Class 7 science light
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
(d) incident ray
2. The image of an object formed by plane the mirror is:
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?
(d) either convex or concave
6. The splitting up of white light into seven colours on passing through a glass prism is called.
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
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.
(a) T (b) F (c) T (d) F (e) T