NCERT Notes for Class 10 Science Chapter 11 The Human Eye and the Colourful World
CBSE Class 10 Science notes will assist students in studying the topic thoroughly and clearly.
These CBSE Class 10 Science notes were written by subject experts who made the study material very basic, both in terms of language and format.
The Human Eye
One of the most valuable and sensitive sense organs in the human body is the eye. It allows us to see the beautiful environment and colours that surround us.
Structure of Human Eye
The human eye has the following main parts:
Corner – It’s the translucent spherical membrane that covers the eye’s front surface. This membrane allows light to enter the eye. The outer surface of the cornea is where the majority of light rays entering the eye are refracted.
Crystalline Lens – A convex lens, similar to a jelly formed of proteins, is made of a transparent, soft, and flexible material.
Iris – Between the cornea and the lens is a black muscular diaphragm. The pupil size is controlled by it. We refer to the colour of the eye as the iris.
Pupil – It is a tiny opening between the iris and the pupil of the eye via which light enters the eye. It expands completely in dim light due to iris muscle expansion, but contracts dramatically in strong light due to iris muscle contraction.
Ciliary Muscles – They hold the lens in place and aid in the modification of the lens’ curvature.
Retino – The image is created on the light-sensitive surface of the eyeball. It is made up of rods and cones, which are light-sensitive cells. Cone cells respond to lighting while rod cells respond to light intensity. That is, fundamental colours. Rod cells outnumber cone cells by a factor of two. Signals are generated by these cells and conveyed to the brain via optic nerves.
Optic nerve – It is responsible for transmitting visual information from the retina to the brain.
Sclero – It is the outer layer of an eye that is opaque, fibrous, and protective, and contains collagen and elastic fibre. It’s also referred to as “eye white.”
Blind Spot – The point at which the optic nerve leaves the eye is known as lt. Because it lacks rods and cones, any image created here is not conveyed to the brain.
Aqueous Humor – We have a space between the cornea and the eye lens that is filled with a translucent liquid called aqueous humour, which helps refracted light focus on the retina. It also keeps intraocular pressure in check.
Vitreous Humor – Another liquid termed vitreous humour fills the gap between the eye lens and the retina.
Formation of an Image
Refractions at the cornea, aqueous humour, lens, and vitreous humour combine to generate an image on the retina. It has been re(diminished and reintroduced into nature.
When the retina is illuminated, the light-sensitive cells become active and emit electrical signals. The optic nerve subsequently transmits these impulses to the brain. The brain decodes these signals and then processes the data, allowing us to see things as they are.
Colour of Objects
The retina’s rod-shaped cells respond to light intensity, or the degree of brightness or blackness, but not to colours. Primary colours like red, blue, and green have varied degrees of sensitivity in cone cells.
Terms Related to Human Eye
- Accommodation : It is the ability or property of the eye lens to alter its focal length to focus on both close and far things. However, if the item is held too close (i.e. less than 25 cm) or too far from the eye, the focal length cannot be decreased or increased beyond a certain point, beyond which a healthy person cannot see clearly.The ciliary muscles aid in the curvature of the eye lens being changed. When muscles relax, the lens thins out and the focus length lengthens. This allows us to clearly perceive distant objects. The ciliary muscles contract when observing adjacent objects. As a result, the lens thickens and the focus length shortens.
- Accommodations power : It is the maximum fluctuation in eye lens strength for clearly focusing close or far objects at the retina. The accommodation power of a young adult with normal eyesight is roughly 4D. With ageing, the eye’s ability to accommodate changes diminishes.
- The eye’s farthest point : It’s the highest place that the eye can see plainly. For a normal eye, it is infinite.
- The eye’s near point : The least distance of distinct vision is the shortest distance at which an object may be seen clearly without exertion. It is 25 cm for an adult’s normal eye. It’s also known as the “near point of the eye.”
- Vision Persistence : Persistence of vision refers to the length of time that an object’s impression or experience remains in the eye. It is approximately 1 / 16th of a second, which means that the minimum period for which we should view an object in order to generate a clear image on the retina is 1 / 16th of a second.
