Class 10 Science Chapter 11 The Human Eye and the Colourful World

📝 Class 10 Science  Chapter 11 The Human Eye and the Colourful World 

The Eye

Construction and working

The eye performs three functions:

1. It makes adjustments to admit the appropriate amount of light.

2. It bends the rays of light to form a sharp image.

3. It collects and sends information about the image to the brain for further processing. 

 Human Eye (Image credit: Foundation Science Physics Class 10)

The different parts of the eye are contained within the spherical eyeball, which is about 2.5 cm in diameter. An opaque, white protective membrane called the sclera covers most of the outer part of the eyeball. this is what we call the 'white of the eye'. A small portion at the front of the eyeball bulges out. A transparent protective membrane called the cornea covers this portion. Behind the cornea, the major structure of the eye are the iris, which controls the amount of light entering the eye. The crystalline lens helps in bending light and the retina on which the image is formed.  As you can see from the diagram, the lens divides the eye into two chambers. The smaller of these two is between the cornea and the lens. It is filled with a watery fluid called the aqueous humour . The larger chamber between the lens and the retina is filled with jelly-like fluid called the vitreous humour. These fluids helps in maintaining the shape of the eyeball. 

Power of Accommodation

The eye lens is composed of a fibrous, jelly-like material. Its curvature can be modified to some extent by the ciliary muscles. The change in the curvature of the eye lens can thus change its focal length. When the muscles are relaxed, the lens becomes thin. Thus, its focal length increases. This enables us to see distant objects clearly. When you are looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens. The eye lens then becomes thicker. Consequently, the focal length of the eye lens decreases. This enables us to see nearby objects clearly.

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

👉 The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision. It is also called the near point of the eye. For a young adult with normal vision, the near point is about 25 cm.

👉 The farthest point up to which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye.

Defects of vision and their correction

Sometimes, the eye may gradually lose its power of accommodation. In such conditions, the person cannot see the objects distinctly and comfortably.

There are mainly three common refractive defects of vision. These are 

(i) myopia or near-sightedness, 

(ii) Hypermetropia or far - sightedness, 

(iii) Presbyopia.

i) Myopia 

Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.
A person with this defect has the far point nearer than infinity. Such a person may see clearly upto a distance of a few metres. In a myopic eye, the image of a distant object is formed in front of the retina [Fig. (b)] and not at the retina itself.

Reasons for defect

This defect may arise due to

 (i) excessive curvature of the eye lens, or

(ii) elongation of the eyeball.

Correction

This defect can be corrected by using a concave lens of suitable power. This is illustrated in Fig. (c). A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected

Refraction of light through a prism 

A triangular glass prism has two triangular bases and three rectangular lateral surfaces. These surfaces are inclined to each other. The angle between its two lateral faces is called the angle of the prism

The figure shows the passage of light through a triangular prism ABC. The angles of incidence and refraction at the first face AB are `i` and `r_1`, while the angle of incidence (from glass to air) at the second face AC is `r_2` and the angle of refraction or emergence `e`. The angle between the emergent ray RS and the direction of the incident ray PQ is called the angle of deviation, `delta`

In the quadrilateral AQNR, two of the angles (at the vertices Q and R) are right angles. Therefore, the sum of the other angles of the quadrilateral is 180°.

`angleA + angleQNR` = 180°

From the `triangleQNR`,

`r_1 + r_2 + angleQNR` = 180°

Comparing these two equations, we get

`r_1 + r_2 = angleA` 

The total deviation `delta` is the sum of deviations at the two faces,

`delta = (i – r_1 ) + (e – r_2 )`

that is,

`delta = i + e  – angleA`

When a ray of light passes through a prism, it bends towards the base of the prism. 

Dispersion of white light by a glass prism

The inclined refracting surfaces of a glass prism show an exciting phenomenon.

In the year 1665, Newton discovered by his experiments with glass prisms that white light consists of a mixture of seven colours. He found that if a beam of white light is passed through a triangular glass prism, the white light splits to form a band of seven colours. The band of the coloured components of a light beam is called its spectrum. The seven colours of the spectrum are Violet, Indigo, Blue, Green, Yellow, Orange, and Red. The acronym VIBGYOR will help you to remember the sequence of colours.  

You have seen that white light is dispersed into its seven-colour components by a prism. The splitting of light into its component colours is called dispersion. The dispersion of white light occurs because colours of white light travel at different speeds through the glass prism. Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. The red light bends the least while the violet the most. The red colour has the maximum speed in glass prism, so the red colour deviates the least. Thus, the rays of each colour emerge along different paths and thus become distinct. It is the band of distinct colours that we see in a spectrum.

Re-combination of Spectrum Colours to Give White Light

Newton placed a second identical prism in an inverted position with respect to the first prism, as shown in Fig. This allowed all the colours of the spectrum to pass through the second prism. He found a beam of white light emerging from the other side of the second prism. This observation gave Newton the idea that the sunlight is made up of seven colours. Any light that gives a spectrum similar to that of sunlight is often referred to as white light.

The Rainbow

A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by the dispersion of sunlight by tiny water droplets, present in the atmosphere. A rainbow is always formed in a direction opposite to that of the Sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection, different colours reach the observer’s eye.

Atmospheric Refraction 

The refraction of light caused by the earth's atmosphere (having air layers of varying optical densities) is called atmospheric refraction. 

You might have observed the apparent random wavering or flickering of objects seen through a turbulent stream of hot air rising above a fire or a radiator. The air just above the fire becomes hotter than the air further up. The hotter air is lighter (less dense) than the cooler air above it, and has a refractive index slightly less than that of the cooler air. Since the physical conditions of the refracting medium (air) are not stationary, the apparent position of the object, as seen through the hot air, fluctuates. This wavering is thus an effect of atmospheric refraction (refraction of light by the earth’s atmosphere) on a small scale in our local environment. 

Twinkling of stars

The twinkling of a star is due to the atmospheric refraction of starlight.

When the light coming from a star enters the earth's atmosphere, it undergoes refraction due to the varying optical densities of air at various altitudes. 

References:

1. NCERT Science Class 10 
2. Physics (S. Chand) Class 10 

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