These DAV Class 7 Science Notes and DAV Class 7 Science Chapter 12 Notes – Light act as excellent revision resources, particularly in preparation for board exams.
Light Class 7 DAV Notes
→ Reflection of Light: When light falls on a shining surface, there is a change in direction of propagation of light. This phenomenon is called reflection of light.
→ Regular Reflection: When light falls on a smooth surface, the reflection is called regular reflection or plane reflection.
→ Irregular Reflection: When light falls on a rough surface, the reflection is called irregular or diffused reflection.
→ Laws of Reflection:
→ First Law: The incident ray, the normal at the point of incidence and the reflected ray; all lie in the same plane.
→ Second Law: Angle of incidence and angle of reflection are equal.
→ Formation of Image by Plane Mirror:
- In case of a plane mirror, the image is of the same size as object.
- Image is laterally inverted.
- In case of plane mirror, virtual image is formed.
- The image distance is same as object distance from the plane mirror.
→ Multiple Images: When two or more than two plane mirrors are kept at an angle with each other, multiple images are formed. This property is utilised to make kaleidoscope.
→ Spherical Mirror: When a mirror is a part of a sphere, it is called spherical mirror. Spherical mirrors are of two types:
(a) Concave mirror: When the reflecting surface is on the inside of spherical surface, it is called a concave mirror.
(b) Convex mirror: When the reflecting surface is on the outside of spherical surface, it is called a convex mirror.
→ Terms used in connection with spherical mirror:
→ Centre of curvature: The centre of sphere (of which the mirror is a part) is called centre of curvature of the spherical mirror.
→ Radius of curvature: The radius of sphere (of which the mirror is a part) is called the radius of curvature of the spherical mirror.
→ Pole: The central point of the spherical mirror is called its pole.
→ Principal axis: The line joining the pole to centre of curvature is called the pole of the spherical mirror.
→ Normal: The perpendicular to the mirror; at the point of incidence is called normal.
→ Focus: In case of a concave mirror, light rays coming from infinity converge at a point after reflection. In case of a convex mirror, light rays coming from infinity appear to be diverging from a point after reflection. This point is called the focus of the mirror. Focal length is half of the radius of curvature for a spherical mirror.
So, f = r/2
Laws of Reflection and Spherical Mirrors
→ Laws of reflection are obeyed in spherical mirrors as well. Following are the important points in case of spherical mirror:
(a) A ray; parallel to the principal axis; passes through the focus of the mirror or appears to diverge from the focus; after reflection.
(b) A ray; directed towards the focus of the mirror; comes out as parallel to the principal axis after reflection.
(c) A ray; directed towards the centre of curvature; retraces its path after reflection.
Table
→ Uses of Concave Mirror:
- As reflector in headlights and flashlights.
- By ENT specialists, to throw a beam of light inside ear.
- By dentists, to see enlarged image of the tooth.
- By hairdresser, to see enlarged image of the face.
→ Uses of Convex Mirror:
- At hairpin bends, so that a driver can see the vehicles coming from other side of the bend.
- As rear view mirror in vehicles.
→ Mirror: A surface with one polished surface and another reflecting surface.
→ Normal: Perpendicular to the reflecting surface at the point of incidence.
→ Incident ray: The light ray falling on a surface.
→ Reflected ray: The light ray reflecting from a surface.
→ Converging mirror: Concave mirror is also known as converging mirror.
→ Diverging mirror: Convex mirror is also known as diverging mirror.
→ Kaleidoscope: A device which shows beautiful patterns because of multiple images.
→ Real image: An image which can be obtained on screen.
→ Virtual image: An image which cannot be obtained on screen.
→ Plane mirror: A mirror with plane reflecting surface.
→ Concave mirror: A spherical mirror with inwardly curved reflecting surface.
→ Convex mirror: A spherical mirror with outwardly curved reflecting surface.
→ Pole: the central point of the given spherical mirror.
→ Focus:The rays of light meet at a point (in concave mirror) or appear to diverge from a point (in case of convex mirror). This point is called the focus of the spherical mirror.
| Image formation by concave mirror | ||
| Object location | Image location | Nature of image |
| At infinity | At focus | Real, inverted and highly diminished |
| Between infinity and C | Between F and C | Real, inverted and diminished |
| At C | At C | Real, inverted and same size |
| Between C and F | Beyond C | Real, inverted and enlarged |
| At F | At infinity | Real inverted and highly enlarged |
| Between F and P | Behind the mirror | Virtual, erect and enlarged |
| Image formation by convex mirror | ||
| Object location | Image location | Nature of image |
| Between infinity and C | Behind the mirror, between P and F | Virtual, erect, diminished |