Defects of Vision and their Correction
Vision defects are those that cause a person to be unable to perceive an object clearly and comfortably.
The following are the most common visual problems:
- Myopia, also known as nearsightedness or nearsightedness, is a condition in which a person’s vision is blurred.
- Hypermetropia (far/long vision)
Myopia or Near /Short Sightedness
This deficiency causes a person to be able to see adjacent objects clearly but not distant objects. In this situation, the image is formed before the retina rather than on it.
Causes : A person with this flaw has a distant point that is closer to infinity than infinity. This flaw comes as a result of the lens’s focal length decreasing as a result of
- elongation of the eyeball,
- abnormal curvature of the eye lens
As a result, the image is produced prior to the formation of the retina.
Remedy Concave lenses can be used to rectify this flaw. A concave lens of appropriate power will restore the image on the retina.
Hypermetropia or Far /Long Sightedness
A person with this impairment can clearly see distant objects but not nearby objects. The near point of a person with this condition is further away from the typical near point (25 cm). The image is created outside of the retina in this scenario.
Causes: The following factors contribute to this flaw:
The focal length of the ocular lens increases.
- Because the eyeball becomes too short,
- The picture is created behind the retina.
Remedy A convex lens of appropriate power can be used to fix this flaw. This will restore the image to the retina.
It is found in the elderly. With ageing, the near point for most people gradually fades away. A person can have both myopia and hypermetropia at the same time.
Causes: The following factors contribute to this flaw:
- Ciliary muscle weakness, hardening or lack of flexibility of the eye lens
Remedy Bifocal or varifocal lenses, which have both convex and concave lenses, can be used to remedy this deficiency. A concave lens (for myopia) is in the upper portion, and a convex lens is in the lower portion (for hypermetropia)
There are a variety of numerical difficulties based on myopia and hypermetropia that include determining the focal length and power of the lens that will be utilised to rectify the defect. The lens formula is used to tackle these issues.
1/f = 1/v – 1/u
where f is the lens’ focal length (if f is positive, the lens is convex; if it is negative, the lens is concave.)
v = image distance and u = object distance
Always remember that the far point for a healthy human eye is infinity, and the near point is the shortest distance of distinct vision, which is 25 cm.
Refraction of Light Through a Prism
A prism is a transparent refracting material with at least two lateral surfaces that are at an angle to each other. It has three rectangular lateral surfaces and two triangular bases. The angle of the prism is the angle formed by two lateral surfaces (A).
A ray of light PQ enters from the air through the first surface AB in the diagram above. On refraction, the light ray bends away from the normal. The light ray penetrates from glass and air on the second surface AC, so it bends away from the normal. The diagram above depicts refraction through a prism.
PQ = incident ray, MR = emergent ray, QM= refracted ray,
i= angle of incidence, r= angle of refraction, e = angle of emergence, D = angle of deviation. A = angle of prism
Angle of Deviation (D)
It’s the angle formed by the incident ray and the emerging ray (stretched rearward) ( extended forward). It is given by LD =Li+ Le – LA and is dependent on the prism angle, i.e. (LA), angle of incidence, and angle of emergence.
Dispersion of White Light by a Glass Prism
When white light passes through a prism, it splits into its constituent colours, which is known as dispersion.
This band of seven colours obtained, the VIBGYOR (V = violet, I= indigo, B = blue, G = green, Y = yellow, 0 = orange and R = red) is called spectrum.
The first person to utilise a glass prism to obtain the spectrum of light was Isaac Newton.
Cause Of Dispersion
In vacuum and air, light beams of different colours move at the same speed, but in any other medium, they travel at different speeds and bend at different angles, resulting in light dispersion.
The wavelength of red light is the longest, while the wavelength of violet light is the shortest. As a result, red light travels the fastest and deviates the least in any medium, while violet light travels the slowest and deviates the most. I.e
Recombination of White Light
Newton demonstrated that the inverse of light dispersion is also feasible.
He kept two prisms in close proximity to each other, one upright and the other inverted. When light travels through the first prism, it disperses.
