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Class VII - science: Light
One Word Answer Questions:
Q) Name the image that cannot be obtained on a screen?
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Q) Which mirror image is erect and smaller in size than the object?
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Q) An image formed on a screen is called?
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Q) Which type of mirror can form a real image?
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Q) Which type of lens forms always a virtual image?
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Q) Objects through which we can see clearly are called?
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Q) Give an example of a non-luminous object?
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Q) When an opaque object comes in the path of light, what is formed?
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Q) Give an example of an opaque object?
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Short Answer Questions:
Q) What is a virtual image? Give one situation where a virtual image is formed?
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Q) Explain the path of light with an example?
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Q) What is reflection of light?
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Q) Define concave lens?
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Q) What is a rainbow? How it is formed?
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Q) State the differences between a convex and a concave lens?
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Q) How to create a Newton's disc?
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Q) What is a pinhole camera? How is it useful?
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Q) What is reflection?
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Q) Define shadow?
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Q) What are luminous objects? Explain with an example?
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Q) Differentiate between a shadow and mirror image?
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Long Answer Questions:
Q) Explain the image formed by a plane mirror?
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Q) What is a convex lens? What are its applications?
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Q) What is white light? Explain the dispersion of light through a prism?
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Q) What is reflection? Explain the two types of reflection?
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Q) Define opaque, trasparent and translucent objects with examples?
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Q) What are the differences between luminous and non-luminous objects? Explain?
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Light

Light
Light: Light is a form of energy. We are able to see the world around us because of light.

Propagation Of Light

  • Light always travels in a straight line. The following experiment proves that light always travels in a straight line.
  • When the candle is viewed through a straight tube, its flame can be seen. In case of a bent tube, the flame cannot be seen.
  • Seeing A Candle Through A Straight Tube And A Bent Tube
Reflection of Light

    Reflection: When light falls on a shiny surface, it bounces back. This phenomenon is called reflection.
    We are able to see images in mirrors or in water, because of reflection.

water
Reflection Of Objects In Water

Laws of Reflection: There are two laws of reflection.

  • The incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
  • Angle of incident and angle of reflection are equal.

Whenever an image is formed by a mirror or by any reflecting surface; the laws of reflection are obeyed.


reflection
Reflection Of Light From A Mirror
  • Real Image: When the image can be obtained on a screen, it is called real image. Real image is formed in front of mirror. Images formed on the retina are real images. Images formed on the film of a camera are real images.
  • Virtual Image: When the image cannot be obtained on a screen, it is called virtual image. Virtual image is formed behind the mirror.
mirror
Real Image And Virtual Image

    Image formation in Plane Mirror:
  • Image is of the same size as object.
  • The distance of image and object from the plane mirror is same. Image is erect and virtual.

plain
Image Formation In Plain Mirror

    Lateral Inversion in Plane Mirror:
  • A plane mirror makes laterally inverted image. This is the reason, your right hand looks like the left hand of your image.
  • The word ambulance is written in laterally inverted form on the front of the ambulance. This is deliberately done so that the driver in a vehicle ahead can easily spot the ambulance and can give way.
  • ambulance
    Ambulance Written Reverse
Spherical Mirror

When the mirror is a part of a sphere, it is called spherical mirror

Spherical mirrors are of two types.

  • Concave mirror
  • Convex mirror

In concave mirror, the reflective surface is inside the sphere, i.e. is depressed. In convex mirror, the reflective surface is outside the sphere, i.e. is bulged or protruded.

concave
Concave Surface/Convex Surface

Image formation in Concave Mirror: In case of concave mirror, the image is usually inverted, real and smaller than the object. When the object is kept too close to the concave mirror, the image is erect, virtual and larger than the object.

Uses of Concave Mirror:
  • Concave mirror is used by dentists and ENT specialists to focus a beam of light to see inside a patients mouth or ears.
  • concavemirror
    Concave Mirror Use By Dentists
  • Concave mirror is used in solar furnace, because rays of sunlight converge at a point once they reflect from the concave mirror.
  • Concave mirror is used as barbers mirror, because it shows a larger image when object is too close.

Image formation in Convex Mirror: In case of convex mirror, the image is erect, virtual and smaller than the object.

Uses of Convex Mirror:
  • Convex mirror is used in rear view mirrors, as it shows smaller images from a bigger field of view.

convex
Small Images Of Cars Can Be Seen Using Convex Mirror
  • Convex mirror is used on hairpin bends, to see the vehicles coming from other side of the bend.

Lens
  • Lens is made of two transparent surfaces in which at least one surface is curved. Convex and concave lenses in which both surfaces are either convex or concave are the most common types.

lenses
Small Images Of Cars Can Be Seen Using Convex Mirror

Image formation in Convex Lens: In case of convex lens, the image is usually inverted, real and smaller than the object. When the object is kept too close to the convex lens, the image is erect, virtual and larger than the object.

convexlens
Convex Lens Image Formation

Image formation Concave Lens:In case of concave lens, the image is erect, virtual and smaller than the object.

concavelens
Concave Lens Image Formation

Uses of Lens:Convex lens is used in microscope, telescope and camera. Convex lens is also used in reading glasses. Concave lens is used in spectacles.

Sunlight-White Or Colored

Components of White Light: The white light, or visible spectrum, is composed of seven colours. These colours are VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange and Red). When white light passes through a prism, it breaks down into its component colours. This is the reason rainbows show all the colours of the visible spectrum.

sevencolors
Prism Splits Sunlight Into Seven Colours

Newtons Disc: Since Newton was the first to make this; it is called Newtons Disc. It is a circular disc on which seven colours of the rainbow are painted on seven different sectors. When the disc is rotated at certain speeds, it appears to be white. This shows how the seven colours make the white light.

disc
On The Left Is,A Disc With Seven Colours,And On The Right The Disc Appears
White On Rotating
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