yellow
• G – Green
• B – Blue
• I – Indigo
• V – Violent

Spectrum of Light

• The band of colors that is obtained is called the spectrum of light.

This spectrum of white light can be seen in the following:

• Rainbows
• Soap bubbles
• The surface of a CD
• Prisms

How a Rainbow is formed?

The rainbow is a natural phenomenon that is formed due to the dispersion of sunlight.

The small water droplets present in the air after rain causes the white light to split into its component colors.

The different colors thus formed appear in the form of a large curve in the sky called a rainbow.

Newton Disc

• A circular disc containing 7 colors.
• It is also called a disappearing color disc.
• It is used to demonstrate that white light consists of 7 colors (rainbow colors).

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