Class 7 science light
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light
KEYWORDS
Rectilinear propagation: Travelling of light in a straight line.
Reflection of light: The process of sending back the light rays which fall on the surface of an object.
Luminous object: The object which emits their own light.
Real image: The image which can be obtained on the screen.
Virtual image: The image which cannot be obtained on the screen.
Concave mirror: Reflecting surface curved inwards.
Convex mirror: Reflecting surface curved outwards.
Lens: A piece of any transparent material bound by two curved surfaces.
Dispersion of light: Splitting up of white light into its seven component colours.
VIBGYOR: A band of seven colours. Rainbow: An arch of seven colours formed by dispersion of white light (sunlight) through rainwater droplets. White light: Mixture of seven colours (VIBGYOR).
POINTS TO REMEMBER
• Light travels in straight lines. This is called the rectilinear propagation of light.
• The bouncing back of light rays from a surface is called reflection.
• The incident ray and the reflected ray make equal angles with a plane mirror.
• An image which can be formed on a screen is called real image.
• Virtual images can’t be formed on a screen.
• The image formed by a plane mirror is always erect, virtual and appears at the same distance behind the mirror as it is in front of the mirror.
• The image formed by a plane mirror is of the same size as that of the object and laterally inverted.
• A mirror with the curved reflecting surface is known as a spherical mirror.
• The spherical mirror whose reflecting surface is curved inwards is called a concave mirror.
• The spherical mirror whose outer surface is reflecting surface is called a convex mirror.
• A concave mirror forms a real or virtual image depending on the distance of the object from the mirror.
• A convex mirror always forms a virtual/diminished image of an object.
• Concave mirrors are used by dentists, in optical instruments, as reflectors in projectors, in-car head lights, as shaving mirror etc.
• Convex mirrors are used as rearview mirrors in cars and for viewing images of larger areas.
• A lens is a piece of transparent material with curved surfaces that are used to form images.
• A magnifying glass and a reading glasses have a convex lens, Glasses used to see distant objects have concave lenses.
• The focal length of a lens is the distance between the principal focus and the optical the centre of the lens.
• The images formed by a concave lens for all the positions of an object are virtual, erect and smaller than the object.
• The splitting up of white light into its component colours is called dispersion of light.
• The set of colours formed by the splitting of white light is called the spectrum of white light.
Exercises for class 7 science light
Multiple Choice Questions (MCQs)
1. A ray of light falling on a mirror is a
(a) reflected ray
(b) normal
(c) deflection
(d) incident ray
2. The image of an object formed by plane the mirror is:
(a) virtual
(b) real
(c) diminished
(d) upside-down
3. A diverging mirror is:
(a) a plane mirror
(b) a convex mirror
(c) a concave mirror
(d) a shaving mirror
4. If the image formed is always virtual, the mirror can be:
(a) concave or convex
(b) concave or plane
(c) concave or plane
(d) only convex
5. Which of the following mirror can form a real image of an object?
(a) convex
(b) concave
(c) plane
(d) either convex or concave
6. The splitting up of white light into seven colours on passing through a glass prism is called.
(a) refraction
(b) deflection
(c) dispersion
(d) scattering
7. If you are standing 2 m away from a plane mirror, the distance between you and your image are:
(a) 2 m
(b) 4 m
(c) 6 m
(d) 8 m
8. A spherical mirror having reflecting surface curved outward is a
(a) plane mirror
(b) concave mirror
(c) convex mirror
(d) either concave or convex depending on which way you look at it
Answers:
Multiple Choice Questions (MCQs)
1. (d) 2. (a) 3. (b) 4. (d ) 5. ( b) 6. (c) 7. (b) 8. (c)
State true (T) or false (F).
(a) Light is a form of energy.
(b) Light travels at the same speed in all medium
(c) The image formed by a plane mirror is always erect, virtual and laterally inverted
(d) A concave lens always forms a virtual image
(e) White light is composed of seven different colours.
Answers:
(a) T (b) F (c) T (d) F (e) T