Solved examples for class 8 science : Chapter 16

    Light

    I. Multiple Choice Questions (MCQs).

    Q.1. The bouncing of light after striking the reflecting surface is called
    a) reflection
    b) refraction
    c) dispersion
    d) none of these

    Answer. (a) reflection

    Q.2. We can see objects because light travels
    a) from source of light to object
    b) from object to the eye
    c) from eye to the object
    d) from object to air

    Answer. b) from object to the eye

    Q.3. Which of the following is not correct regarding a real image?
    a) It can be taken on the screen
    b) It is always inverted
    c) Rays after reflection or refraction actually meet
    d) It is erect but laterally inverted

    Answer. (d) It is erect but laterally inverted

    Q.4. The perpendicular drawn at the point of incidence is known as
    a) normal
    b) tangent
    c) ray
    d) none of these

    Answer. a) normal

    Q.5. Eye lens is a
    a) concave lens
    b) convex lens
    c) bifocal lens
    d) cylindrical lens

    Answer: b) convex lens

    II. Fill in the blanks by selecting appropriate words from the given list:

    List: abnormalities, Iris, every, regular, retina
    Q.1. Law of reflection is valid for……………………………….. surface.
    Q.2. Images in mirrors are formed by ……………………..reflection.
    Q.3. …………………………….controls the amount of light that enters the eyes by adjusting the size of the pupil.
    Q.4. The eye always forms the image on the…………………………. of objects at varying distances from the eye.
    Q.5. The………………………….. in the normal vision of the eye are called defects of vision.

    Answer.
    1- every,
    2- regular,
    3- Iris,
    4- retina,
    5- abnormalities
    III. Match the items of Column A with those of Column B.


    Answer. 1-e, 2-a, 3-d, 4-b, 5- c

    IV. Write ‘T’ for the true and ‘F’ for the false statements.
    Q.1. We can see those things/ objects which emit or reflect light that reaches our eyes.
    Q.2. If mirror rotates, then reflected light does not rotate.
    Q.3. The minimum height of mirror needs to be only half of the height of the person for him/her to be able to see himself/herself.
    Q.4. Regular reflection is obtained when parallel rays are reflected from a rough surface.
    Q.5. Plane mirrors are used for making periscope and kaleidoscope.

    Answer. 1-T, 2-F, 3-T, 4-F,5-T

    V. Very Short Answer Questions.

    Q.1. Name the kind of beam when rays originate at a point.
    Answer. Converging beam

    Q.2. Name the kind of beam when rays diverge in different directions.
    Answer. Diverging beam

    Q.3. Name the kind of image which is obtained on a screen.
    Answer. Real image

    Q.4. What will be the angle of incidence if the angle between the incident ray and the reflected ray is 90°?
    Answer. 45°

    Q.5. Name the lens which is used for myopic eye.

    Answer. Concave lens

    VI. Short Answer Questions.
    Q.1. Define beam and rays.
    Answer. Beam: A beam of light consists of a number of light rays. Thus a beam of light is group of parallel rays of light.
    Ray: A ray of light is a path along which light travels. It is represented by a line with an arrow to indicate the direction.

    Q.2. What do you mean by lateral inversion?

    Answer.: The phenomenon of left appearing right and right appearing left on reflection in a plane mirror is known as lateral inversion.

    Q.3. ‘A’ is standing 5 m from a plane mirror. How far will the image seem to be from ‘A’?
    Answer. 10 m

    Q.4. Describe laws of reflection. How can these laws be experimentally verified?
    Answer.
    Laws of reflection:
    i. Incident ray, the reflected ray, and the normal to the surface of the mirror all lie in the same plane.
    ii. The angle of refection is equal to the angle of incidence

    To verified laws of reflection experimentally, we required, a torch light, a plane mirror fixed on a stand, a white plane sheet of paper, a card board with a slit.

    Procedure: Fix a white sheet of paper on a table. Now take a cardboard having slit and adjust it vertically on the white sheet of paper with the help of any support. Shine a torch just behind the slit. You can see the path of light on the sheet of paper. Keep the mirror vertically in the path of light. The ray of Light after striking the mirror, moves (reflected) in another direction.


    Make the position of the mirror MN with a straight Line. Using a scale and a pencil, mark the path of the incident ray AO and the reflected ray OB. Draw a perpendicular OP making an angle of 90° with Line MN. This perpendicular is known as normal.
    Measure < i = <r. Vary the value of <i and for this value measure <r respectively.



    Put the values of
    You will find that these angles are equal. Thus, we can conclude that, the reflected ray leaves the
    mirror at the same angle at which the incident ray strikes it. In other words, Angle of incidence = Angle of reflection

    You notice that reflected ray, incident ray and normal are drawn on plane sheet of paper. In this case, white sheet of paper acts as a plane, in which incident ray, reflected ray and normal lies, which is the second law of reflection

    Q.5. Discuss formation of image by a plane mirror. Describe characteristics of such image.
    Answer. The image which can be seen only into a plane mirror but cannot be obtained on a screen is called a virtual image. It is formed when the Light rays after reflection or refraction do not actually intersect each other but they appear to diverge from it. Geometrically, they intersect when they are produced in backward direction.

