Light For Class 8 Science Important Questions Answers
Q 1 – A smooth shining surface, which rebounds the light back in same or in different direction, is called (a) a mirror (b) a lens (c) reflection of light (d) point of incidence
Q 2 – Which one of the following statements is correct regarding rods and cones in the human eye? (a) Cones are sensitive to dim light (b) Cones are sensitive to bright light (c) Rods are sensitive to bright light (d) Rods can sense colour
Q 7 – An owl can see clearly at night but not day time because it has (a) More rods and few cones (b) Less rod and more cones (c) More rods and more cone (d) Less rods and less cones
Q 11 – If you hold a pen in your right hand and stand in front of the mirror, the pen will be in the left hand in the image. This phenomenon is called (a) lateral inversion (b) diffraction (c) reflection (d) inversion
Q 13 – Two mirrors A and B are placed at right angles to each other. A ray of light incident on mirror A at an angle of 25° falls on mirror B after reflection. The angle of reflection for the ray reflected from mirror B would be (a) 25° (b) 50° (c) 65° (d) 115°
Q 16 – Which of the following parts of an eye controls the amount of light entering the eye by contracting or dilating? (a) Retina (b) Cornea (c) Pupil (d) Iris
Q 17 – We can see ourselves in a mirror or a polished surface but not on walls because of (a) regular reflection (b) normal reflection (c) irregular reflection (d) specular reflection
Q 19 – The property of a plane mirror to make ‘right appear as left’ and vice versa, is called (a) vertical inversion (b) lateral inversion (c) reflection (d) refraction
Ans. The most popular resource for visually challenged persons is Braille. Braille system was developed by Louis Braille. He himself was a visually challenged person. There is Braille code for common languages, mathematics and scientific notation. Many Indian languages can be read using the Braille system.
Braille system has 63 dot patterns or characters. Each character represents a letter, a combination of letters, a common word or a grammatical sign. Dots are arranged in cells of two vertical rows of three dots each. Patterns of dots to represent some English letters and some common words are shown in Fig. 16.26.
These patterns when embossed on Braille sheets help visually challenged persons to recognise words by touching. To make them easier to touch, the dots are raised slightly.
Visually challenged people learn the Braille system by beginning with letters, then special characters and letter combinations. Methods depend upon recognition by touching. Each character has to be memorised. Braille texts can be produced by hand or by machine. Typewriter-like devices and printing machines have now been developed.
Q 28 – Make your own mirror. Take a glass strip or glass slab, clean it and put it on a white sheet of paper. See yourself in the glass. Next put the glass slab on a black sheet of paper. Again look into the glass. In which case do you see yourself better and why?
Ans. Eye ia a sense organ that enables us to see the world around us. It is roughly spherical in shape.
The first part that is bulged outward is called ‘cornea. It protects the eye.
Behind the cornea, the coloured part of the eye, iris is present. It controls the size of the pupil.
Pupil is a small opening in the cornea which allows the light to enter the eye.
Behind the iris, eye lens is present which is a convex lens. It focus the image on retina, by bending the light rays.
Retina is the inner back surface of the eye which acts as a screen to form image. It is sensitive to light.
The sensation of the image formed on the retina is carried to the brain by the optic nerve.
Optic nerve is connection between the eye and thebrain.
Q 30 – Boojho planned an activity to observe an object A through pipes as shown in the given figure, so that he could see objects which he could not directly see.
(a) How many mirrors should he use to see the object? (b) Indicate the positions of the mirrors in the figure. (c) What must be the angle with respect to the incident light at which he should place the mirrors? (d) Indicate the direction of rays in the figure. (e) If any of the mirrors is removed, will he be able to see the object?
Ans. Raw carrots, broccoli and green vegetables (such as spinach) and cod liver oil are rich in vitamin A. Eggs, milk, curd, cheese, butter and fruits such as papaya and mango are also rich in vitamin A.
Q 32 – A book lying on a table in a room can be seen from all the parts of the room. Give reason?
Ans. A book lying on a table in a room can be seen from all the parts of the room because the light is reflected by the book and it reaches our eye at any part of the room.
Q 33 – What are rods and cones in the retina of an eye?
Ans. There are two kinds of light sensitive cells on the retina. (i) cones, which are sensitive to bright light and (ii) rods, which are sensitive to dim light. Besides, cones sense colour.
Q 34 – What happens when a ray of light falls perpendicularly on the surface of a plane mirror?
Ans. The angle of incidence for such a ray of light is zero. Since the angle of incidence is zero, so according to the law of reflection, the angle of reflection should also be zero. This means that the reflected ray will also travel back from the mirror along the normal.
Q 35 – How many images of the coin will be seen when two plane mirrors are set at right angles to each other and a coin is placed in-between these two plane mirrors?
Ans. Sometimes, particularly in old age, eyesight becomes foggy. It is due to the eye lens becoming cloudy. When it happens, persons are said to have cataract. There is a loss of vision, sometimes extremely severe. It is possible to treat this defect. The opaque lens is removed and a new artificial lens is inserted. Modern technology has made this procedure simpler and safer.
Q 37 – How does eye adjust itself to deal with light of varying intensity?
Ans. The iris controls the amount of light entering into the eye by automatically adjusting the size of the pupil according to the intensity of the light that the eye receives. If the amount of light is high, the iris contracts the pupil and reduces the amount of light entering the eyes. If the amount of light is less, the iris expands the pupil so that more light can enter the eye and the things can be viewed clearly.
Q 38 – Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Iris – Iris controls the size of the pupil. The iris is the part of that eye which gives it its distinctive colour.
Eye-lens – The lens focuses light on the back of the eye, on a layer called retina.
Retina – Retina receives the light focused by the lens. Retina contains several nerve cells. Sensations felt by the nerve cells are then transmitted to the brain.
Optic nerve – The job of the optic nerve is to transfer visual information from the retina to the brain.
(i) Cataract: A cataract is a clouding of the lens in the eye leading to a decrease in vision. It can affect one or both eyes. Often it develops slowly. Symptoms may include faded colours, blurry vision, halos around light, trouble with bright lights, and trouble seeing at night. This may result in trouble driving, reading, or recognising faces. Cataracts are the cause of half of blindness and 33% of visual impairment worldwide. Cataract is treated by replacing the opaque lens with a new artificial lens.
(ii) Myopia: Near-sightedness or myopia, is the most common refractive error of the eye. Myopia occurs when the eyeball is too long, relative to the focusing power of the cornea and lens of the eye. This causes light rays to focus at a point in front of the retina, rather than directly on its surface. It can be corrected using spectacles made of concave lens.
(iii) Hypermetropia: Hypermetropia or long-sightedness occurs when eyeball is too short or the cornea or crystalline lens does not refract the light enough. This lead to formation of the image of a nearby object behind the retina. A hypermetropic person may have blurred vision when looking at objects close to them, and clearer vision when looking at objects in the distance. By placing a convex (plus powered) lens in front of a hypermetropic eye, the image is moved forward and focuses correctly on the retina.
(iv) Astigmatism: It is a defect in the eye or in a lens caused by a deviation from spherical curvature, which results in distorted images, as light rays are prevented from meeting at a common focus. It can be corrected by using a convex lens or concave lens or both.
2 = 50
