CBSE Class 10 Science Chapter 11 The Human Eye & the Colorful World

Class Notes of Ch 11 : The Human Eye & the Colorful World

Class 10th Science

 The Human Eye & the Colorful World  

      Topics:

•  Introduction
• The Human Eye
• Power of Accommodation
• Defects of Vision and their Correction
• Refraction through a triangular prism
• Dispersion of White Light by a Glass Prism
• Formation of Rainbow
• Atmospheric Refraction
• Tyndall Effect

Introduction

We see many beautiful things around us - rainbow, colourful butterfly, forest, beautiful flowers and so on. What enables us to see these things? Eyes and Light. No light, no sight. In this chapter we will study about our eyes and some interesting properties of light.

The Human Eye

Let's understand how the human eye helps us in seeing objects. The eye is roughly spherical in shape.

• Cornea: Light mainly falls on the cornea, the white clear front part of the eye which is slightly bulging forward. It is tough and protects the interior of the eye from accidents.
• Iris: The iris(a muscular structure behind the cornea) of the eye controls the amount of light reaching the interior of the eye by automatically adjusting the size of the pupil. The iris is that part of the eye which gives it the color such as black, brown, green eyes commonly called cat-eyes etc
• Pupil: The pupil regulates and controls the amount of light entering the eye.
• Lens: The eye's crystalline biconvex shaped lens is located behind the pupil. It further focuses the light on the retina.
• Ciliary Muscles: The lens is contracted and relaxed by the ciliary muscles. The focal length is varied by this movement.
• Retina: The lens forms an inverted real image of the object on the retina. The retina contains several nerve cells which convert the optical images to signals to the brain via the optic nerve.
• Nerve Cells: There are 2 types of nerve cells. (i) Cones : which are sensitive to bright light and help us recognize color (ii) Rods: which are sensitive to dim light.
• Brain: The brain again inverts the image, decodes this signal and then we see the image.

Note : The light is focused by the cornea, then the iris by controlling the opening of the pupil and then the lens. In dim light, your pupil expands to allow more light to enter your eye. In bright light, it contracts.

Power of Accomodation

• The ability of the human eye to adjust itself such that it can see objects which are nearby and faraway is called Accommodation.
• It primarily involves the lens and ciliary muscles.
• The ciliary muscles are capable of modifying the curvature of the lens and thereby affecting the focal length of the lens.
• When viewing objects which are nearby, the ciliary muscles contract, the lens becomes thicker. It is squeezed into a more convex shape and the focal length decreases (curvature increases). The images again falls on the retina. Note that here the eye is 'accommodating' by taking a thicker shape so that the image falls on the retina.

• When viewing objects which are far away, the ciliary muscles expand, the lens becomes thinner. It is relaxed/loosened into a less convex shape and the focal length increases (curvature decreases). The images again falls on the retina. Note that here the eye is 'accommodating' by taking a thinner shape so that the image falls on the retina.
• However, the focal length of the eye cannot be reduced below a certain minimum limit. If you read a book very close to your eye, it will appear blurred. The minimum distance, at which objects can be seen most distinctly without strain is called the 'Least Distance of Distinct Vision' or the 'Near point of the eye'. For a young adult with normal vision, the near point is about 25cm.
• The farthest point upto which the eye can see objects clearly is called the 'Far point of the eye'. For a normal eye, it is infinity.

Defects of  Vision and their Correction

Three common refractive defects of the eye:

• Myopia or Near-sightedness
• Hypermetropia or Farsightedness
• Presbyopia

Myopia

• Near-sightedness : A person with Myopia can see nearby objects clearly
• A person with myopia cannot see faraway objects clearly.
• The far point for the myopic eye is nearer than infinity
• Occurs due to
  • Excessive curvature of the eye lens
  • Elongation of eyeball
• The image of a distance object is formed in front of the retina and not on the retina
• Defected is corrected by using Concave lenses such that the lens will bring the image back on to the retina.

 Hypermetropia

• Far sightedness : A person with Hypermetropia can see faraway objects clearly.
• A person with Hypermetropia cannot see nearby objects clearly.
• The near point of the eye is more than 25cm
  • This arises mostly during latter stages in life, as a result of the weakening of the ciliary muscles and/or the decreased flexibility of the lens.
  • Focal length of the eye lens is too long
  • Eyeball has become too small. 
• The image of a distance object is formed in behind the retina and not on the retina
• Defected is corrected by using Convex lenses such that the lens will bring the image back on to the retina.