    Characteristics of the image formed by a plane mirror

    (i) The image is as far behind the mirror as the object is in front of it.
    (ii) The image formed is erect.
    (iii) The image is of the same s1ze as the object.
    (iv) The image formed is virtual, which means, such an image cannot be caught on a screen. (v) The image is laterally inverted that is the right side of the object appears as the left side of the image.

    Q.6. (a) What do you mean by regular and irregular reflections?
    (b) What is multiple images? Explain with an example.

    Answer. (a) (i) When, all the parallel rays reflected from a rough surface are not parallel, the reflection is known as irregular or diffused reflection.
    (ii) When all the parallel rays reflected from a smooth surface are parallel in one direction, is known as regular reflection.

    (b) Multiple images are the images that are formed when two or more mirrors are placed in front of each other and an object is placed between them.

    When an object is placed in front of two plane mirrors placed at right angles to each other, three images are formed as shown in the given figure.


    VII. Long Answer Questions.

    Q.1. Describe human eye with well labelled diagram. Also explain its main parts.

    Answer.


    The main parts of the eye are :
    (i) Cornea: This is the front portion of the eyeball. It is made up of a transparent material. The light enters the eye through cornea. It protects the eyes, at the same time it helps in focussing light. (ii) Pupil and iris: Iris is situated behind the cornea. It has a hole in the centre known as pupil. It is a dark coloured muscular diaphragm. The pupil looks black as no light is reflected from it. Iris controls the amount of light that enters the eyes by adjusting the size of the pupil.
    (iii) Eye lens: It is a convex lens made of a transparent jelly like material. Eye lens is held in position with the help of ciliary muscles and changes the focal length of the eye lens.
    (iv) Aqueous humour: This is a transparent liquid, which fills the region between the cornea and the eye lens. This in fact, is also the part of the eye lens.
    (v) Vitreous humour: It is a jelly-like substance which fills the space between retina and the lens.
    (vi)Retina: It consists of fine nerve tissue which lines the inside wall of the eyes and act like the film in the cornea. Its function is to convert light into electrical signals that are later sent down the optic nerve to the brain.
    (vii) Optic nerve: The optic nerve carries image of objects from the retina to the brain.

    Q.1. (a) What do you mean by accommodation of the eyes?
    (b) Describe defects of vision. How are these defects corrected?

    Answer. a) The ability of the eye to alter the focal Length of its Lens to see objects clearly within a certain range is called accommodation of the eye.

    b) Abnormalities in the normal vision of the eye are called defects of vision.
    The common defects of vision are:
    i. Myopia or shortsightedness
    ii. Hypermetropia or longsightedness

    Myopia
    The person who suffers from this defect, he/she can see nearby objects clearly, but distant objects appear blurred.

    Causes of this defect
    (i) Eye lens becomes too thick.
    (ii) The ciliary muscles do not relax sufficiently to make the eye Lens thin when viewing distant objects.

    Correction of Myopia
    Spectacles with concave Lenses of the required power are used. The concave Lens diverges the parallel rays coming from a distant object in such a way that they appear to come from the far point of the objective eye. In other words, the Lens forms a virtual image of a far off object at the far point of the defective eye. The eye then can see the distant object clearly.


    Hypermetropia
    This is also known as Long sightedness. A person suffering from this defect can see distant objects clearly but nearby objects appear blurred.

    Cause of this defect
    (i) This defect of vision is caused due to the eye Lens being too thin.
    (ii) The ciliary muscles are unable to thicken the eye Lens enough to see nearby objects clearly. This defect generally occurs in old age when the ciliary muscles become weak.

    Correction of Hypermetropia

    Spectacles with convex lenses of the required power are used. The convex lens converges the rays from a nearby object in such a way that they appear to come from the near point of the defective eye and forms a sharp image of the object on the retina.


    Q.3. (a) Describe some steps which are required to maintain our eyes in healthy state.
    (b) What do you mean by Braille system?

    Answer. a) we must take the following steps to maintain our eyes in healthy state:
    (i) Do not read, write or do any work in dim light or very bright light. This can cause defect to vision.
    (ii) If any injury is caused to your eyes, consult a doctor immediately.
    (iii) Do not read in moving vehicle because eyes are strained and in the long run can cause serious defects to vision.
    (iv) Wash and clean your eyes with clean cold water at least thrice a day.
    (v) Avoid working continuously in long stretches if the work requ1res a close look. (vi) Do not play carelessly and avoid playing with pointed and sharp objects, which may damage your eyes.
    (vii) If anything gets into your eyes, do not rub your eyes. Use clean and cold water to clean your eyes.

    b) The Braille system is a way of writing things. It is named after Louis Braille. It is a method that is widely used by blind people to read and write. Braille designed a coding system, based on patterns of raised dots which the blind could read by touch. Each Braille character or cell is made up of six dot or raised points. The Braille system consists of 63 possible combinations. It is used in modified form for printing, writing and musical notation for the blind.

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