DAV Class 8 Science Solution Chapter 11 The Human Eye
DAV School Books Class 8 Science Solution Chapter 11 The Human Eye all Question Answer. DAV Class 8 11th Chapter The Human Eye full Chapter explanation by expert teacher.
DAV School Books Class 8 Science Solution Chapter 11 The Human Eye
A.) Fill in the blanks.
1.) The ___________ of the human eye determines the colour of a person’s eye.
2.) The light sensitive screen of the human eye is called ___________.
3.) The cone-shaped cells, on the retina, respond to the ___________.
4.) The defect of vision, in which eyeball becomes smaller than its normal size, is called ___________.
5.) A ___________ lens is used to correct myopia and a ___________ lens is used to correct hypermetropia.
Answer: (1) iris (2) retina (3) bright light (4) hypermetropia (5) concave, convex
B.) Write True or False for the following statements.
1.) Iris is the muscular assembly that controls the size of the pupil.
2.) The eye lens forms a virtual, erect and diminished image of an object, on the retina.
3.) The retina of the human eye has a large number of cone-shaped and rod-shaped cells. The rods can respond even in dim light.
4.) Cinematography makes use of the persistence of vision of the human eye.
5.) For a normal human eye, the range of vision is from (nearly) 25 cm to infinity.
6.) Myopia is caused due to excessive curvature of the cornea.
Answer: (1) True (2) False, the eye len forms an inverted image (3) True (4) True (5) True (6) True
C.) Tick the correct option.
1.) The change in focal length of an eye lens, to focus the image of objects at varying distances, is brought about by the action of-
Answer: ciliary muscles
2.) The impression of the image formed on the retina of the human eye remains there for about-
Answer: 1/16th second
3.) The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This ability of the eye is known as-
Answer: power of accommodation
4.) A near sighted person will face difficulty while reading-
Answer: a street signboard across the street.
5.) The defect of the human eye, in which the eye lens of a person gets progressively cloudy, resulting in blurred vision, is called-
Answer: cataract
6.) The Braille system, for visually challenged persons, is a-
Answer: tactual aid
D.) Answer the following questions in brief.
1.) Name the part of the human eye which acts as a protective layer for the eye.
Answer: Cornea
2.) The pupil appears black. Why?
Answer: When the pupil is dilated, light from the surroundings cannot enter the rear of the eye and reflect off the retina, which causes the pupil to appear black. The pupil appears black as a result.
3.) Differentiate between:
(a) rod-shaped and cone-shaped cells
Answer: Compared to cone-shaped cells, rod-shaped cells are longer and narrower. Cone-shaped cells have such a round triangular shape, but rod-shaped cells often have a cylindrical shape. Additionally, rod-shaped cells are frequently bacteria, whereas cone-shaped cells are frequently eukaryotic.
(b) near point and far point of human eye
Answer: Nearsightedness, or myopia, causes the ability to see clearly up close but blurry at a distance.
Farsightedness, or hypermetropia, is the condition in which nearby things are indistinct but distant ones are plainly visible.
Near point of human eye |
Far point of human eye |
The near point of the eye is the minimum distance an object must be from the eye before it can be clearly seen without any stress | The far point of the eye is the greatest distance to which the eye can see items plainly. |
This distance of the near point is 25 cm in case of a normal person’s eye. | In a normal person’s eye, the far point is infinity. |
4.) State the difference between:
a) myopia and hypermetropia
Answer:
(b) tactual aids and auditory aids
Answer: Tactual aids are tools that use touch to help persons with vision impairment learn and comprehend. Braille, big print, raised charts, textured images, and tactile books are a few examples of tactile assistance.
With the aid of auditory aids, people with hearing loss can learn and comprehend by using their hearing. Cochlear implants, FM system, as well as assistive listening aids are a few examples of auditory aids.
5.) Define the terms:
(a) Blind spot on the retina
Answer: There is a region of the retina known as a “blind spot” that lacks light-sensitive cells. The blind spot most people have experienced when moving their eyes is caused by this region in the back of the eye. Where the optic nerve exits the eye and sends visual data to the brain is known as the blind spot.
(b) Persistence of vision of human eye
Answer: The ability of the human eye to keep an afterimage for a little amount of time after the initial image has vanished is known as human visual persistence of vision. The impression of motion in movies as well as animation is based on this phenomena.
6.) Give the meaning of the terms:
(a) Power of accommodation of the human eye
Answer: The capacity of the eye to change its focus in order to see objects at various distances is known as accommodation power in humans. This is made possible by the ciliary muscles of the eye altering the lens’s shape to direct light onto the retina.
