# Chapter 16 Light Class 8 Notes

## Light Class 8 Notes

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### Definition of Light

What is Light?

Light is a form of energy that enables us to see objects in our surroundings or Light gives us the sensation of vision.

• Light is an electromagnetic wave.
• Light travels in a straight line.
• It is reflected from all surfaces.

Question: What makes things visible?

The things around us are visible to us because light is reflected from it or it is a source of light. We cannot see things if light is not reflected from an object. The light from the object then enters our eyes and our brain interprets it.

### Important Definitions

#### Incident Ray

What is an Incident Ray?

The incoming light ray from a light source, which strikes or falls on any surface,  is called the incident ray.

#### Reflected Rays

What is a Reflected Ray?

The reflected rays are the rays that bounce after falling on a surface.

#### Normal

What is Normal?

It is an imaginary line drawn perpendicular to the surface between Incident ray and reflected ray.

#### Reflection of Light

What do you understand by Reflection of Light?

When light falls on an object and the light from that object comes back or bounce back, it is called reflection of light.

• Smooth and polished surfaces are good reflectors of light.

#### Angle of Incidence

The angle between the incident ray and the normal is known as the angle of incidence (∠i).

#### Angle of Reflection

The angle between the normal and the reflected ray is known as the angle of reflection (∠r).

### Laws of Reflection

The reflection of light follows two laws. The two laws of Reflection are-

1. The Angle of Incidence (∠i) is always equal to the Angle of Reflection (∠r).

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

### Regular and Diffused Reflection

When light falls on an object or a surface, the light reflects in two ways. So, there are two types of Reflection –

1. Regular Reflection
2. Diffused Reflection

#### Regular Reflection

The types of reflection in which the reflected rays after reflection are parallel to each other.

• This type of reflection takes place when the surface is smooth.

#### Diffused Reflection

In this type of reflection if the incident rays are parallel to each other then after reflection the reflected rays are non-parallel.

• This type of reflection takes place if the reflecting surface is rough or irregular.
• This type of reflection is also called Irregular reflection.

Scattering: process in which air molecules absorb sunlight and reemit it in various directions.

### Multiple Reflection of Light

What do you understand by multiple reflection of light?

Multiple reflection of light means it is the reflection of the reflected light. When the reflected light is again reflected one or more times, it is called multiple reflection of light.

Give some examples of Multiple Reflection of Light?

Two or more plane mirrors used in Barber Shop, Kaleidoscope, Periscope, etc.

### Kaleidoscope

What is a Kaleidoscope?

Kaleidoscope is a device which produces numerous beautiful patterns. I is used by designers to get an idea of different patterns or designs. It uses the principle of multiple reflection.

It consists of 3 mirrors kept at 60º with each other inside a hollow cylindrical box. One end of the cylindrical box is used to view the pattern and the other end is used for allowing the light to get inside the box.

### Structure of Human Eye

Human is roughly spherical in shape. It has three layers: Sclera, Choroid and Retina.

Sclera: It is the tough outer coat that protects the entire eyeball.

Choroid: Middle Layer containing blood vessels or vascular layer of the eye. It nourishes the inner parts of the eye.

Cornea: Outer layer of the eye (clear circular structure in the Sclera). It is White in color and transparent. Cornea refracts the light. It protects the inner part of the eye also.

Iris: colored or dark muscular structure behind cornea, or around the pupil. It determines color of the eye. and controls the amount of light entering the eye.

Pupil: small opening at the center of the iris. It controls the amount of light entering.

Aqueous Humor: viscous liquid between cornea and eye lens.

Lens: light through pupil falls on the lens. It is a convex lens.

Vitreous Humor: jelly like substances between eye lens and the retina. It helps in maintaining shape of the eye.

Retina: Image of the object is formed at retina. Retina is the inner boundary where light converges to form image. Retina contains numerous nerve cells to sense light.

These are of two types Rods (dim light sensitive) and Cones (Bright light sensitive)

Optic Nerves: Message or the sensation is transferred to Brain through optic nerves.

### Accommodation

What is Accommodation of Eye?

• Accommodation is the ability of the of the lens to change its focal length accordingly to form sharp image on retina.
• The minimum Distance is 25 cm

Blind Spot: small region in the retina where the optic nerve enters the eye. it is insensitive to light.

### Eyes Defect

If we do not take care or take proper precautions, we may face problems or defects in our eyes. There are many defects associated with eyes. Let us discuss some defects that people generally face.

#### Myopia or Near Sightedness

• Cannot see distant objects.
• Image is formed in front of the retina
• can be corrected by the use of concave lens.

#### Hypermetropia or Hyperopia or Farsightedness

• Distant objects are clear but near objects are blurry.
• Image is formed behind the retina.
• It is usually due to shortening of eyeballs.
• can be corrected with the help of convex lens.

#### Astigmatism

• Due to the distortion of cornea (slightly elongated in the middle) is termed as Astigmatism.
• In this condition all the light is not focused at one point.
• Objects appears blurry.
• Image is formed behind or in front of Retina.

#### Presbyopia

• condition related to age. Distant objects clear but not near.
• It is similar to Hyperopia, but not due to shortening of eyeball.
• With age power accommodation of lens decreases (loses flexibility)

### Braille System

What is the Braille System?

A system developed by Louis Braille for visually impaired persons so that they can read and write.

• The Braille system has 63 dot patterns or characters and each character represents a letter, a combination of letters, a common word or a grammatical sign.
• Dots are arranged in cells of two vertical rows of three dots each.

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