CONTENTS

The book covers-

1)The Human Eyes

2)Dispersion of white light through a glass prism

3)Applications of dispersion of light

4)Atmospheric Refraction and its application

5)Scattering of light and its application

6)Recapitulation

THE HUMAN EYE

The human eye is the most fascinating and complicated part of the human body. It is the most valuable and sensitive organ of our body, that enables us to see this beautiful world. Hence the eye is the most significant of all the sense organs. There are so many tiny parts that work synchronously to enable us to see this colorful world around us. It works like a camera. Isn't that interesting. In this book we shall explore how the human eye works and some amazing phenomena associated with light.

PARTS OF THE HUMAN EYE

The eye consists of-

1.LENS

2.CORNEA

3.RETINA

4.IRIS

5.PUPIL

6.OPTIC NERVE

WORKING OF THE EYE

The light enters the eye through a transparent, thin membrane on the front of the eye called the CORNEA. Most of the refraction for the light rays takes place at the outer surface of the cornea.

The crystalline LENS helps to adjust the focal length required to focus the objects on the retina.

The IRIS at the back of the cornea is a dark muscular diaphragm that controls the size of the pupil.

The PUPIL regulates and controls the amount of light entering our eyes.

The lens forms an inverted, real image on the RETINA, which is a delicate membrane having enormous number of light-sensitive cells. When upon illumination gets activated and send an electric signal through the OPTIC NERVE to the brain.

 The brain interprets these signals and we perceive the object.

FUN-FACT

When we enter from bright lightroom to a dimly lighted room we find it hard to see the object for some time. This is because the pupil is an aperture whose size is controlled by the iris. In bright light, the iris reduces the size of the pupil to allow less light to enter the eye and hence constricts, whereas in dim light the iris increases the size of the pupil to allow more light to enter the eye and hence our eyes dilate.

INTERESTING FACT-POWER OF ACCOMMODATION

The ability of the eye lens to adjust its focal length is called accommodation.

The eye lens being jelly-like can change its curvature to some extent with the help of ciliary muscles.

When the muscles are relaxed the lens becomes thin, its focal length increases and it enables us to see distant objects clearly.

When the muscles contract, the lens becomes thicker, and the focal length decreases which enables us to see nearby objects clearly.

INTERESTING TO KNOW....

The focal length of the lens cannot be decreased below a certain limit. We can't read printed material when it is held too close to our eye. We must hold the material at about 25 cm to read it comfortably. The minimum distance at which objects can be seen distinctly without strain is called the least distance of distinct vision. It is also called the near point of the eye and it is at 25 cm for humans.

The farthest point up to which our eyes can see is called the far point of our eyes and it is infinity for a normal eye.

Why do we have two eyes instead of one?

There are 4 main reasons for having two eyes instead of one-

1. They increase our field of vision to 180 degrees.

2. They increase the ability to detect faint objects.

3. It enables us to see 3-D objects.

4. Each eye sees a slightly different image, both combine into one in our brains, giving us extra information about how close or far  away things are.

DEFECT OF THE EYES AND THEIR CORRECTION

MYOPIA OR NEARSIGHTEDNESS-

Symptoms- The person can see near objects distinctly but has trouble seeing far away objects. He can see clear up to 2 meters only as the image is formed at the front of the retina and not at the retina itself.

Causes-Excessive curvature of the lens and elongation of the eyeball.

Correction-Can be corrected using a concave lens of suitable power.

HYPERMETROPIA OF FAR-SIGHTEDNESS

Symptoms-Can see distant objects clearly but has trouble seeing near objects clearly. He has to keep the object at a distance less than 25 cm. The light rays are focused behind the retina.

Causes-Focal length of the eye lens is too long and the eyeball has become too small.

Correction-Can be corrected using a convex lens of appropriate power.

PRESBYOPIA

The power of accommodation usually decreases with age. For most people, the near point recedes away. It becomes difficult to see near objects clearly and corrective glasses are required. It happens due to the weakening of the ciliary muscles and reduced flexibility of the eyes.

ASTIGMATISM-

It is a defect in which we cannot see simultaneously horizontal and vertical lines with both eyes. It can be corrected using cylindrical lenses.

CATARACT-

Due to the growth of a membrane over the eye lens, the lens becomes hazy or even opaque leading to reduced vision of even loss of vision. It can be corrected only with surgery.

REFRACTION OF LIGHT THROUGH A PRISM

Refraction is the change in direction of light as it travels from one medium to another.

Let us recall that when light travels through a rectangular glass slab it would come out of the slab parallel to the incident light ray, but laterally displaced.

Unlike a glass slab, the surfaces of a prism are inclined at an angle called the angle of prism.

DISPERSION OF WHITE LIGHT BY A PRISM

A prism has 2 triangular surfaces and 3 rectangular surfaces inclined at an angle with each other.

We all know that white light is made up of 7 different colours namely violet, indigo, blue, green, yellow, orange, red. But have you wondered what will happen to this white light if it passes through a prism?

Isacc Newton was the first to obtain a spectrum of sunlight when it passes through a prism. This splitting of white light into its constituent colours is called DISPERSION.

Observe that white light has split into 7 colors (summarized by the word VIBGYOR). The first color is red and the last is violet. This happens because each colour bends at a different angle w.r.t. the angle of incidence on entering the glass prism and hence emerge out at different paths and become distinct. The red colour bends the least and the blue/violet light bends the most. This band of colours is called spectrum of light.

What do think will happen if we keep another inverted prism near the first prism?

Newton was the first to split sunlight into different colours, he used a second prism to split light further but this did not work. He then placed an inverted prism next to the first prism and observed that white light emerged out.

REMEMBER- A prism followed by an inverted prism acts like a rectangular glass slab.

DO YOU KNOW HOW RAINBOWS ARE FORMED?

When it rain, little droplets are suspended in the air. These tiny droplets act like little prisms, they disperse the sunlight that falls on them. The incident light gets refracted inside the drop, then undergoes internal reflection and refracts a second time before emerging out of the droplet. Due to the internal reflection and dispersion of light, the different colours reach the observer's eyes.

Have you observed objects wavering through hot air above a fire?

The air above fire gets hotter and lighter, this reduces the refractive index of the hot air lower than cooler air. Due to changing conditions of the refractive medium (air), the position of the object as seen through the hot air fluctuate. This is called atmospheric refraction.

MORE EXAMPLES OF ATMOSPHERIC REFRACTION.

Have you wondered why stars twinkle?

Observe the image on the previous page. The starlight entering the earth's atmosphere undergoes refraction several times before reaching our eyes. This is due to changing refractive index of the atmosphere. The atmosphere bends the light towards the normal and hence the apparent position of the star is different from the actual position.

Further, this apparent position is not stationary but keeps changing as the physical conditions of the refractive medium keep changing.