(b) Least distance of distinct vision
Answer:The closest range that a human could see clearly and recognise two points as different objects is known as the least distance of separate vision.
(c) Range of vision
Answer: The ability to detect or interpret things within a specific distance or area is known as range of vision. It is the furthest distance or region that a person’s unaided eye can perceive.
7.) What happens to the image distance in the eye when the distance of the object from the eye increases?
Answer: As the distance of the object from the eye increases, the image distance in the eye decreases. This is because the eye’s lens is focusing the light rays from the object onto the retina, and as the object gets farther away, the light rays spread out and the image appears smaller on the retina.
8.) When Sahil tried to read a book by keeping it at a distance of 25 cm, the text print appeared to be blurred.
(a) Is the image of the text getting formed before the retina, or beyond the retina ofhis eye?
Answer: Beyond the retina.
(b) What conclusion can be drawn about the least distance of distinct vision’ of Sahil’s eye?
Answer: Sahil’s least distinct vision distance has gotten longer due to his near sightedness.
9.) A person, with a myopic eye, cannot see objects beyond 1.2 m distinctly. What can be said about the likely size of his eyeball? State the nature of the corrective lens that is needed to restore his proper vision?
Answer: The size of the myopic eye is likely smaller than a normal eye as the eye has difficulty focusing on objects beyond 1.2 m. The corrective lens that is needed to restore proper vision is a concave lens, which diverges light and focuses it to the back of the eye.
E.) Answer the following questions.
1.) Draw a labelled sketch of the human eye. Explain the function of the following parts of the human eye: (a) cornea (b) iris (c) pupil (d) eye lens (e) ciliary muscles (f) retina (g) optic nerves.
Answer:
(A) Cornea: The cornea, a transparent, curving layer surrounding the outside of the eye, aids in optimal focus by refracting light entering the eye.
(b) Iris: The iris, a ring of delicate tissue surrounding the pupil that regulates the quantity of light entering the eye, is in charge of doing this.
(c) Pupil: The iris’s central black opening, known as the pupil, allows light to penetrate the eye.
(d) eye lens: The translucent eye lens, which is situated behind the iris, aids in sharpening the light that enters the eye.
e) Ciliary Muscles: A collection of muscles known as the ciliary muscles surround the eye lens and work to change the shape of the lenses depending on the object’s distance.
f)Retina: The tissue that covers the back of the eye, called the retina, is light-sensitive. It transforms light into brain-transmitting electrical signals.
(g):Optic nerves: The eye and the brain are connected by two sizable nerves called the optic nerves, which are found at the rear of the eye. They are in charge of communicating visual data to the brain.
2.) Explain how we perceive objects as they are.
Answer:Humans use their five senses to perceive things as they actually are. Humans can observe an object’s form, size, colour, and texture through their sense of sight. Humans may detect an object’s sound through hearing. Humans are able to identify an object’s odour through smell. Humans may taste a thing to feel its flavour. Humans are able to sense an object’s texture, temperature, as well as texture through touch. Humans also frequently blend different senses to perceive an object more thoroughly, such as the taste and aroma of food.
3.) State any five precautions that are needed to take care of our eyes.
Answer: The five precautions are as follows:
- When working with tools or participating in sports, wear safety goggles.
- In order to protect the eyes when outside, people should use sunglasses.
- It is important to take regular eye exams.
- In order to prevent any eye disease it is important to have nutritious and healthy meals.
- Using special working glasses while working on computer and mobiles
4.) Explain how optical and non-optical aids help visually challenged persons to develop their skills to a higher level.
Answer: Visually impaired people can benefit from optical devices including magnifying lenses, telescoping lenses, as well as vision aides. Text can be enlarged using these methods to make it simpler to read and comprehend. Braille and speech software are examples of non-optical aids that can help people who are blind or visually impaired access information and interact with others. By enabling visually impaired people to interact with their surroundings, the employment of both optical as well as non-optical aids can help them advance their skills.
5.) What is the Braille system? How does it help the visually challenged persons?
People who are blind or visually challenged utilise the Braille system of writing. Raised dots that stand in for characters, numerals, and symbols are used to create it. One can comprehend the text by feeling the dots. People with vision problems can read a book, newspaper, journals, and other printed items on their own thanks to the Braille system. They benefit from improved interpersonal communication as well as assistance with routine tasks like arranging their possessions.