NCERT Class 7 Science Chapter 15 - Light Notes

Chapter 15 - Light Notes

1. Light is a form of energy which enables us to see objects from which it comes or from which it is reflected.

2. Properties of Light:
• Light travels in a straight line.
• The speed of light is faster than sound. Light travels at a speed of 3 x 10⁸ m/s.
• Reflection of light.

3. The objects which emit light on their own are known as luminous objects. Example: The sun, lamp, candle, etc.

4. The objects which do not emit light on their own are known as non-luminous objects. Example: chair, table, window, etc.

5. Light always travels in a straight line and this property of light is called rectilinear propagation of light.

6. Light falls on the surface of an object and the object sends the light back. This process of sending back the rays of light which fall on the surface of an object is called reflection of light.

7. The ray of light which falls on an object is called incident ray and the ray of light which is sent back by an object is called reflected ray.

8. A highly polished surface which is smooth enough to reflect a good fraction of light incident on it is called mirror.

9. Materials which don't allow light to pass through them completely are called opaque objects. Example: Wood etc

10. Materials which allow light to pass through them partially are called translucent objects. Example: Butter Paper etc

11. Materials which allow light to pass through them completely are called transparent objects. Example: Glass etc

12. An image that can be formed on the screen is called real image.

13. An image that can’t be formed obtained on the screen is called virtual image.

14. An erect image is the image that appears right side up. An image formed by a plane mirror is erect and of the same size as the object.

15. The image will be called inverted if it is formed upside - down compared to the object.

16. The characteristics of the image formed by a plane mirror are:
• Image distance and object distance are equal.
• Size of the objects and images are equal.
• The image formed is erect and virtual.

17. A mirror whose reflecting surface is the part of a hollow sphere of glass is known as a spherical mirror. There are 2 types of spherical mirror:
• Concave mirror
• Convex mirror

18. A concave mirror is a spherical mirror whose reflecting surface is on the inner side. It is also known as a converging mirror, as a parallel beam of light after reflection from a concave mirror converges at a point. The image formed by concave mirror is always real, inverted, and always different from object size. It is also virtual, erect when close the mirror.

19. A convex mirror is a spherical mirror whose reflecting surface is on the outer side. It is also known as a diverging mirror, as the parallel beam of light after reflection from a convex mirror diverges. The image formed by convex mirror is always virtual, erect, and always smaller than object size.

20. A lens is a type of mirror which refracts the light instead of reflection. There are two types of lenses:
• Convex lens
• Concave lens

21. Convex lenses are also called converging lenses since they converge a given parallel beam of light passing through them. They are thicker at the middle and thinner at the edges. The image formed by convex lens is always real, inverted, always lesser or greater than object size. It is also virtual, erect when object close to lens.

22. Concave lenses are also called diverging lenses since they diverge a parallel beam of light passing through them. They are thinner at the middle and thicker at the edges. The image formed by concave lens is virtual, erect and always smaller than object size.

23. When a beam of white light is passed through a prism, it splits into its component colours. This phenomenon is called dispersion of light.

24. White light is actually made up of seven colours. The order of colours is Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR).

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