Solved examples for class 7 science : Chapter 15

    Light

    I. Multiple Choice Questions (MCQs).
    Q.1. Light travels in a
    a) straight line
    b) curved line
    c) either (a) or (b)
    d) none of these
    Answer: (a) straight line
    Q.2. The bouncing back of light from a mirror is called
    a) refraction of light
    b) reflection of light
    c) dispersion of light
    d) none of these
    Answer: (b) reflection of light
    Q.3. An image which can be obtained on a screen is called a
    a) real image
    b) virtual image
    c) either (a) or (b)
    d) none of these
    Answer: (a) real image
    Q.4. A spherical mirror in which the reflecting surface is the outer side is called
    a) concave mirror
    b) convex mirror
    c) either (a) or (b)
    d) none of these
    Answer: (b) convex mirror
    Q.5. A curved and polished piece of glass bound by two curved surfaces or by one curved and one plane surfaces is known as
    a) mirror
    b) prism
    c) lens
    d) all of these
    Answer: (c) lens

    II. Fill in the blanks by selecting appropriate words from the given list :
    List : two, prisms, glass slab, convex, erect, same, invisible

    Q.1. Light is an ______________ energy, but it makes the dust particles in path of light visible as the light falls on them.
    Q.2. Reflection of light is of ______________ kinds depending on the nature of reflecting surfaces.
    Q.3. An image formed by a plane mirror is ______________ and of the ______________ size as the object.
    Q.4. Lenses are considered as a combination of a two _________ and a __________.
    Q.5. A magnifying glass is a ______________ lens of short focal length.

    Answer:
    1- invisible,
    2- two,
    3- erect, same
    4- prisms, glass slab
    5- convex

    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. The property of light of travelling in straight lines in a medium is called rectilinear propagation of light.
    Q.2. Any polished or shiny surface acts like a mirror.
    Q.3. Reflection of light does not depend on the nature of the reflecting surfaces.
    Q.4. Convex mirrors are used as reflectors in the headlight of cars and torches.
    Q.5. The outward bending of the ray of light by concave lens is called its diverging action.
    Answer: 1-T, 2-T, 3-F, 4-F, 5-T
    V. Very Short Answer Questions.
    Q.1. Why is the image formed by irregular reflection not sharp?
    Answer: The image formed by irregular reflection not sharp because the reflected light falls over a large area.

    Q.2. Which image can be obtained on a screen?
    Answer: Real image can be obtained on a screen.
    Q.3. Name the phenomenon in which left of an object appears right and right appears left on reflection in a plane mirror.
    Answer: Lateral inversion.
    Q.4. Name the mirror which forms real image.
    Answer: Concave Mirror except the case when the object is held close to a concave mirror.
    Q.5. Name the lens which forms real image.

    Answer: Convex lens except the case when the object is held close to a lens.
    VI. Short Answer Questions.
    Q.1. Why is light not visible when looking through a bent tube?
    Answer: It is because light travels along a straight line.

    Q.2. Why is the word written on an ambulance car ‘AMBULANCE’?
    Answer: in an ambulance car, the word “AMBULANCE” is written as .It is so because when the driver of a vehicle ahead of an ambulance looks in her/his rear view mirror, he/she can read “AMBULANCE” written on it and may give way to it immediately.

     

    Q.3. How can you use a concave mirror to burn a paper?
    Answer: The concave mirror has a real focal point. So, whenever a parallel beam of light rays has a point of incidence on the concave mirror, then all the rays will meet at the focal point. So, when light coming from the sun has a point of incidence on a concave mirror, then all the rays that hit the mirror will be focused onto one point. But sunlight not only consists of light, but also heat radiation. The heat carried by each ray is focused at the focal point, so if we place a piece of paper in front of it, the paper will burn.
    Q.4. What do you mean by reflection of light? Discuss types of reflection with suitable examples.
    Answer: Reflection of light is the phenomenon of bouncing back of light from a smooth polished surface.
    i. Reflection of light is of two kinds depending on the nature of the reflecting surfaces. When a ray of light falls on a smooth reflecting surface, e.g., a mirror, it gets reflected along a particular direction. That is why a smooth surface gives a sharp and clear image. Reflection of light from a smooth surface [mirror] is called regular reflection.
    ii. When light rays falls on a rough and not so smooth reflecting surface they get reflected in different directions. As a result, the reflected light falls over a large area and the image formed is not sharp and clear. This type of reflection is called irregular (or diffused) reflection.