Presbyopia

• The power of accommodation of the eye usually decreases with ageing. The ciliary muscles weaken and thereby the flexibility of the eye lens reduces.
• The near point moves away.
• Spectacles with convex lenses are recommended

Myopia and Hypermetropia

• Sometimes a person may suffer from both near sightedness and far-sightedness.
• Such people are advised to use bifocal lenses.
• Bifocal lenses consists of concave on the upper portion and convex on the lower portion.
• Concave supports distinct vision and convex supports near vision

Refractive eye defects can also be corrected using contact lenses or through specific surgical procedures.

Refraction through a Triangular Prism

Prism

• The above figure shows a triangular prism with 2 triangular bases and 3 lateral rectangular surfaces.
• The surfaces are inclined at an angle of 60 degrees to each other.
• This is called the 'Angle of the prism'

Refraction

• When a ray of light moves from one surface to another it bends. This is called Refraction
• The ray of light from air is incident on the glass surface AB. On hitting the surface AB, it bends towards the normal.
• At the surface AC, the light ray moves from glass to air. It again bends.
• T he angle of incidence (i), the angle of refraction (r) and the angle of emergence (e) is as shown in the figure.
• PQ is the incident ray
• QR is the refracted ray
• RS is the emergent ray.

Dispersion of White Light by a Glass Prism

• Sunlight or White Light is made of 7 colors as in VIBGYOR(Violet, Indigo, Blue, Green, Yellow, Orange, Red). When white light is passed through a prism, we can see it split into the components that it is made of. This is called the Spectrum.
• Violet bends the most and Red bends the least
• Newton first discovered this and then he passed this spectrum of light through an another prism as shown and he observed that again white light emerged. This proves that white light is made of VIBGYOR colors.

Formation of Rainbow

• A rainbow is a natural spectrum that usually appears after a rainfall, always in a direction to that of the sun.
• The sunlight gets dispersed by tiny droplets of water in the atmosphere, which act like small prisms.

Sometimes, when you look at the sun through a water fall or a fountain (even on a sunny day), you see the rainbow. Here again the water droplets refract sunlight.

Atmospheric Refraction

Twinkling of Stars

• The twinkling of stars is also due to atmospheric refraction.
• The air in the earth's atmosphere is such that it has increasing refractive index
• When starlight enters earth's atmosphere it undergoes multiple refraction continuously before it reaches the earth
• Because of the increasing refractive index, the starlight bends downwards (ie towards the normal) and thereby the apparent position of the star is slightly higher than its actual position.
• Also, this apparent position of the star is not fixed. It keeps on changing slightly since the atmospheric conditions of the earth keep varying
• But however the stars are so distant that they appear fixed to the naked eye.

Early Sunset and Delayed Sunrise

• Atmospheric Refraction is the same reason that the sun is seen about 2 minutes before actual sunrise and is seen for about 2 minutes after sunset.
• The actual position of the sun is slightly shifted from the actual position of the sun

Tyndall Effect

• The earth's atmosphere is a mixture of many minute particles such as smoke, water droplets, dust etc. When a beam of light strikes these fine particles, the path taken by that beam becomes visible. Light gets reflected continuously by these particles and then reaches us.
• This phenomenon of scattering of light by particles is the Tyndall effect.
• Tyndall effects explain why we see the sky as blue, forests as green and so on.

Why  does the Sky appear clear blue?

• The air molecules and other fine particles have sizes smaller than the wavelength of visible light.
• Such particles scatter light of shorter wavelengths ie blue more effectively than longer wavelengths( red)
• When sunlight passes through the atmosphere, the blue end of the spectrum is scattered more.
• This enters our eyes and hence we see the sky as blue.

SCIENCE Revision Notes

Chapter:01  Chemical Reaction & Equation
Chapter:02  Acid Base & Salt
Chapter:03  Metals & Non Metals
Chapter:04  Carbon & its Components
Chapter:05  Periodic Classification of Elements
Chapter:06  Life Processes
Chapter:07  Control & Coordinates
Chapter:08  How do Organisms Reproduce
Chapter:09  Heridity & Evolution
Chapter:10  Light Reflection Refraction
Chapter:11  The Human Eye & the Colourful World
Chapter:12  Electricity
Chapter:13  Magnetic Effect of Electric Current
Chapter:14  Source Of Energy
Chapter:15  Our Environment
Chapter:16  Management of Natural Resource

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