The second prism collects all seven coloured photons from the first prism and recombines them into white light.
This discovery demonstrates that sunlight has seven distinct colours. White light is any light that has a spectrum similar to that of sunlight.
- After a rain shower, a rainbow is a natural spectrum that appears in the sky.
- It’s created by sunlight being dispersed by microscopic water droplets in the atmosphere.
- A rainbow always appears to form in the opposite direction of the Sun. Water droplets function as tiny prisms.
- They refract and disperse incident sunlight, then internally reflect it, and ultimately refract it as it exits the raindrop.
- Different colours reach the observer’s eye due to light dispersion and internal reflection.
- On a sunny day, you can view a rainbow by staring at the sky through a waterfall or a water fountain with the Sun behind you.
The density of the Earth’s atmosphere changes as we move up or below; it is not homogeneous throughout. It can be thought of as a collection of layers of varying densities that serve as a rarer or denser medium in relation to one another.
As a result, light rays undergo refraction as they pass through the earth’s atmosphere. Atmospheric refraction is the refraction of light caused by these layers.
Some Phenomena Based on Atmospheric Refraction
- Twinkling of Stars
Starlight is refracted by the atmosphere, which causes it to twinkle. The light from the star refracts as it enters the earth’s atmosphere due to different optical densities of air at different altitudes.
The light is refracted differently by the constantly shifting atmosphere. In this fashion, the starlight reaching our eyes alternates between increasing and decreasing, giving the appearance of a twinkling star at night.
- The Stars Seem Higher than They Actually Are
Due to atmospheric refraction, light from a star enters the Earth’s atmosphere and undergoes refraction, bending towards the normal each time. As a result, the star’s apparent position differs slightly from its true position.
When viewed near the horizon, the star appears to be slightly higher than its real position.
- Planets do not Twinkle
Planets are larger than stars and are much closer to Earth, so they can be thought of as a collection of many little point sources of light. The total fluctuation in the amount of light entering our eye from all of these small point sources will average out to zero, thereby cancelling out the sparkling effect. Planets, as a result, do not sparkle.
- Advance Sunrise and Delayed Sunset
About two minutes before sunrise and two minutes after dusk, the Sun is visible to us. Atmospheric refraction is to blame for this.
When the Sun is somewhat below the horizon, sunlight is refracted downwards as it travels from less dense to more dense air. As a result, the Sun appears to have risen above the horizon and can be seen two minutes before sunrise.
Similarly, even after the Sun has set below the horizon, the Sun can be visible for around two minutes due to atmospheric refraction.
Scattering of Light
Scattering of light is the reflection of light from an object in aJI directions. The wavelength of light and the size of scattering particles determine the colour of scattered light.
Fine particles scatter mostly blue light, whereas larger particles scatter light with greater wavelengths (red light). The dispersed light may seem white if the scattering particles are large enough in size.
Some Phenomena Based on Scattering of Light
- Tyndall Effect
Light does not disperse when it passes through a genuine solution. The Tyndall effect is the dispersion of light as it passes through a colloidal solution.
The earth’s atmosphere contains a heterogeneous collection of minute smoke particles, microscopic water droplets, suspended dust particles, and air molecules that are visible due to light scattering.
- Why the Colour of Sky is Blue?
The sky appears blue during the day. Because the size of the particles in the atmosphere is less than the wavelength of visible light, the light of shorter wavelengths is scattered.
Our eyes are drawn to the scattered blue light. It should be mentioned that passengers travelling at higher altitudes see the sky as black since light scattering is less noticeable at such altitudes due to the lack of particles.
- Colour of Sun at Sunrise and Sunset
The Sun and the sky seem red during sunrise and sunset. Before reaching our eyes, light from the Sun towards the horizon goes through thick layers of air and travels a greater distance in the atmosphere.
The particles scatter much of the blue light and shorter wavelengths toward the horizon. As a result, the wavelengths of light that reach our eyes are longer. The Sun and the sky take on a crimson hue as a result of this.
The light from the Sun overhead, on the other hand, would go a shorter distance at midday. As a result, it seems white since only a few flecks of blue and violet are visible.