    Q.5. Describe image formation by a plane mirror and also explain the nature of image formed by a plane mirror.
    Answer: When an object is placed in front of the mirror, we are able to see the image of the object. Image in the mirror is of the same size as that of the object and at the same distance from the mirror where the object has been kept. If the object is moved back the image also moves behind in the mirror. The image formed in the mirror is laterally inverted i.e. left side seems to be on the right side and vice versa.
    It is not possible to take the image on a screen. This is because the image is virtual.
    Nature of the image formed in a plane mirror:

    Nature of the image formed by a plane mirror is
    • Always virtual,
    • Upright, and
    • Of the same shape and size as the object it is reflecting.

    Q.6. What do you mean by spherical mirror? Describe its kinds and nature of image formed by them.
    Answer: A curved mirror which is part of a hollow sphere is called spherical mirror.
    It is of two kinds
    i. Concave Mirror
    ii. Convex Mirror

    Nature of image formed by a concave mirror:

    When the object is moved away from the concave mirror it forms a real, inverted and enlarged image. But if the distance becomes more, image gradually decreases in size only. When the object is held close to a concave mirror, it forms a virtual, erect and enlarged image.

    Nature of image formed by a convex mirror:

    Convex mirror always forms a virtual, erect and diminished image. When the object moves towards a convex mirror, the size of the image increases, but is never more than the object.

    VII. Long Answer Questions.

    Q.1. What is a lens? Describe its kinds and nature of image formed by them briefly. Write the uses of lens also.
    Answer: A lens is a curved and polished piece of glass or any other transparent material bound by two curved surfaces or by one curved and one plane surfaces.

    Nature of image formed by a Convex Lens

    Image formed by convex lens is real, inverted and diminished when an object is fairly far away from it. The image becomes larger as the object is slowly moved towards the lens till a stage comes when the size of the image is equal to the size of the object. When the object is again moved towards the lens, the size of the image becomes bigger than the size of the object.

    Nature of image formed by a Concave Lens

    The image formed by concave lens is always virtual and erect. However, the size of image becomes large, but is never equal to the size of the object.

    Application of lenses

    • In cameras
    • In the telescopes
    • In compound microscopes
    • In magnifying glass or simple microscopes.

    Q.2. (a) What do you mean by recomposition of the dispersed white light? (b) How will you show that white light is made up of seven colours?
    Answer.(a) When two glass prisms are placed opposite to each other, the spectrum produced by the first glass prism is recombined when these rays (VIBGYOR) are allowed to pass through the oppositely placed second glass prism. This is known as recomposition of the dispersed white light.

    b) To show that white light is made up of seven colours we need a circular cardboard disc; a ball pen Seven colours.
    Procedure : Take a circular cardboard disc of about 10 cm diameter. Divide this disc into seven segments. Paint these segments with seven rainbow colours or paste seven coloured papers [of rainbow) in these segments. Make a small hole in the centre of the disc and fix it loosely on the tip of the refill of a ball-pen. Take care that the disc rotates on the ball pen freely. Rotate the disc fast in the daylight. The seven colours get mixed up and a whitish patch appears on the disc which shows that light is made up of seven colours.

     

    Q.3. (a) Show the formation of real image of the sun by a concave mirror. Explain the process briefly.
    (b) Give main uses of concave and convex mirrors.

    Answer: To show the formation of real image of the sun by a concave mirror. Requirements: A concave mirror, white sheet of paper.

    Procedure : Take a concave mirror and hold it facing the sun. The concave mirror reflects the light. Hold the plane paper sheet in another hand. Try to get the light reflected by the mirror or try to obtain the image of the sun reflected from the mirror onto the white sheet. Adjust the distance between mirror and paper until you get a sharp bright spot. Hold the mirror and sheet of paper steady for a few minutes. After some time, you will notice that paper starts burning with a lot of smoke. This bright spot is, in fact, is the real image of the sun.

    b) Uses of concave mirrors
    Some of the main uses of concave mirrors are given below:

    • Concave mirrors are used as reflectors in headlights of cars and in torches.
    • Concave mirrors are used by dentists to focus on the tooth to be examined.
    • Concave mirrors are used as radiation collections in solar heating devices.
    • Concave mirrors are used as shaving mirrors and as make-up mirrors to see the enlarged, erect image of the face.
    Uses of convex mirrors
    • Convex mirrors are used in the shops to view what is going on in the shop and hence prevent shop-Lifting.
    • Big convex mirrors are fixed at the sharp, bends of certain roads to see the traffic coming from the other side to avoid accidents.
    • Convex-mirrors are used as rear-view mirrors in cars, scooters, buses and trucks. The drivers look into the rear-view mirror to see the traffic on the road behind them.

     

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