Light Class 8 Science Notes - Chapter 13 NCERT Solutions

Chapter 13: Light

What Makes Things Visible?

We see objects because of light. We can see an object when light emitted by it or reflected by it enters our eyes. Objects that emit their own light, like the Sun, fire, and a candle, are called luminous objects. Objects that do not emit their own light but are visible because they reflect light are called illuminated objects (e.g., the moon, a table).

Reflection of Light

Reflection is the phenomenon of bouncing back of light after striking a surface. A smooth, shiny surface, like a mirror, reflects light well.

Laws of Reflection:

1. The angle of incidence is always equal to the angle of reflection. (∠i = ∠r)
2. The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.

The image formed by a plane mirror is erect, virtual, of the same size as the object, and is at the same distance behind the mirror as the object is in front of it. It is also laterally inverted (right appears left and left appears right).

Regular and Diffused Reflection

Regular Reflection

When all the parallel rays reflected from a smooth surface are parallel, it is called regular reflection. It forms clear images. Example: Reflection from a plane mirror.

Diffused or Irregular Reflection

When all the parallel rays reflected from a rough or irregular surface are not parallel, it is known as diffused reflection. This type of reflection enables us to see objects that are not luminous. Example: Reflection from a cardboard surface.

Multiple Images: When two plane mirrors are placed at an angle to each other, they can form multiple images. This principle is used in devices like a kaleidoscope to create beautiful patterns.

Sunlight: White or Coloured?

Sunlight, which appears white, is actually composed of seven colours. The splitting of light into its constituent colours is known as dispersion. A rainbow is a natural phenomenon showing dispersion.

What is inside Our Eyes?

The human eye is one of our most important sense organs. Its main parts are:

• Cornea: The transparent front part of the eye that covers the iris and pupil.
• Iris: The coloured part of the eye that controls the amount of light entering the pupil.
• Pupil: The small opening in the center of the iris through which light enters.
• Lens: A convex lens located behind the pupil that focuses light onto the retina.
• Retina: The light-sensitive layer at the back of the eye where images are formed. It contains two types of cells: cones (sensitive to bright light and colours) and rods (sensitive to dim light).
• Optic Nerve: Transmits visual information from the retina to the brain.

The impression of an image persists on the retina for about 1/16th of a second. This is called the persistence of vision.

Care of the Eyes

It is necessary to take proper care of our eyes. Some tips include:

• Read at a normal distance for vision (about 25 cm).
• Avoid reading in very dim or very bright light.
• Do not look at the sun or a powerful light source directly.
• Wash your eyes frequently with clean water.
• Include food rich in Vitamin A (like carrots, broccoli, and green vegetables) in your diet.

Visually Challenged Persons and the Braille System

Some people have limited or no vision. They develop their other senses more sharply and use non-optical aids to read and write.

The Braille system is the most popular resource for visually challenged persons. It was developed by Louis Braille in 1821. Braille uses a system of raised dots that can be read with the fingers. Each character (letter, number, punctuation) is represented by a unique arrangement of six dots in a small rectangular cell.

Frequently Asked Questions (FAQ)

What are the laws of reflection?

The two laws are: (1) The angle of incidence equals the angle of reflection. (2) The incident ray, the reflected ray, and the normal all lie in the same plane.

What is the difference between rods and cones in the retina?

Cones are nerve cells that are sensitive to bright light and allow us to see colour. Rods are sensitive to dim light and are responsible for vision in low-light conditions.

What is the Braille system?

It is a tactile writing system used by people who are visually impaired. It uses patterns of raised dots, representing letters and numbers, which are read by touch.