DAV Class 7 Science Chapter 12 Notes – Light

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